We, the Quantuz team, (trying to join the GT community) offer our translation of the section of the particleadventure.org website dedicated to the Higgs boson. In this text we have excluded uninformative pictures (for the full version, see the original). The material will be of interest to anyone interested in the latest achievements of applied physics.

The role of the Higgs boson

The Higgs boson was the last particle discovered in the Standard Model. This is a critical component of the theory. His discovery helped confirm the mechanism of how fundamental particles acquire mass. These fundamental particles in the Standard Model are quarks, leptons, and force-carrying particles.

1964 theory

In 1964, six theoretical physicists hypothesized the existence of a new field (like an electromagnetic field) that fills all space and solves a critical problem in our understanding of the universe.

Independently, other physicists developed a theory of fundamental particles, eventually called the Standard Model, which provided phenomenal accuracy (the experimental accuracy of some parts of the Standard Model reaches 1 in 10 billion. This is equivalent to predicting the distance between New York and San Francisco with an accuracy of about 0.4 mm). These efforts turned out to be closely interconnected. The Standard Model needed a mechanism for particles to acquire mass. Field theory was developed by Peter Higgs, Robert Brout, Francois Englert, Gerald Guralnick, Carl Hagen and Thomas Kibble.

boson

Peter Higgs realized that, by analogy with other quantum fields, there must be a particle associated with this new field. It must have a spin equal to zero and, thus, be a boson - a particle with an integer spin (unlike fermions, which have a half-integer spin: 1/2, 3/2, etc.). And indeed it soon became known as the Higgs Boson. Its only drawback was that no one saw it.

What is the mass of the boson?

Unfortunately, the theory that predicted the boson did not specify its mass. Years passed until it became clear that the Higgs boson must be extremely heavy and most likely beyond the reach of facilities built before the Large Hadron Collider (LHC).

Remember that according to E=mc 2, the greater the mass of the particle, the more energy is needed to create it.

At the time the LHC began collecting data in 2010, experiments at other accelerators showed that the mass of the Higgs boson should be greater than 115 GeV/c2. During experiments at the LHC it was planned to look for evidence of a boson in the mass range 115-600 GeV/c2 or even higher than 1000 GeV/c2.

Every year, it was experimentally possible to exclude bosons with higher masses. In 1990 it was known that the required mass should be greater than 25 GeV/c2, and in 2003 it turned out that it was greater than 115 GeV/c2

Collisions at the Large Hadron Collider could produce a lot of interesting things

Dennis Overbye in the New York Times talks about recreating the conditions of a trillionth of a second after the Big Bang and says:

« ...the remains of [the explosion] in this part of the cosmos have not been seen since the Universe cooled 14 billion years ago - the spring of life is fleeting, over and over again in all its possible variations, as if the Universe were participating in its own version of the movie Groundhog Day»

One of these “remains” may be the Higgs boson. Its mass must be very large, and it must decay in less than a nanosecond.

Announcement

After half a century of anticipation, the drama became intense. Physicists slept outside the auditorium to take their seats at a seminar at the CERN laboratory in Geneva.

Ten thousand miles away, on the other side of the planet, at a prestigious international conference on particle physics in Melbourne, hundreds of scientists from all corners of the globe gathered to hear the seminar broadcast from Geneva.

But first, let's take a look at the background.

Fireworks 4th of July

On July 4th, 2012, the directors of the ATLAS and CMS experiments at the Large Hadron Collider presented their latest results in the search for the Higgs boson. There were rumors that they were going to report more than just a results report, but what?

Sure enough, when the results were presented, both collaborations that carried out the experiments reported that they had found evidence for the existence of a “Higgs boson-like” particle with a mass of about 125 GeV. It was definitely a particle, and if it is not the Higgs boson, then it is a very high-quality imitation of it.

The evidence was not inconclusive; the scientists had five-sigma results, meaning there was less than a one in a million chance that the data was simply a statistical error.

The Higgs boson decays into other particles

The Higgs boson decays into other particles almost immediately after it is produced, so we can only observe its decay products. The most common decays (among those that we can see) are shown in the figure:

Each decay mode of the Higgs boson is known as a "decay channel" or "decay mode". Although the bb mode is common, many other processes produce similar particles, so if you observe bb decay, it is very difficult to tell whether the particles are due to the Higgs boson or something else. We say that the bb decay mode has a “broad background”.

The best decay channels for searching for the Higgs boson are the channels of two photons and two Z bosons.*

*(Technically, for a 125 GeV Higgs boson mass, decay into two Z bosons is not possible, since the Z boson has a mass of 91 GeV, causing the pair to have a mass of 182 GeV, greater than 125 GeV. However, what we observe is a decay into a Z-boson and a virtual Z-boson (Z*), whose mass is much smaller.)

Decay of the Higgs boson to Z + Z

Z bosons also have several decay modes, including Z → e+ + e- and Z → µ+ + µ-.

The Z + Z decay mode was quite simple for the ATLAS and CMS experiments, with both Z bosons decaying in one of two modes (Z → e+ e- or Z → µ+ µ-). The figure shows four observed decay modes of the Higgs boson:

The end result is that sometimes the observer will see (in addition to some unbound particles) four muons, or four electrons, or two muons and two electrons.

What the Higgs boson would look like in the ATLAS detector

In this event, the “jet” (jet) appeared going down, and the Higgs boson was going up, but it decayed almost instantly. Each collision picture is called an "event".

Example of an event with a possible decay of the Higgs boson in the form of a beautiful animation of the collision of two protons in the Large Hadron Collider, you can view it on the source website at this link.

In this event, a Higgs boson can be produced and then immediately decays into two Z bosons, which in turn immediately decay (leaving two muons and two electrons).

Mechanism that gives mass to particles

The discovery of the Higgs boson is an incredible clue to how fundamental particles acquire mass, as claimed by Higgs, Brout, Engler, Gerald, Karl and Kibble. What kind of mechanism is this? This is a very complex mathematical theory, but its main idea can be understood by a simple analogy.

Imagine a space filled with the Higgs field, like a party of physicists calmly communicating with each other with cocktails...
At one point, Peter Higgs enters and creates excitement as he moves across the room, attracting a group of fans with every step...

Before entering the room, Professor Higgs could move freely. But after entering a room full of physicists, his speed decreased. A group of fans slowed his movement across the room; in other words, he gained mass. This is analogous to a massless particle acquiring mass when interacting with the Higgs field.

But all he wanted was to get to the bar!

(The idea for the analogy belongs to Prof. David J. Miller from University College London, who won the prize for an accessible explanation of the Higgs boson - © CERN)

How does the Higgs boson get its own mass?

On the other hand, as the news spreads around the room, they also form groups of people, but this time exclusively of physicists. Such a group can slowly move around the room. Like other particles, the Higgs boson gains mass simply by interacting with the Higgs field.

Finding the mass of the Higgs boson

How do you find the mass of the Higgs boson if it decays into other particles before we detect it?

If you decide to assemble a bicycle and want to know its mass, you should add up the masses of the bicycle parts: two wheels, frame, handlebars, saddle, etc.

But if you want to calculate the mass of the Higgs boson from the particles it decayed into, you can't simply add up the masses. Why not?

Adding the masses of Higgs boson decay particles does not work, since these particles have enormous kinetic energy compared to the rest energy (remember that for a particle at rest E = mc 2). This occurs due to the fact that the mass of the Higgs boson is much greater than the masses of the final products of its decay, so the remaining energy goes somewhere, namely, into the kinetic energy of the particles that arise after the decay. Relativity tells us to use the equation below to calculate the "invariant mass" of a set of particles after decay, which will give us the mass of the "parent", the Higgs boson:

E 2 =p 2 c 2 +m 2 c 4

Finding the mass of the Higgs boson from its decay products

Quantuz note: here we are a little unsure of the translation, since there are special terms involved. We suggest comparing the translation with the source just in case.

When we talk about decay like H → Z + Z* → e+ + e- + µ+ + µ-, then the four possible combinations shown above could arise from both Higgs boson decay and background processes, so we need to look at the histogram of the total mass of the four particles in these combinations.

The mass histogram implies that we are observing a huge number of events and noting the number of those events when the resulting invariant mass is obtained. It looks like a histogram because the invariant mass values ​​are divided into columns. The height of each column shows the number of events in which the invariant mass is in the corresponding range.

We might imagine that these are the results of the decay of the Higgs boson, but this is not the case.

Higgs boson data from background

The red and purple areas of the histogram show the "background" in which the number of four-lepton events expected to occur without the participation of the Higgs boson.

The blue area (see animation) represents the "signal" prediction, in which the number of four-lepton events suggests the result of the decay of the Higgs boson. The signal is placed at the top of the background because in order to get the total predicted number of events, you simply add up all the possible outcomes of events that could occur.

The black dots show the number of observed events, while the black lines passing through the dots represent the statistical uncertainty in these numbers. The rise in data (see next slide) at 125 GeV is a sign of a new 125 GeV particle (Higgs boson).

An animation of the evolution of data for the Higgs boson as it accumulates is on the original website.

The Higgs boson signal rises slowly above the background.

Data from the Higgs boson decaying into two photons

Decay into two photons (H → γ + γ) has an even wider background, but nevertheless the signal is clearly distinguished.

This is a histogram of the invariant mass for the decay of the Higgs boson into two photons. As you can see, the background is very wide compared to the previous chart. This is because there are many more processes that produce two photons than there are processes that produce four leptons.

The dashed red line shows the background, and the thick red line shows the sum of the background and the signal. We see that the data are in good agreement with a new particle around 125 GeV.

