Standard theory. Standard model. See what "Standard Model" is in other dictionaries

The world of elementary particles obeys quantum laws and is still not fully understood. The defining concept in constructing various models of interaction of elementary particles is the concept of symmetry, understood as the mathematical property of the invariance of interaction processes under various transformations of coordinates or internal parameters of the model. Such transformations form groups called symmetry groups.

It is on the basis of the concept of symmetry that the Standard Model is built. First of all, it has space-time symmetry with respect to rotations and shifts in space-time. The corresponding symmetry group is called the Lorentz (or Poincaré) group. This symmetry corresponds to the independence of predictions from the choice of reference frame. In addition, there are groups of internal symmetry - symmetry with respect to rotations in “isospin” and “color” space (in the case of weak and strong interactions, respectively). There is also a group of phase rotations associated with electromagnetic interactions. These symmetries correspond to the conservation laws electric charge, “colored” charge, etc. Full group internal symmetry of the Standard Model, obtained from the analysis of numerous experimental data, is the product of unitary groups SU(3) x SU(2) x U(1). All Standard Model particles belong to different ideas symmetry groups, and particles of different spins never mix.

Standard model – modern theory structure and interactions of elementary particles, the theory is based on a very small number of postulates and allows one to theoretically predict the properties of various processes in the world of elementary particles. To describe the properties and interactions of elementary particles, the concept of a physical field is used, which is associated with each particle: electron, muon, quark, etc. A field is a specific form of distribution of matter in space. The fields associated with elementary particles are of a quantum nature. Elementary particles are quanta of the corresponding fields. The working tool of the Standard Model is quantum field theory. Quantum field theory (QFT) is the theoretical basis for describing microparticles, their interactions and interconversions. The mathematical apparatus of quantum field theory (QFT) allows us to describe the birth and destruction of a particle at each space-time point.

The Standard Model describes three types of interaction: electromagnetic, weak and strong. Gravitational interaction is not part of the Standard Model.

The main question for describing the dynamics of elementary particles is the question of choosing a system of primary fields, i.e. about the choice of particles (and, accordingly, fields) that should be considered the most fundamental (elementary) in describing the observed particles of matter. The standard model selects as fundamental particles structureless particles with spin ½: three pairs of leptons ( , ( and three pairs of quarks, usually grouped into three generations.

“We ask ourselves why a group of talented and dedicated people would dedicate their lives to chasing objects so tiny that they can’t even be seen? In fact, what particle physicists do is about human curiosity and the desire to know how the world we live in works." Sean Carroll

If you are still afraid of the phrase quantum mechanics and still don’t know what the standard model is, welcome to the cat. In my publication I will try to explain the basics of the quantum world, as well as elementary particle physics, as simply and clearly as possible. We will try to figure out what the main differences between fermions and bosons are, why quarks have such strange names, and finally, why everyone wanted to find the Higgs Boson so much.

What are we made of?

Well, we will begin our journey into the microworld with a simple question: what are the objects around us made of? Our world, like a house, consists of many small bricks, which, when combined in a special way, create something new, not only appearance, but also in its properties. In fact, if you look closely at them, you will find that there are not so many different types of blocks, they just connect to each other in different ways each time, forming new forms and phenomena. Each block is an indivisible elementary particle, which will be discussed in my story.

For example, let’s take some substance, let’s say it’s the second element periodic table Mendeleev, inert gas, helium. Like other substances in the Universe, helium consists of molecules, which in turn are formed by bonds between atoms. But in this case, for us, helium is a little special because it consists of just one atom.

What does an atom consist of?

The helium atom, in turn, consists of two neutrons and two protons, which make up the atomic nucleus, around which two electrons revolve. The most interesting thing is that the only thing absolutely indivisible here is electron.

Interesting moment of the quantum world
How less the mass of an elementary particle, the more she takes up space. It is for this reason that electrons, which are 2000 times lighter than a proton, take up much more space compared to the nucleus of an atom.

Neutrons and protons belong to the group of so-called hadrons(particles subject to strong interaction), and to be even more precise, baryons.

Hadrons can be divided into groups
Baryons, which consist of three quarks

Mesons, which consist of a particle-antiparticle pair

The neutron, as its name suggests, is neutrally charged and can be divided into two down quarks and one up quark. A proton, a positively charged particle, splits into one down quark and two up quarks.

Yes, yes, I'm not kidding, they are really called upper and lower. It would seem that if we discovered the up and down quark, and even the electron, we could use them to describe the entire Universe. But this statement would be very far from the truth.

The main problem is that the particles must somehow interact with each other. If the world consisted only of this trinity (neutron, proton and electron), then the particles would simply fly around the vast expanses of space and would never gather into larger formations, such as hadrons.