Disadvantages of the first data

The data were compelling but not perfect and had significant limitations. By July 4, 2012, there were not enough statistics to determine the rate at which a particle (the Higgs boson) decays into the various sets of less massive particles (the so-called "branching proportions") predicted by the Standard Model.

The "branching ratio" is simply the probability that a particle will decay through a given decay channel. These proportions are predicted by the Standard Model and measured by repeatedly observing the decays of the same particles.

The following graph shows the best measurements of branching proportions we can make as of 2013. Since these are the proportions predicted by the Standard Model, the expectation is 1.0. The points are the current measurements. Obviously, the error bars (red lines) are mostly still too large to draw serious conclusions. These segments are shortened as new data is received and the points may possibly move.

How do you know that a person is observing a candidate event for the Higgs boson? There are unique parameters that distinguish such events.

Is the particle a Higgs boson?

While the new particle had been detected to decay, the rate at which it was happening was still unclear by July 4th. It was not even known whether the discovered particle had the correct quantum numbers—that is, whether it had the spin and parity required for the Higgs boson.

In other words, on the 4th of July the particle looked like a duck, but we needed to make sure it swam like a duck and quacked like a duck.

All results from the ATLAS and CMS experiments of the Large Hadron Collider (as well as the Tevatron collider at Fermilab) after July 4, 2012 showed remarkable agreement with the expected branching proportions for the five decay modes discussed above, and agreement with the expected spin (equal to zero) and parity (equal to +1), which are the fundamental quantum numbers.

These parameters are important in determining whether the new particle is truly the Higgs boson or some other unexpected particle. So all available evidence points to the Higgs boson from the Standard Model.

Some physicists considered this a disappointment! If the new particle is the Higgs boson from the Standard Model, then the Standard Model is essentially complete. All that can now be done is to take measurements with increasing precision of what has already been discovered.

But if the new particle turns out to be something not predicted by the Standard Model, it will open the door to many new theories and ideas to be tested. Unexpected results always require new explanations and help push theoretical physics forward.

Where did mass come from in the Universe?

In ordinary matter, the bulk of the mass is contained in atoms, and, to be more precise, is contained in a nucleus consisting of protons and neutrons.

Protons and neutrons are made of three quarks, which gain their mass by interacting with the Higgs field.

BUT... the quark masses contribute about 10 MeV, which is about 1% of the mass of the proton and neutron. So where does the remaining mass come from?

It turns out that the mass of a proton arises from the kinetic energy of its constituent quarks. As you, of course, know, mass and energy are related by the equality E=mc 2.

So only a small fraction of the mass of ordinary matter in the Universe belongs to the Higgs mechanism. However, as we will see in the next section, the Universe would be completely uninhabitable without the Higgs mass, and there would be no one to discover the Higgs mechanism!

If there were no Higgs field?

If there was no Higgs field, what would the Universe be like?

It's not that obvious.

Certainly nothing would bind the electrons in the atoms. They would fly apart at the speed of light.

But quarks are bound by a strong interaction and cannot exist in a free form. Some bound states of quarks might be preserved, but it is not clear about protons and neutrons.

All of this would probably be nuclear-like matter. And maybe all this collapsed as a result of gravity.

A fact of which we are certain: the Universe would be cold, dark and lifeless.
So the Higgs boson saves us from a cold, dark, lifeless universe where there are no people to discover the Higgs boson.

Is the Higgs boson a boson from the Standard Model?

We know for sure that the particle we discovered is the Higgs boson. We also know that it is very similar to the Higgs boson from the Standard Model. But there are two points that are still not proven:

1. Despite the fact that the Higgs boson is from the Standard Model, there are small discrepancies indicating the existence of new physics (currently unknown).
2. There are more than one Higgs bosons, with different masses. This also suggests that there will be new theories to explore.

Only time and new data will reveal either the purity of the Standard Model and its boson or new exciting physical theories.

The Higgs boson, its place in the series of elementary particles and theoretically predicted properties. The importance of the search for the boson for the physical picture of the world. Experiments...

From Masterweb

10.06.2018 14:00

In physics, the Higgs boson is an elementary particle that scientists believe plays a fundamental role in the formation of mass in the Universe. Confirming or disproving the existence of this particle was one of the main goals of using the Large Hadron Collider (LHC), the most powerful particle accelerator in the world, which is located at the European Particle Physics Laboratory (CERN) near Geneva.

Why was it so important to find the Higgs boson?

In modern particle physics there is a certain standard model. The only particle that this model predicts, and which scientists have struggled to detect for a long time, is the boson named. The standard model of particles (according to experimental data) describes all interactions and transformations between elementary particles. However, the only “blank spot” remained in this model - the lack of an answer to the question of the origin of mass. The importance of mass is beyond doubt, because without it the Universe would be completely different. If the electron did not have mass, then atoms and matter itself would not exist, there would be no biology and chemistry, and, ultimately, there would be no man.

To explain the concept of the existence of mass, several physicists, including the British Peter Higgs, hypothesized the existence of the so-called Higgs field back in the 60s of the last century. By analogy with the photon, which is a particle of the electromagnetic field, the Higgs field also requires the existence of its carrier particle. Thus, Higgs bosons, in simple words, are particles from the multitude of which the Higgs field is formed.

The Higgs particle and the field it creates

All elementary particles can be divided into two types:

• Fermions.
• Bosons.

Fermions are those particles that form the matter we know, such as protons, electrons and neutrons. Bosons are elementary particles that determine the existence of various types of interactions between fermions. For example, bosons are the photon - the carrier of the electromagnetic interaction, the gluon - the carrier of the strong or nuclear interaction, the Z and W bosons, which are responsible for the weak interaction, that is, for transformations between elementary particles.

If we talk in simple terms about the Higgs boson and the meaning of the hypothesis that explains the appearance of mass, then we should imagine that these bosons are distributed in the space of the Universe and form a continuous Higgs field. When any body, atom or elementary particle experiences “friction” about this field, that is, interacts with it, then this interaction manifests itself as the existence of mass for this body or particle. The more a body “rubs” a particle against the Higgs field, the greater its mass.

How to detect and where to dig for the Higgs boson

This boson cannot be detected directly, since (according to theoretical data) after its appearance it instantly decays into other more stable elementary particles. But the particles that appeared after the decay of the Higgs boson can already be detected. They are the “traces” indicating the existence of this important particle.

Scientists collided high-energy beams of protons to detect the Higgs boson particle. The enormous energy of protons during a collision can turn into mass, according to Albert Einstein’s famous equation E = mc2. In the proton collision zone in the collider, there are many detectors that make it possible to record the appearance and decay of any particles.

The mass of the Higgs boson was not theoretically established, but only a possible set of its values ​​was determined. To detect a particle, powerful accelerators are required. The Large Hadron Collider (LHC) is currently the most powerful accelerator on planet Earth. With its help, it was possible to collide protons with an energy close to 14 tetraelectronvolts (TeV). It currently operates at energies of about 8 TeV. But even these energies turned out to be enough to detect the Higgs boson or the God particle, as many also call it.

Random and real events

In particle physics, the existence of an event is assessed with a certain probability "sigma", which determines the randomness or reality of this event obtained in the experiment. To increase the likelihood of an event, it is necessary to analyze a large number of data. The search for and discovery of the Higgs boson is one of these types of probable events. To detect this particle, the LHC generated about 300 million collisions per second, so the amount of data that needed to be analyzed was enormous.

We can speak about a real observation of a specific event with confidence if its “sigma” is equal to 5 or more. This is equivalent to the event of a coin (if you flip it and it lands on heads 20 times in a row). This result corresponds to a probability of less than 0.00006%.

Once this “new” real event is discovered, it is necessary to study it in detail, answering the question of whether this event exactly corresponds to the Higgs particle or is it some other particle. To do this, it is necessary to carefully study the properties of the decay products of this new particle and compare them with the results of theoretical predictions.

LHC experiments and discovery of the mass particle

Searches for the mass particle, which were carried out at the LHC colliders in Geneva and the Tevatron at Fermilab in the USA, established that the God particle must have a mass greater than 114 gigaelectronvolts (GeV), if expressed in energy equivalent. For example, let's say that the mass of one proton approximately corresponds to 1 GeV. Other experiments that were aimed at searching for this particle found that its mass cannot exceed 158 GeV.

The first results of the search for the Higgs boson at the LHC were presented back in 2011, thanks to the analysis of data that was collected at the collider over the course of one year. During this time, two main experiments were carried out on this problem - ATLAS and CMS. According to these experiments, the boson has a mass between 116 and 130 GeV or between 115 and 127 GeV. It is interesting to note that in both of these experiments at the LHC, according to many features, the boson mass is in a narrow region between 124 and 126 GeV.

Peter Higgs, together with his colleague Frank Englert, received the Nobel Prize on October 8, 2013 for the discovery of a theoretical mechanism for understanding the existence of mass in elementary particles, which was confirmed in the ATLAS and CMS experiments at the LHC at CERN (Geneva), when the experimentally predicted boson was discovered.

The importance of the discovery of the Higgs particle for physics

To put it simply, the discovery of the Higgs boson marked the beginning of a new stage in particle physics, as this event provided new ways for further exploration of the phenomena of the Universe. For example, the study of the nature and characteristics of black matter, which, according to general estimates, makes up about 23% of the entire known Universe, but whose properties remain a mystery to this day. The discovery of the God particle made it possible to think through and carry out new experiments at the LHC that will help clarify this issue.

Boson properties

Many of the properties of the God particle that are described in the standard model of elementary particles are now fully established. This boson has zero spin, no electrical charge and no color, so it does not interact with other bosons such as the photon and gluon. However, it interacts with all particles that have mass: quarks, leptons, and the weak interaction bosons Z and W. The greater the mass of the particle, the stronger it interacts with the Higgs boson. In addition, this boson is its own antiparticle.

The particle's mass, its average lifetime, and the interaction between bosons are not predicted by the theory. These quantities can only be measured experimentally. The results of experiments at the LHC at CERN (Geneva) established that the mass of this particle lies in the range of 125-126 GeV, and its lifetime is approximately 10-22 seconds.

Discovered boson and space apocalypse

The discovery of this particle is considered one of the most important in the history of mankind. Experiments with this boson continue, and scientists are obtaining new results. One of them was the fact that a boson could lead the Universe to destruction. Moreover, this process has already begun (according to scientists). The essence of the problem is this: the Higgs boson can collapse on its own in some part of the Universe. This will create an energy bubble that will gradually spread, absorbing everything in its path.

When asked whether the world will end, every scientist answers positively. The fact is that there is a theory called the “Stellar Model”. It postulates an obvious statement: everything has its beginning and its end. According to modern ideas, the end of the Universe will look like this: the accelerated expansion of the Universe leads to the dispersion of matter in space. This process will continue until the last star goes out, after which the Universe will plunge into eternal darkness. No one knows how long it will take for this to happen.

With the discovery of the Higgs boson, another doomsday theory emerged. The fact is that some physicists believe that the resulting boson mass is one of the possible temporary masses; there are other values. These mass values ​​can also be realized, since (in simple terms) the Higgs boson is an elementary particle that can exhibit wave properties. That is, there is a possibility of its transition to a more stable state corresponding to a larger mass. If such a transition occurs, then all natural laws known to man will take on a different form, and therefore the end of the Universe known to us will come. In addition, this process could already have occurred in some part of the Universe. Humanity does not have much time left for its existence.

The benefits of the LHC and other particle accelerators for society

Technologies that are being developed for particle accelerators are also useful for medicine, computer science, industry, and the environment. For example, collider magnets made of superconducting materials, with the help of which elementary particles are accelerated, can be used for medical diagnostic technologies. Modern detectors of various particles produced in the collider can be used in positron tomography (a positron is the antiparticle of an electron). In addition, technologies for forming beams of elementary particles in the LHC can be used to treat various diseases, for example, cancer.

As for the benefits of research using the LHC at CERN (Geneva) for information technology, it should be said that the global computer network GRID, as well as the Internet itself, owe their development largely to experiments with particle accelerators, which produced huge amounts of data. The need to share this data among scientists around the world led to the creation at CERN of the World Wide Web (WWW) language, on which the Internet is based, by Tim Bernels-Lee.

Beams of particles, which were and are being formed in various types of accelerators, are currently widely used in the industry for studying the properties of new materials, the structure of biological objects and chemical industry products. Achievements in particle physics are used to design solar energy panels, reprocess radioactive waste, and so on.

The impact of the discovery of the Higgs particle on literature, cinema and music

The following facts indicate the sensational nature of the news of the discovery of a mass particle in physics:

• Following the discovery of this particle, the popular science book "The God Particle: If the Universe is the Answer, What is the Question" was published? Lev Liederman. Physicists say calling the Higgs boson a God particle is an exaggeration.
• The movie Angels and Demons, which is based on the book of the same name, also uses the name "God particle" boson.
• The sci-fi movie Solaris, starring George Clooney and Natascha McElhone, puts forward a theory that mentions the Higgs field and its important role in stabilizing subatomic particles.
• In the science fiction book Flashforward, written by Robert Sawyer in 1999, two scientists cause global disaster when they conduct experiments to detect the Higgs boson.
• The Spanish series "Ark" tells the story of a global catastrophe in which all continents were flooded as a result of experiments at the Large Hadron Collider, and only the people on the ship "Polar Star" survived.
• The musical group from Madrid "Aviador Dro" in their album "Voice of Science" dedicated a song to the discovered mass boson.
• Australian singer Nick Cave in his album "Push the Sky Away" called one of the songs "Blue Higgs Boson".

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We can bet a large sum that most of you (including people interested in science) do not have a very good idea of ​​what physicists found at the Large Hadron Collider, why they looked for it for so long, and what will happen next.

Therefore, a short story about what the Higgs boson is.

We need to start with the fact that people are generally very poor at imagining in their minds what is happening in the microcosm, on the scale of elementary particles.

For example, many people from school imagine that electrons are small yellow balls, like mini-planets, revolving around the nucleus of an atom, or it looks like a raspberry made up of red and blue protons-neutrons. Those who are somewhat familiar with quantum mechanics from popular books imagine elementary particles as blurry clouds. When we are told that any elementary particle is also a wave, we imagine waves on the sea (or in the ocean): the surface of a three-dimensional medium that periodically oscillates. If we are told that a particle is an event in a certain field, we imagine a field (something humming in the void, like a transformer box).

This is all very bad. The words “particle”, “field” and “wave” reflect reality extremely poorly, and there is no way to imagine them. Whatever visual image comes to your mind will be incorrect and will interfere with understanding. Elementary particles are not something that can in principle be seen or “touched”, and we, the descendants of monkeys, are designed to imagine only such things. It is not true that an electron (or photon, or Higgs boson) “is both a particle and a wave”; this is something third, for which there have never been words in our language (as unnecessary). We (in the sense, humanity) know how they behave, we can make some calculations, we can arrange experiments with them, but we cannot find a good mental image for them, because things that are at least approximately similar to elementary particles are not found at all on our scale.

Professional physicists do not try to visually (or in any other way in terms of human feelings) imagine what is happening in the microworld; this is a bad path, it leads nowhere. They gradually develop some intuition about what objects live there and what will happen to them if they do this and that, but a non-professional is unlikely to be able to duplicate it.

So, I hope you don't think about little balls anymore. Now about what they were looking for and finding at the Large Hadron Collider.

The generally accepted theory of how the world works on the smallest scales is called the Standard Model. According to her, our world works like this. It contains several fundamentally different types of matter that interact with each other in different ways. It is sometimes convenient to talk about such interactions as the exchange of certain “objects” for which one can measure speed, mass, accelerate them or push them against each other, etc. In some cases it is convenient to call them (and think of them) as carrier particles. There are 12 types of such particles in the model. I remind you that everything I am writing about now is still inaccurate and profanation; but, I hope, still much less than most media reports. (For example, “Echo of Moscow” on July 4 distinguished itself with the phrase “5 points on the sigma scale”; those in the know will appreciate it).

One way or another, 11 of the 12 particles of the Standard Model have already been observed before. The 12th is a boson corresponding to the Higgs field - what gives many other particles mass. A very good (but, of course, also incorrect) analogy, which was not invented by me: imagine a perfectly smooth billiard table on which there are billiard balls - elementary particles. They easily scatter in different directions and move anywhere without interference. Now imagine that the table is covered with some kind of sticky mass that impedes the movement of particles: this is the Higgs field, and the extent to which a particle sticks to such a coating is its mass. The Higgs field does not interact in any way with some particles, for example, with photons, and their mass, accordingly, is zero; One can imagine that photons are like a puck in air hockey, and the coating is not noticed at all.

This whole analogy is incorrect, for example, because mass, unlike our sticky coating, prevents the particle from moving, but from accelerating, but it gives some illusion of understanding.

The Higgs boson is the particle corresponding to this “sticky field.” Imagine hitting a pool table very hard, damaging the felt and crushing a small amount of the sticky substance into a bubble-like fold that quickly flows back out. This is it.

Actually, this is exactly what the Large Hadron Collider has been doing all these years, and this is roughly what the process of obtaining the Higgs boson looked like: we hit the table with all our might until the cloth itself begins to transform from a very static, hard and sticky surface into something something more interesting (or until something even more wonderful happens, not predicted by theory). That is why the LHC is so large and powerful: they have already tried to hit the table with less energy, but without success.

Now about the notorious 5 sigma. The problem with the above process is that we can only knock and hope something comes of it; There is no guaranteed recipe for obtaining the Higgs boson. Worse, when he is finally born into the world, we must have time to register him (naturally, it is impossible to see him, and he exists only for an insignificant fraction of a second). Whatever detector we use, we can only say that it seems we may have observed something similar.

Now imagine that we have a special die; it falls randomly on one of the six faces, but if the Higgs boson is near it at that very time, then the six will never fall out. This is a typical detector. If we throw the dice once and at the same time hit the table with all our might, then no result at all will tell us anything at all: did it come up as a 4? Quite a probable event. Did you roll a 6? Perhaps we simply hit the table slightly at the wrong moment, and the boson, although existing, did not have time to be born at the right moment, or, conversely, managed to decay.

But we can do this experiment several times, and even many times! Great, let's roll the dice 60,000,000 times. Let's say that the six came up "only" 9,500,000 times, and not 10,000,000; Does this mean that a boson appears from time to time, or is it just an acceptable coincidence - we do not believe that the die should be a six smooth 10 million times out of 60?

Well uh. Such things cannot be assessed by eye; you need to consider how large the deviation is and how it relates to possible accidents. The greater the deviation, the less likely it is that the bone just lay down like that by accident, and the greater the likelihood that from time to time (not always) a new elementary particle arose that prevented it from lying like a six. It is convenient to express the deviation from the average in “sigmas”. “One sigma” is the level of deviation that is “the most expected” (its specific value can be calculated by any third-year student in the Faculty of Physics or Mathematics). If there are quite a lot of experiments, then a deviation of 5 sigma is the level when the opinion “randomness is unlikely” turns into an absolutely firm confidence.

Physicists announced the achievement of approximately this level of deviations on two different detectors on July 4. Both detectors behaved very similarly to how they would behave if the particle produced by hitting the table hard was actually a Higgs boson; Strictly speaking, this does not mean that it is he who is in front of us; we need to measure all sorts of other characteristics of it with all sorts of other detectors. But there are few doubts left.

Finally, about what awaits us in the future. Has “new physics” been discovered, and has a breakthrough been made that will help us create hyperspace engines and absolute fuel? No; and even vice versa: it became clear that in that part of physics that studies elementary particles, miracles do not happen, and nature is structured almost as physicists had assumed all along (well, or almost so). It's even a little sad.

The situation is complicated by the fact that we know with absolute certainty that in principle it cannot be structured exactly like this. The Standard Model is purely mathematically incompatible with Einstein’s general theory of relativity, and both simply cannot be true at the same time.

And where to dig now is not yet very clear (it’s not that there are no thoughts at all, rather, on the contrary: there are too many different theoretical possibilities, and there are much fewer ways to test them). Well, maybe it’s clear to someone, but certainly not to me. I already went beyond my competence in this post a long time ago. If I lied badly somewhere, please correct me.

- What will the new particle give to scientists and ordinary people?

The main directions of development of modern fundamental physics are the physics of elementary particles and cosmology - the science of the evolution of the Universe. In the last 10–15 years, it has become clear that the devices of the micro- and macroworld are closely connected with each other. A discovery in one area gives a strong impetus to the development of another.

The discovery of the Higgs boson will allow scientists to confirm that the basis of modern physics - the Standard Model - is a reliable basis for the further development of our ideas about Nature. The prediction of the existence of the Higgs particle was not confirmed experimentally for decades, which was a dark spot in all of particle physics. The discovery of the Higgs boson confirms the correctness of the main direction of development and greatly narrows the possibilities of alternative theories in both the micro- and macro-worlds. This will allow for more efficient use of budget funds.

- Where is it possible to apply the discovery of a new boson?

It's too early to talk about this. First of all, you need to thoroughly study its properties and only then think about application. The possibilities of using Higgs particles to explain the earliest stage of the formation of the Universe are already being explored. And also the phenomenon of dark energy. The latter, as yet unexplained, phenomenon was discovered in 1998 while observing the accelerated retreat of quasars, the brightest objects in the Universe. This effect can be explained only by assuming unusual properties of the matter filling the Universe.

- What impetus to the development of new technologies can this particle give?

It is known from the history of science that fundamental discoveries do not immediately lead to the emergence of new technologies. A well-known example is Michael Faraday's discovery of the laws of electromagnetic induction, the application of which in technology seemed extremely doubtful. Now, almost 200 years later, it is difficult to imagine our world without electricity. Another example is the neutrino, discovered in 1933, which interacts so weakly with matter that it can pass through the Earth without even noticing it. For a long time it seemed that a particle with such a property would be difficult to find application for. However, now scientists are already trying to use neutrinos to transmit signals through dense media and detect traces of nuclear reactions at great distances.

The situation is similar with the Higgs particle. Apparently, more than a dozen years must pass before the possibilities of using this phenomenon in technology become obvious. First of all, related fields of science will develop, then the influence will spread further. It may turn out that only future generations will be able to benefit from the fruits of this discovery, just as we now benefit from Faraday’s discoveries.

The development of modern science is occurring at an accelerated pace and in a variety of directions. Thus, the Russian heavy ion accelerator, Nika, is being built in Dubna. It will operate in the energy region that is not covered by any of the existing installations in the world, including the Large Hadron Collider. It is in this energy region that there is a chance to obtain a mixed phase of nuclear matter - a state in which particles released from the nucleus - quarks and gluons - simultaneously exist. So far, no one in the world has been able to “catch” free quarks.

Academician Valery Rubakov, Institute of Nuclear Research RAS and Moscow State University.

On the Fourth of July 2012, an event of outstanding significance for physics occurred: at a seminar at CERN (European Center for Nuclear Research), the discovery of a new particle was announced, which, as the authors of the discovery carefully declare, corresponds in its properties to the theoretically predicted elementary boson of the Standard Model of elementary physics particles. It is usually called the Higgs boson, although this name is not entirely adequate. Be that as it may, we are talking about the discovery of one of the main objects of fundamental physics, which has no analogues among the known elementary particles and occupies a unique place in the physical picture of the world (see “Science and Life” No. 1, 1996, article “Boson Higgs is necessary!").

The LHC-B detector is designed to study the properties of B-mesons - hadrons containing a b-quark. These particles quickly disintegrate, having time to fly away from the particle beam only a fraction of a millimeter. Photo: Maximilien Brice, CERN.

Elementary particles of the Standard Model. Almost all of them have their own antiparticles, which are designated by a symbol with a tilde on top.

Interactions in the microworld. Electromagnetic interaction occurs due to the emission and absorption of photons (a). Weak interactions are of a similar nature: they are caused by the emission, absorption or decay of Z-bosons (b) or W-bosons (c).

The Higgs boson H (spin 0) decays into two photons (spin 1), whose spins are antiparallel and add up to 0.

When a photon is emitted or a Z-boson is emitted by a fast electron, the projection of its spin onto the direction of motion does not change. The circular arrow shows the internal rotation of the electron.

In a uniform magnetic field, an electron moves in a straight line along the field and in a spiral in any other direction.

A photon of long wavelength and, therefore, low energy is not capable of resolving the structure of the π-meson - a quark-antiquark pair.

Particles accelerated to enormous energies in the Large Hadron Collider collide, generating many secondary particles - reaction products. Among them, the Higgs boson was discovered, which physicists had been hoping to find for almost half a century.

English physicist Peter W. Higgs proved in the early 1960s that in the Standard Model of elementary particles there must be another boson - a quantum of the field that creates mass in matter.

What happened at the seminar and before it

The announcement of the seminar was made at the end of June, and it immediately became clear that it would be extraordinary. The fact is that the first indications of the existence of a new boson were received back in December 2011 in the ATLAS and CMS experiments conducted at the Large Hadron Collider (LHC) at CERN. In addition, shortly before the seminar, a message appeared that data from experiments at the Tevatron proton-antiproton collider (Fermilab, USA) also indicate the existence of a new boson. All this was not yet enough to talk about a discovery. But since December, the amount of data collected at the LHC has doubled and the methods for processing it have become more advanced. The result was impressive: in each of the ATLAS and CMS experiments separately, the statistical reliability of the signal reached a value that is considered the discovery level in particle physics (five standard deviations).

The seminar was held in a festive atmosphere. In addition to researchers working at CERN and students studying there in summer programs, it was “visited” via the Internet by participants of the largest conference on high-energy physics, which opened in Melbourne on the same day. The seminar was broadcast over the Internet to research centers and universities around the world, including, of course, Russia. After impressive performances by the leaders of the CMS collaborations - Joe Incandela and ATLAS - Fabiola Gianotti, CERN Director General Rolf Heuer concluded: “I think we have it!” (“I think we have it in our hands!”).

So what is “in our hands” and why did theorists come up with it?

What is a new particle?

The minimal version of microworld theory is awkwardly called the Standard Model. It includes all known elementary particles (we list them below) and all known interactions between them. Gravitational interaction stands apart: it does not depend on the types of elementary particles, but is described by Einstein’s general theory of relativity. The Higgs boson remained the only element of the Standard Model that had not been discovered until recently.

We called the Standard Model minimal precisely because there are no other elementary particles in it. In particular, it has one, and only one, Higgs boson, and it is an elementary particle, not a composite one (other possibilities will be discussed below). Most aspects of the Standard Model - with the exception of the new sector to which the Higgs boson belongs - have been tested in numerous experiments, and the main task in the LHC work program is to find out whether the minimal version of the theory is actually implemented in nature and how fully it describes the microworld.

During the implementation of this program, a new particle was discovered, quite heavy by the standards of microworld physics. In this field of science, mass is measured in units of energy, bearing in mind the relationship E = mс 2 between mass and rest energy. The unit of energy is the electronvolt (eV) - the energy that an electron acquires after passing through a potential difference of 1 volt, and its derivatives - MeV (million, 10 6 eV), GeV (billion, 10 9 eV), TeV (trillion, 10 12 eV) . The mass of an electron in these units is 0.5 MeV, a proton is approximately 1 GeV, and the heaviest known elementary particle, the t-quark, is 173 GeV. So, the mass of the new particle is 125-126 GeV (the uncertainty is associated with the measurement error). Let's call this new particle N.

It has no electrical charge, is unstable and can decay in different ways. It was discovered at the CERN Large Hadron Collider by studying decays into two photons, H → γγ, and into two electron-positron and/or muon-antimuon pairs, H → e + e - e + e - , H → e + e - μ + μ - , H → μ + μ - μ + μ-. The second type of process is written as H → 4ℓ, where ℓ denotes one of the particles e +, e -, μ + or μ - (they are called leptons). Both CMS and ATLAS also report some excess of events, which can be explained by H → 2ℓ2ν decays, where ν is a neutrino. This excess, however, does not yet have high statistical significance.

In general, everything that is now known about the new particle is consistent with its interpretation as the Higgs boson, predicted by the simplest version of the theory of elementary particles - the Standard Model. Using the Standard Model, it is possible to calculate both the probability of Higgs boson production in proton-proton collisions at the Large Hadron Collider and the probabilities of its decays, and thereby predict the number of expected events. The predictions are well confirmed by experiments, but, of course, within the limits of error. Experimental errors are still large, and there are still very few measured values. Nevertheless, it is difficult to doubt that it is the Higgs boson or something very similar to it that has been discovered, especially considering that these decays should be very rare: 2 out of 1000 Higgs bosons decay into two photons, and 1 out of 10,000 decay into 4ℓ .

In more than half of the cases, the Higgs boson should decay into a b-quark - b-antiquark pair: H → bb̃. The birth of a bb̃ pair in proton-proton (and proton-antiproton) collisions is a very frequent phenomenon even without any Higgs boson, and it has not yet been possible to isolate the signal from it from this “noise” (physicists say background) in experiments at the LHC. This was partly achieved at the Tevatron collider, and although the statistical significance there is noticeably lower, these data are also consistent with the predictions of the Standard Model.

All elementary particles have spin - internal angular momentum. The spin of a particle can be integer (including zero) or half-integer in units of Planck's constant ћ. Particles with integer spin are called bosons, and particles with half-integer spin are called fermions. The spin of an electron is 1/2, the spin of a photon is 1. From the analysis of the decay products of a new particle, it follows that its spin is integral, that is, it is a boson. From the conservation of angular momentum in the decay of a particle into a pair of photons H → γγ it follows: the spin of each photon is integer; The total angular momentum of the final state (pair of photons) always remains intact. This means that the initial state is also intact.

In addition, it is not equal to unity: a particle of spin 1 cannot decay into two photons with spin 1. What remains is spin 0; 2 or more. Although the spin of the new particle has not yet been measured, it is extremely unlikely that we are dealing with a particle of spin 2 or greater. It is almost certain that the spin of H is zero, and as we will see, this is exactly what the Higgs boson must be.

Concluding the description of the known properties of the new particle, let’s say that by the standards of microworld physics it lives for quite a long time. Based on experimental data, a lower estimate of its lifetime gives ТH > 10 -24 s, which does not contradict the prediction of the Standard Model: ТH = 1.6·10 -22 s. For comparison: the lifetime of a t-quark is T t = 3·10 -25 s. Note that direct measurement of the lifetime of a new particle at the LHC is hardly possible.

Why another boson?

In quantum physics, each elementary particle serves as a quantum of a certain field, and vice versa: each field has its own quantum particle; the most famous example is the electromagnetic field and its quantum, the photon. Therefore, the question posed in the title can be reformulated as follows:

Why is a new field needed and what are its expected properties?

The short answer is that the symmetries of the theory of the microworld - be it the Standard Model or some more complex theory - prohibit elementary particles from having mass, and the new field breaks these symmetries and ensures the existence of particle masses. In the Standard Model - the simplest version of the theory (but only in it!) - all the properties of the new field and, accordingly, the new boson, with the exception of its mass, are unambiguously predicted, again based on symmetry considerations. As we said, the available experimental data are consistent with the simplest version of the theory, but these data are still quite scarce, and there is a lot of work ahead to figure out exactly how the new sector of elementary particle physics works.

Let us consider, at least in general terms, the role of symmetry in the physics of the microworld.

Symmetries, conservation laws and prohibitions

A common property of physical theories, be it Newtonian mechanics, the mechanics of special relativity, quantum mechanics or the theory of the microworld, is that each symmetry has its own conservation law. For example, symmetry with respect to shifts in time (that is, the fact that the laws of physics are the same at every moment of time) corresponds to the law of conservation of energy, symmetry with respect to shifts in space corresponds to the law of conservation of momentum, and symmetry with respect to rotations in it (all directions in space are equal) — law of conservation of angular momentum. Conservation laws can also be interpreted as prohibitions: the listed symmetries prohibit changes in the energy, momentum and angular momentum of a closed system during its evolution.

And vice versa: each conservation law has its own symmetry; This statement is absolutely accurate in quantum theory. The question arises: what symmetry corresponds to the law of conservation of electric charge? It is clear that the symmetries of space and time that we just mentioned have nothing to do with it. Nevertheless, in addition to the obvious, space-time symmetries, there are non-obvious, “internal” symmetries. One of them leads to the conservation of electric charge. It is important for us that this same internal symmetry (only understood in an expanded sense - physicists use the term “gauge invariance”) explains why the photon has no mass. The lack of mass in a photon, in turn, is closely related to the fact that light has only two types of polarization - left and right.

To clarify the connection between the presence of only two types of polarization of light and the absence of mass in the photon, let us digress for a moment from talking about symmetries and again recall that elementary particles are characterized by spin, half-integer or integer in units of Planck’s constant ћ. Elementary fermions (half-integer spin particles) have spin 1/2. These are electron e, electron neutrino ν e, heavy analogues of the electron - muon μ and tau lepton τ, their neutrinos ν μ and ν τ, quarks of six types u, d, c, s, t, b and antiparticles corresponding to all of them (positron e + , electron antineutrino ν̃ e, antiquark ũ, etc.). U and d quarks are light, and they make up the proton (quark composition uud) and neutron (udd). The remaining quarks (c, t, s, b) are heavier; they are part of short-lived particles, for example, K-mesons.

Bosons, particles of whole spin, include not only the photon, but also its distant analogues - gluons (spin 1). Gluons are responsible for the interactions between quarks and bind them into a proton, neutron and other constituent particles. In addition, there are three more spin-1 particles - electrically charged W +, W - bosons and a neutral Z-boson, which will be discussed below. Well, the Higgs boson, as already mentioned, must have zero spin. Now we have listed all the elementary particles found in the Standard Model.

A massive particle of spin s (in units of ћ) has 2s + 1 states with different spin projections onto a given axis (spin is internal angular momentum - a vector, so the concept of its projection onto a given axis has the usual meaning). For example, the spin of an electron (s = 1/2) in its rest frame can be directed, for example, up (s 3 = +1/2) or down (s 3 = -1/2). The Z boson has non-zero mass and spin s = 1, so it has three states with different spin projections: s 3 = +1, 0 or -1. The situation is completely different with massless particles. Since they fly at the speed of light, it is impossible to move to the reference system where such a particle is at rest. Nevertheless, we can talk about its helicity—the projection of the spin onto the direction of movement. So, although the photon spin is equal to unity, there are only two such projections - in the direction of motion and against it. These are the right and left polarizations of light (photons). The third state with zero spin projection, which would have to exist if the photon had mass, is prohibited by the deep internal symmetry of electrodynamics, the very symmetry that leads to the conservation of electric charge. Thus, this internal symmetry prohibits the existence of mass in the photon!

Is there something wrong

What is of interest to us, however, is not photons, but W ± - and Z-bosons. These particles, discovered in 1983 at the Spp̃S proton-antiproton collider at CERN and predicted long before by theorists, have quite a large mass: the W ± boson has a mass of 80 GeV (about 80 times heavier than a proton), and the Z boson has a mass of 91 GeV. The properties of W ± - and Z-bosons are well known mainly due to experiments at the electron-positron colliders LEP (CERN) and SLC (SLAC, USA) and the proton-antiproton collider Tevatron (Fermilab, USA): the accuracy of measurements of a number of quantities related to W ± - and Z-bosons, better than 0.1%. Their properties, and other particles too, are perfectly described by the Standard Model. This also applies to the interactions of W ± - and Z-bosons with electrons, neutrinos, quarks and other particles. Such interactions, by the way, are called weak. They have been studied in detail; One of the long-known examples of their manifestation is the beta decays of the muon, neutron and nuclei.

As already mentioned, each of the W ± - and Z-bosons can be in three spin states, and not in two, like a photon. However, they interact with fermions (neutrinos, quarks, electrons, etc.) in principle the same way as photons. For example, a photon interacts with the electric charge of an electron and the electric current created by the moving electron. In the same way, the Z-boson interacts with a certain electron charge and current that arises when the electron moves, only this charge and current are non-electric in nature. Up to an important feature, which will be discussed shortly, the analogy will be complete if, in addition to the electric charge, a Z-charge is also assigned to the electron. Both quarks and neutrinos have their own Z-charges.

The analogy with electrodynamics extends even further. Like the photon theory, the theory of the W ± and Z bosons has deep internal symmetry, close to that which leads to the law of conservation of electric charge. In complete analogy with the photon, it prohibits W ± - and Z-bosons from having a third polarization, and therefore mass. This is where the inconsistency arises: the symmetry ban on the mass of a spin-1 particle works for a photon, but does not work for W ± - and Z-bosons!

Further more. Weak interactions of electrons, neutrinos, quarks and other particles with W ± - and Z-bosons occur as if these fermions had no mass! The number of polarizations has nothing to do with it: both massive and massless fermions have two polarizations (spin directions). The point is how exactly fermions interact with W ± and Z bosons.

To explain the essence of the problem, let's first turn off the mass of the electron (in theory this is allowed) and consider an imaginary world in which the mass of the electron is zero. In such a world, an electron flies at the speed of light and can have a spin directed either in the direction of motion or against it. As for the photon, in the first case it makes sense to talk about an electron with right polarization, or, in short, about a right-handed electron, in the second case - about a left-handed one.

Since we know well how electromagnetic and weak interactions work (and only the electron participates in them), we are quite capable of describing the properties of the electron in our imaginary world. And they are like that.

Firstly, in this world, the right and left electrons are two completely different particles: the right electron never turns into a left one and vice versa. This is prohibited by the law of conservation of angular momentum (in this case, spin), and interactions of an electron with a photon and a Z-boson do not change its polarization. Secondly, only the left electron experiences the interaction of the electron with the W boson, and the right one does not participate in it at all. The third important feature that we mentioned earlier in this picture is that the Z-charges of the left and right electrons are different, and the left electron interacts with the Z boson more strongly than the right one. The muon, the tau lepton, and quarks have similar properties.

We emphasize that in an imaginary world with massless fermions there is no problem with the fact that left and right electrons interact with W- and Z-bosons differently and, in particular, that the “left” and “right” Z-charges are different. In this world, left and right electrons are different particles, and that’s the end of it: it doesn’t surprise us, for example, that an electron and a neutrino have different electrical charges: -1 and 0.

By including the mass of the electron, we immediately arrive at a contradiction. A fast electron, whose speed is close to the speed of light, and whose spin is directed against the direction of motion, looks almost the same as the left electron from our imaginary world. And it should interact in almost the same way. If its interaction is associated with the Z-charge, then the value of its Z-charge is “left-handed”, the same as that of the left-handed electron from the imaginary world. However, the speed of a massive electron is still less than the speed of light, and you can always switch to a reference system moving even faster. In the new system, the direction of electron motion will be reversed, but the spin direction will remain the same.

The projection of the spin onto the direction of motion will now be positive, and such an electron will look like a right-handed one, not a left-handed one. Accordingly, its Z-charge should be the same as that of the right-handed electron from the imaginary world. But this cannot be: the value of the charge should not depend on the reference system. There is a contradiction. Let us emphasize that we arrived at it assuming that the Z-charge is conserved; There is no other way to talk about its significance for a given particle.

This contradiction shows that the symmetries of the Standard Model (for definiteness, we will talk about it, although everything said applies to any other version of the theory) should prohibit the existence of masses not only in W ± - and Z-bosons, but also in fermions. But what does symmetry have to do with it?

Despite the fact that they should lead to the conservation of the Z-charge. By measuring the Z-charge of an electron, we could definitely tell whether the electron is left-handed or right-handed. And this is only possible when the mass of the electron is zero.

Thus, in a world where all the symmetries of the Standard Model would be realized in the same way as in electrodynamics, all elementary particles would have zero masses. But in the real world they have masses, which means something must happen with the symmetries of the Standard Model.

Symmetry breaking

Speaking about the connection of symmetry with conservation laws and prohibitions, we lost sight of one circumstance. It lies in the fact that conservation laws and symmetry prohibitions are satisfied only when symmetry is explicitly present. However, symmetries can also be broken. For example, in a homogeneous sample of iron at room temperature there may be a magnetic field directed in some direction; the sample is then a magnet. If there were microscopic creatures living inside it, they would discover that not all directions of space are equal. An electron flying across a magnetic field is affected by the Lorentz force from the magnetic field, but an electron flying along it is not affected by the force. An electron moves along a magnetic field in a straight line, across the field in a circle, and in the general case, in a spiral. Therefore, the magnetic field inside the sample breaks the symmetry with respect to rotations in space. In this regard, the law of conservation of angular momentum is not satisfied inside the magnet: when an electron moves in a spiral, the projection of angular momentum onto the axis perpendicular to the magnetic field changes with time.

Here we are dealing with spontaneous symmetry breaking. In the absence of external influences (for example, the Earth's magnetic field), in different samples of iron the magnetic field can be directed in different directions, and none of these directions is preferable to another. The original symmetry with respect to rotation is still present and it manifests itself in the fact that the magnetic field in the sample can be directed anywhere. But once the magnetic field arose, a preferred direction also appeared, and the symmetry inside the magnet was broken. At a more formal level, the equations governing the interaction of iron atoms with each other and with a magnetic field are symmetrical with respect to rotations in space, but the state of the system of these atoms—the iron sample—is asymmetrical. This is the phenomenon of spontaneous symmetry breaking. Note that we are talking here about the most advantageous state, which has the least energy; This state is called basic. This is where the iron sample will eventually end up, even if it was initially unmagnetized.

So, spontaneous breaking of some symmetry occurs when the equations of the theory are symmetrical, but the ground state is not. The word “spontaneous” is used in this case due to the fact that the system itself, without our participation, chooses an asymmetric state, since it is this state that is energetically most favorable. From the above example it is clear that if symmetry is spontaneously broken, then the conservation laws and prohibitions arising from it do not work; in our example this refers to the conservation of angular momentum. Let us emphasize that the complete symmetry of the theory can only be partially broken: in our example, out of complete symmetry with respect to all rotations in space, the symmetry with respect to rotations around the direction of the magnetic field remains clear and unbroken.

Microscopic creatures living inside a magnet might ask themselves the question: “In our world, not all directions are equal, angular momentum is not conserved, but is space really asymmetrical with respect to rotations?” Having studied the movement of electrons and built the corresponding theory (in this case, electrodynamics), they would understand that the answer to this question is negative: its equations are symmetrical, but this symmetry is spontaneously broken due to the magnetic field “spread” everywhere. Developing the theory further, they would predict that the field responsible for the spontaneous breaking of symmetry should have its own quanta, photons. And, having built a small accelerator inside a magnet, we would be happy to see that these quanta really exist - they are born in collisions of electrons!

In general terms, the situation in particle physics is similar to the one described. But there are also important differences. Firstly, there is no need to talk about any medium like a crystal lattice of iron atoms. In nature, the state with the lowest energy is vacuum (by definition!). This does not mean that in a vacuum - the basic state of nature - there cannot be uniformly “diffused” fields, similar to the magnetic field in our example. On the contrary, the inconsistencies that we talked about indicate that the symmetries of the Standard Model (more precisely, part of them) should be spontaneously broken, and this assumes that there is some kind of field in the vacuum that ensures this violation. Secondly, we are not talking about space-time symmetries, as in our example, but about internal symmetries. Space-time symmetries, on the contrary, should not be broken due to the presence of a field in a vacuum. An important conclusion follows from this: unlike the magnetic field, this field should not highlight any direction in space (more precisely, in space-time, since we are dealing with relativistic physics). Fields with this property are called scalar; they correspond to particles of spin 0. Therefore, the field “spread out” in the vacuum and leading to symmetry breaking must be hitherto unknown and new. Indeed, the known fields that we mentioned explicitly or implicitly above - the electromagnetic field, the fields of W ± - and Z-bosons, gluons - correspond to particles of spin 1. Such fields highlight directions in space-time and are called vector, and we need a field scalar. Fields corresponding to fermions (spin 1/2) are also not suitable. Thirdly, the new field should not completely break the symmetries of the Standard Model; the internal symmetry of electrodynamics should remain unbroken. Finally, and this is the most important thing, the interaction of the new field, “diffused” in vacuum, with W ± - and Z-bosons, electrons and other fermions should lead to the appearance of masses in these particles.

The mechanism for the generation of masses of spin-1 particles (in nature these are W ± - and Z-bosons) due to spontaneous symmetry breaking was proposed in the context of elementary particle physics by Brussels theorists Francois Englert and Robert Brout in 1964 and a little later by Edinburgh physicist Peter Higgs .

The researchers relied on the idea of ​​spontaneous symmetry breaking (but in theories without vector fields, that is, without spin 1 particles), which was introduced in 1960-1961 in the works of J. Nambu, who together with J. Jona-Lasinio, V. G. Vaks and A. I. Larkin, J. Goldstone (Yoichiro Nambu received the Nobel Prize for this work in 2008). Unlike previous authors, Engler, Brout, and Higgs considered a theory (at the time speculative) that involved both a scalar (spin 0) and a vector field (spin 1). This theory has an internal symmetry, quite similar to the symmetry of electrodynamics, which leads to the conservation of electric charge and to the prohibition of the photon mass. But unlike electrodynamics, internal symmetry is spontaneously broken by a uniform scalar field present in a vacuum. A remarkable result of Engler, Brout and Higgs was the demonstration of the fact that this violation of symmetry automatically entails the appearance of mass in a spin 1 particle - a quantum of the vector field!

A fairly straightforward generalization of the Engler-Brout-Higgs mechanism, associated with the inclusion in the theory of fermions and their interaction with a symmetry-breaking scalar field, leads to the appearance of mass in fermions. Everything is starting to fall into place! The Standard Model is obtained as a further generalization. It now contains not one, but several vector fields - photons, W ± - and Z-bosons (gluons are a separate story, they have nothing to do with the Engler-Brout-Higgs mechanism) and different types of fermions. The last step is actually quite non-trivial; Steven Weinberg, Sheldon Glashow and Abdus Salam received the Nobel Prize in 1979 for formulating a complete theory of weak and electromagnetic interactions.

Let's go back to 1964. To analyze their theory, Engler and Brout used an approach that is rather elaborate by today's standards. This is probably why they did not notice that, along with a massive spin-1 particle, the theory predicts the existence of another particle - a boson with spin 0. But Higgs noticed, and now this new spinless particle is often called the Higgs boson. As already noted, this terminology is not entirely correct: it was Engler and Brout who first proposed using a scalar field for spontaneous symmetry breaking and generation of masses of spin-1 particles. Without going into more terminology, we emphasize that the new boson with zero spin serves as a quantum of the very scalar field that breaks the symmetry. And this is its uniqueness.

A clarification needs to be made here. Let us repeat that if there were no spontaneous symmetry breaking, then the W ± and Z bosons would be massless. Each of the three bosons W + , W - , Z would have, like a photon, two polarizations. In total, considering particles with different polarizations to be unequal, we would have 2 × 3 = 6 types of W ± - and Z-bosons. In the Standard Model, the W ± and Z bosons are massive, each of them has three spin states, that is, three polarizations, for a total of 3 × 3 = 9 types of particles - quanta of the W ±, Z fields. The question arises, where did the three “extra” types come from? quants? The fact is that the Standard Model needs to have not one, but four Engler-Brout-Higgs scalar fields. The quantum of one of them is the Higgs boson. And the quanta of the other three, as a result of spontaneous symmetry breaking, turn into the three “extra” quanta present in massive W ± - and Z-bosons. They were found a long time ago, since it is known that W ± - and Z-bosons have mass: the three “extra” spin states of W + -, W - and Z-bosons are what they are.

This arithmetic, by the way, is consistent with the fact that all four Engler-Brout-Higgs fields are scalar, their quanta have zero spin. Massless W ± - and Z-bosons would have spin projections on the direction of motion equal to -1 and +1. For massive W ± - and Z-bosons, these projections take values ​​-1, 0 and +1, that is, the “extra” quanta have a zero projection. The three Engler-Brout-Higgs fields from which these “extra” quanta are obtained also have a zero spin projection onto the direction of motion simply because their spin vector is zero. Everything fits together.

So, the Higgs boson is a quantum of one of the four Engler-Brout-Higgs scalar fields in the Standard Model. The other three are eaten by (scientific term!) W ± - and Z-bosons, turning into their third, missing spin states.

Is a new boson really necessary?

The most amazing thing in this story is that today we understand: the Engler-Brout-Higgs mechanism is by no means the only possible mechanism for breaking symmetry in the physics of the microworld and generating masses of elementary particles, and the Higgs boson might not exist. For example, in the physics of condensed matter (liquids, solids) there are many examples of spontaneous symmetry breaking and a variety of mechanisms for this breaking. And in most cases there is nothing like the Higgs boson in them.

The closest solid-state analogue of the spontaneous breaking of symmetry of the Standard Model in a vacuum is the spontaneous breaking of the internal symmetry of electrodynamics in the thickness of a superconductor. It leads to the fact that in a superconductor a photon in a certain sense has mass (like W ± - and Z-bosons in a vacuum). This manifests itself in the Meissner effect - the expulsion of a magnetic field from a superconductor. The photon “does not want” to penetrate inside the superconductor, where it becomes massive: it is “hard” for it there, it is energetically unfavorable for it to be there (remember: E = mс 2). The magnetic field, which can be somewhat conventionally considered a set of photons, has the same property: it does not penetrate the superconductor. This is the Meissner effect.

The effective Ginzburg-Landau theory of superconductivity is extremely similar to the Engler-Brout-Higgs theory (more precisely, on the contrary: the Ginzburg-Landau theory is 14 years older). It also contains a scalar field, which is uniformly “spread” throughout the superconductor and leads to spontaneous symmetry breaking. However, it is not for nothing that the Ginzburg-Landau theory is called effective: it captures, figuratively speaking, the external side of the phenomenon, but is completely inadequate for understanding the fundamental, microscopic reasons for the emergence of superconductivity. In fact, there is no scalar field in a superconductor; it contains electrons and a crystal lattice, and superconductivity is due to the special properties of the ground state of the electron system, arising due to the interaction between them (see “Science and Life” No. 2, 2004, article “ ". - Ed.).

Could a similar picture also take place in the microcosm? Will it turn out that there is no fundamental scalar field “diffused” in the vacuum, and spontaneous symmetry breaking is caused by completely different reasons? If we reason purely theoretically and do not pay attention to experimental facts, then the answer to this question is affirmative. A good example is the so-called technicolor model, proposed in 1979 by the already mentioned Steven Weinberg and - independently - Leonard Susskind.

It contains neither fundamental scalar fields nor the Higgs boson, but instead there are many new elementary particles that resemble quarks in their properties. The interaction between them leads to spontaneous breaking of symmetry and generation of masses of W ± - and Z-bosons. With the masses of known fermions, for example the electron, the situation is worse, but this problem can also be solved by complicating the theory.

An attentive reader may ask the question: “What about the arguments of the previous chapter, which say that it is the scalar field that should break the symmetry?” The loophole here is that this scalar field can be composite, in the sense that the corresponding quanta particles are not elementary, but consist of other, “truly” elementary particles.

Let us recall in this regard the quantum-mechanical Heisenberg uncertainty relation Δх ×Δр ≥ ћ, where Δх and Δр are the uncertainties of the coordinate and momentum, respectively. One of its manifestations is that the structure of composite objects with a characteristic internal size Δх appears only in processes involving particles with sufficiently high momenta р ≥ћ/Δх, and therefore with sufficiently high energies. Here it is appropriate to recall Rutherford, who bombarded atoms with electrons of high energies at that time and thus found out that atoms consist of nuclei and electrons. Looking at atoms through a microscope, even with the most advanced optics (that is, using light - low-energy photons), it is impossible to discover that atoms are composite, and not elementary, point particles: there is not enough resolution.

So, at low energies, a compound particle looks like an elementary particle. To effectively describe such particles at low energies, they can be considered quanta of some field. If the spin of a composite particle is zero, then this field is scalar.

A similar situation is realized, for example, in the physics of π-mesons, particles with spin 0. Until the mid-1960s, it was not known that they consist of quarks and antiquarks (the quark composition of π + -, π - - and π 0 -mesons - these are ud̃, dũ and a combination of uũ and dd̃ respectively).

Then π-mesons were described by elementary scalar fields. We now know that these particles are composite, but the “old” field theory of π mesons remains valid because processes at low energies are considered. Only at energies of the order of 1 GeV and higher does their quark structure begin to appear, and the theory stops working. The energy scale of 1 GeV did not appear here by chance: this is the scale of strong interactions that bind quarks into π-mesons, protons, neutrons, etc., this is the scale of the masses of strongly interacting particles, for example the proton. Note that the π-mesons themselves stand apart: for a reason that we will not talk about here, they have much smaller masses: m π± = 140 MeV, m π0 = 135 MeV.

So, the scalar fields responsible for spontaneous symmetry breaking can, in principle, be composite. This is precisely the situation suggested by the technicolor model. In this case, three spinless quanta, which are eaten by W ± - and Z-bosons and become their missing spin states, have a close analogy with π + -, π - - and π 0 -mesons. Only the corresponding energy scale is no longer 1 GeV, but several TeV. In such a picture, the existence of many new constituent particles is expected - analogues of the proton, neutron, etc. — with masses of the order of several TeV. On the contrary, the relatively light Higgs boson is absent in it. Another feature of the model is that the W ± and Z bosons in it are partly composite particles, since, as we said, some of their components are similar to π mesons. This should manifest itself in the interactions of W ± and Z bosons.

It was the latter circumstance that led to the technicolor model (at least in its original formulation) being rejected long before the discovery of the new boson: precise measurements of the properties of the W ± and Z bosons at LEP and SLC do not agree with the predictions of the model.

This beautiful theory was crushed by stubborn experimental facts, and the discovery of the Higgs boson put an end to it. Nevertheless, for me, as for a number of other theorists, the idea of ​​composite scalar fields is more attractive than the Engler-Brout-Higgs theory with elementary scalar fields. Of course, after the discovery of a new boson at CERN, the idea of ​​composition found itself in an even more difficult position than before: if this particle is composite, it should quite successfully mimic the elementary Higgs boson. And yet, let’s wait and see what experiments at the LHC will show, first of all, more accurate measurements of the properties of the new boson.

The discovery has been made. What's next?

Let us return, as a working hypothesis, to the minimal version of the theory - the Standard Model with one elementary Higgs boson. Since in this theory it is the Engler-Brout-Higgs field (more precisely, fields) that gives the masses to all elementary particles, the interaction of each of these particles with the Higgs boson is strictly fixed. The greater the mass of the particle, the stronger the interaction; The stronger the interaction, the more likely it is that the Higgs boson will decay into a pair of particles of a given type. Decays of the Higgs boson into pairs of real particles tt̃ , ZZ and W+W- are prohibited by the law of conservation of energy. It requires that the sum of the masses of the decay products be less than the mass of the decaying particle (again, remember E = mc 2), and for us, recall, m n ≈ 125 GeV, m t = 173 GeV, m z = 91 GeV and m w = 80 GeV. The next largest mass is the b quark with m b = 4 GeV, and that is why, as we said, the Higgs boson most readily decays into a bb̃ pair. Also interesting is the decay of the Higgs boson into a pair of rather heavy τ-leptons H → τ + τ - (m τ = 1.8 GeV), which occurs with a probability of 6%. The decay H → μ + μ - (m µ = 106 MeV) should occur with an even smaller, but still non-vanishing probability of 0.02%. In addition to the decays discussed above, H → γγ; H → 4ℓ and H → 2ℓ2ν, we note the decay H → Zγ, the probability of which should be 0.15%. All of these probabilities will be measurable at the LHC, and any deviation from these predictions will mean that our working hypothesis, the Standard Model, is incorrect. Conversely, agreement with the predictions of the Standard Model will convince us more and more of its validity.

The same can be said about the creation of the Higgs boson in proton collisions. The Higgs boson can be produced alone from the interaction of two gluons, together with a pair of high-energy light quarks, together with a single W or Z boson, or, finally, together with a tt̃ pair. Particles produced together with the Higgs boson can be detected and identified, so different production mechanisms can be studied separately at the LHC. Thus, it is possible to extract information about the interaction of the Higgs boson with W ± -, Z-bosons and the t-quark.

Finally, an important property of the Higgs boson is its interaction with itself. It should manifest itself in the process Н* → НН, where Н* is a virtual particle. The properties of this interaction are also clearly predicted by the Standard Model. However, its study is a matter of the distant future.

So, the LHC has an extensive program to study the interactions of the new boson. As a result of its implementation, it will become more or less clear whether the Standard Model describes nature or we are dealing with some other, more complex (and possibly simpler) theory. Further progress is associated with a significant increase in measurement accuracy; it will require the construction of a new electron-positron accelerator - an e + e - collider with a record energy for this type of machine. It may very well be that a lot of surprises await us along this path.

Instead of a conclusion: in search of “new physics”

From a “technical” point of view, the Standard Model is internally consistent. That is, within its framework it is possible - at least in principle, and as a rule, in practice - to calculate any physical quantity (of course, related to the phenomena that it is intended to describe), and the result will not contain uncertainties. Nevertheless, many, although not all, theorists consider the state of affairs in the Standard Model, to put it mildly, not entirely satisfactory. And this is primarily due to its energy scale.

As is clear from the previous, the energy scale of the Standard Model is of the order of M cm = 100 GeV (we are not talking here about strong interactions with a scale of 1 GeV, everything is simpler with it). This is the mass scale of the W ± and Z bosons and the Higgs boson. Is it a lot or a little? From an experimental point of view - pretty much, but from a theoretical point of view...

In physics there is another energy scale. It is associated with gravity and is equal to the Planck mass M pl = 10 19 GeV. At low energies, gravitational interactions between particles are negligible, but they increase with increasing energy, and at energies of the order of M pl, gravity becomes strong. Energies above M pl are the region of quantum gravity, whatever that is. It is important for us that gravity is perhaps the most fundamental interaction and the gravitational scale M pl is the most fundamental energy scale. Why then is the Standard Model scale Mcm = 100 GeV so far from Mpl = 1019 GeV?

The identified problem has another, more subtle aspect. It is associated with the properties of physical vacuum. In quantum theory, the vacuum - the basic state of nature - is structured in a very non-trivial way. Virtual particles are constantly being created and destroyed in it; in other words, field fluctuations form and disappear. We cannot directly observe these processes, but they influence the observable properties of elementary particles, atoms, etc. For example, the interaction of an electron in an atom with virtual electrons and photons leads to a phenomenon observed in atomic spectra - the Lamb shift. Another example: the correction to the magnetic moment of an electron or muon (anomalous magnetic moment) is also due to interaction with virtual particles. These and similar effects have been calculated and measured (in these cases with fantastic accuracy!), so that we can be sure that we have the correct picture of the physical vacuum.

In this picture, all the parameters originally included in the theory receive corrections, called radiative ones, due to interaction with virtual particles. In quantum electrodynamics they are small, but in the Engler-Brout-Higgs sector they are huge. This is the peculiarity of the elementary scalar fields that make up this sector; other fields do not have this property. The main effect here is that radiative corrections tend to “pull” the energy scale of the Standard Model M cm towards the gravitational scale M pl. If we remain within the Standard Model, then the only way out is to select the initial parameters of the theory so that, together with radiation corrections, they lead to the correct value of M cm. However, it turns out that the accuracy of the fit should be close to M cm 2 /M pl 2 = 10 -34 ! This is the second aspect of the Standard Model energy scale problem: it seems implausible that such a fit occurs in nature.

Many (though, we repeat, not all) theorists believe that this problem clearly indicates the need to go beyond the Standard Model. Indeed, if the Standard Model stops working or expands significantly on the energy scale of “new physics - NF” M nf, then the required accuracy of fitting the parameters will be, roughly speaking, M 2 cm / M 2 nf, but in fact it is about two orders of magnitude less. If we assume that there is no fine tuning of parameters in nature, then the scale of “new physics” should lie in the region of 1-2 TeV, that is, exactly in the region accessible for research at the Large Hadron Collider!

What could a “new physics” be like? There is no unity among theorists on this matter. One possibility is the composite nature of the scalar fields that provide the spontaneous symmetry breaking already discussed. Another, also popular (so far?) possibility is supersymmetry, about which we will only say that it predicts a whole zoo of new particles with masses in the range of hundreds of GeV - several TeV. Very exotic options are also being discussed, such as additional dimensions of space (for example, the so-called M-theory - see “Science and Life” Nos. 2, 3, 1997, article “Superstrings: on the way to the theory of everything.” - Ed. .).

Despite all efforts, no experimental indications of “new physics” have yet been received. This, in fact, is already beginning to inspire concern: do we understand everything correctly? It is quite possible, however, that we have not yet reached “new physics” in terms of energy and the amount of data collected, and that new, revolutionary discoveries will be associated with it. The main hopes here are again placed on the Large Hadron Collider, which in a year and a half will begin operating at full energy of 13-14 TeV and quickly collect data. Follow the news!

Precise measurement and discovery machines

Particle physics, which studies the tiniest objects in nature, requires giant research facilities where these particles accelerate, collide, and disintegrate. The most powerful of them are colliders.

Collider is an accelerator with colliding particle beams, in which particles collide head-on, for example, electrons and positrons in e + e - colliders. So far, proton-antiproton, proton-proton, electron-proton and nucleus-nucleus (or heavy ion) colliders have also been created. Other possibilities, for example, μ + μ - - collider, are still being discussed. The main colliders for particle physics are proton-antiproton, proton-proton and electron-positron.

Large Hadron Collider (LHC)- proton-proton, it accelerates two beams of protons one towards the other (can also work as a heavy-ion collider). The design energy of the protons in each beam is 7 TeV, so the total collision energy is 14 TeV. In 2011, the collider operated at half this energy, and in 2012, at full energy of 8 TeV. The Large Hadron Collider is a 27 km long ring in which protons are accelerated by electric fields and contained by fields created by superconducting magnets. Proton collisions occur at four locations where detectors are located to record the particles produced in the collisions. ATLAS and CMS are designed for high-energy particle physics research; LHC-b is for studying particles that contain b-quarks, and ALICE is for studying hot and dense quark-gluon matter.

Spp̃S- proton-antiproton collider at CERN. The ring length is 6.9 km, the maximum collision energy is 630 GeV. Worked from 1981 to 1990.

LEP- a ring electron-positron collider with a maximum collision energy of 209 GeV, located in the same tunnel as the LHC. Worked from 1989 to 2000.

SLC— linear electron-positron collider at SLAC, USA. Collision energy 91 GeV (Z-boson mass). Worked from 1989 to 1998.

Tevatron is a ring proton-antiproton collider at Fermilab, USA. The length of the ring is 6 km, the maximum collision energy is 2 TeV. Worked from 1987 to 2011.

When comparing proton-proton and proton-antiproton colliders with electron-positron colliders, you need to keep in mind that the proton is a composite particle; it contains quarks and gluons. Each of these quarks and gluons carries only a fraction of the energy of a proton. Therefore, in the Large Hadron Collider, for example, the energy of an elementary collision (between two quarks, between two gluons, or a quark with a gluon) is noticeably lower than the total energy of colliding protons (14 TeV at design parameters). Because of this, the energy range available for study on it reaches “only” 2-4 TeV, depending on the process being studied. Electron-positron colliders do not have such a feature: the electron is an elementary, structureless particle.

The advantage of proton-proton (and proton-antiproton) colliders is that, even taking this feature into account, it is technically easier to achieve high collision energies with them than with electron-positron colliders. There is also a minus. Because of the composite structure of the proton, and because quarks and gluons interact with each other much more strongly than electrons and positrons, many more events occur in proton collisions that are not interesting from the point of view of the search for the Higgs boson or other new particles and phenomena. Interesting events look more “dirty” in proton collisions; many “extraneous”, uninteresting particles are born in them. All this creates “noise”, from which it is more difficult to isolate a useful signal than at electron-positron colliders. Accordingly, the measurement accuracy is lower. Because of all this, proton-proton (and proton-antiproton) colliders are called discovery machines, and electron-positron colliders are called precision measurement machines.

Standard deviation(standard deviation) σ x - characteristic of random deviations of the measured value from the average value. The probability that the measured value of X will randomly differ by 5σ x from the true value is only 0.00006%. This is why in particle physics a signal deviation from the background by 5σ is considered sufficient to recognize the signal as true.

Particles, listed in the Standard Model, except for the proton, electron, neutrino and their antiparticles, are unstable: they decay into other particles. However, two of the three types of neutrinos should also be unstable, but their lifetime is extremely long. In the physics of the microworld there is a principle: everything that can happen actually happens. Therefore, the stability of a particle is associated with some kind of conservation law. The electron and positron are prohibited from decaying by the law of conservation of charge. The lightest neutrino (spin 1/2) does not decay due to the conservation of angular momentum. The decay of a proton is prohibited by the law of conservation of another “charge,” which is called the baryon number (the baryon number of a proton, by definition, is 1, and that of lighter particles is zero).

Another internal symmetry is associated with the baryon number. Whether it is accurate or approximate, whether the proton is stable or has a finite, albeit very long, lifetime is a subject for a separate discussion.

Quarks- one of the types of elementary particles. In a free state, they are not observed, but are always connected with each other and form composite particles - hadrons. The only exception is the t-quark; it decays before it has time to combine with other quarks or antiquarks into a hadron. Hadrons include proton, neutron, π-mesons, K-mesons, etc.

The b quark is one of six types of quarks, the second in mass after the t quark.

A muon is a heavy unstable analogue of an electron with a mass m μ = 106 MeV. The muon's lifetime T μ = 2·10 -6 seconds is long enough for it to fly through the entire detector without decaying.

Virtual particle differs from the real one in that for a real particle the usual relativistic relationship between energy and momentum E 2 = p 2 s 2 + m 2 s 4 is satisfied, but for a virtual one it is not satisfied. This is possible due to the quantum mechanical relationship ΔE·Δt ~ ħ between the energy uncertainty ΔE and the duration of the process Δt. Therefore, a virtual particle almost instantly decays or annihilates with another (its lifetime Δt is very short), while a real one lives noticeably longer or is generally stable.

Lamb level shift- a slight deviation of the fine structure of the levels of a hydrogen atom and hydrogen-like atoms under the influence of the emission and absorption of virtual photons or the virtual creation and annihilation of electron-positron pairs. The effect was discovered in 1947 by American physicists W. Lamb and R. Rutherford.