Fermions and Bosons

Quite a long time ago, scientists came up with a convenient and concise form of representing elementary particles, called the standard model. It turns out that all elementary particles are divided into fermions, from which all matter consists, and bosons that carry various types interactions between fermions.

The difference between these groups is very clear. The fact is that fermions need some space to survive according to the laws of the quantum world, but for bosons the presence of free space is almost unimportant.

Fermions

A group of fermions, as already mentioned, creates visible matter around us. Whatever we see, wherever we see it, is created by fermions. Fermions are divided into quarks, strongly interacting with each other and locked inside more complex particles like hadrons, and leptons, which exist freely in space independently of their fellows.

Quarks are divided into two groups.

Top type. Top quarks, with charge +2\3, include: top, charm and true quarks
Bottom type. The quarks of the lower type, with a charge of -1\3, include: bottom, strange and charm quarks

The up and down quarks are the largest quarks, and the up and down quarks are the smallest. Why quarks were given such unusual names, or, more correctly, “flavors,” is still a matter of debate for scientists.

Leptons are also divided into two groups.

The first group, with charge “-1”, includes: electron, muon (heavier particle) and tau particle (the most massive)
The second group, with a neutral charge, contains: electron neutrino, muon neutrino and tau neutrino

A neutrino is a small particle of matter that is almost impossible to detect. Its charge is always 0.

The question arises whether physicists will find several more generations of particles that will be even more massive than the previous ones. It is difficult to answer, but theorists believe that the generations of leptons and quarks are limited to three.

Don't you see any similarities? Both quarks and leptons are divided into two groups, which differ from each other in charge by one? But more on that later...

Bosons

Without them, fermions would fly around the universe in a continuous stream. But by exchanging bosons, fermions communicate to each other some type of interaction. The bosons themselves practically do not interact with each other.
In fact, some bosons still interact with each other, but this will be discussed in more detail in future articles on the problems of the microworld

The interaction transmitted by bosons is:

Electromagnetic, particles are photons. Light is transmitted using these massless particles.
Strong nuclear, particles are gluons. With their help, quarks from the atomic nucleus do not break up into individual particles.
Weak nuclear, particles - ±W and Z bosons. With their help, fermions transfer mass, energy, and can transform into each other.
Gravitational , particles - gravitons. An extremely weak force on a microscopic scale. Becomes visible only on supermassive bodies.

Clause about gravitational interaction.
The existence of gravitons has not yet been experimentally confirmed. They exist only as a theoretical version. In most cases they are not considered in the standard model.

That's it, the standard model is assembled.

The problems have just begun

Despite the very beautiful representation of particles in the diagram, two questions remain. Where do particles get their mass from and what are they? Higgs boson, which stands out from the rest of the bosons.

In order to understand the idea of using the Higgs boson, we need to turn to quantum field theory. Speaking in simple language, it can be argued that the whole world, the entire Universe, consists not of the smallest particles, but of many different fields: gluon, quark, electron, electromagnetic, etc. In all these fields, slight fluctuations constantly occur. But we perceive the strongest of them as elementary particles. Yes, and this thesis is very controversial. From the point of view of particle-wave dualism, the same object of the microworld in different situations behaves either as a wave or as an elementary particle; it only depends on how it is more convenient for the physicist observing the process to model the situation.

Higgs field

It turns out that there is a so-called Higgs field, the average value of which does not want to approach zero. As a result, this field tries to take on some constant non-zero value throughout the Universe. The field constitutes an omnipresent and constant background, as a result of strong oscillations of which the Higgs Boson appears.
And it is thanks to the Higgs field that particles are endowed with mass.
The mass of an elementary particle depends on how strongly it interacts with the Higgs field, constantly flying inside it.
And it is precisely because of the Higgs Boson, or more precisely because of its field, that the standard model has so many similar groups of particles. The Higgs field forced the creation of many additional particles, such as neutrinos.

Results

What I have shared is the most superficial concepts about the nature of the standard model and why we need the Higgs Boson. Some scientists still hope deep down that the Higgs-like particle found in 2012 at the LHC was simply a statistical error. After all, the Higgs field breaks many of the beautiful symmetries of nature, making physicists’ calculations more confusing.
Some even believe that the standard model is reaching its end. recent years because of its imperfection. But this has not been proven experimentally, and the standard model of elementary particles remains a working example of the genius of human thought.

The modern understanding of particle physics is contained in the so-called Standard Model . The Standard Model (SM) of particle physics is based on quantum electrodynamics, quantum chromodynamics and the quark-parton model.
Quantum electrodynamics (QED), a high-precision theory, describes processes occurring under the influence of electromagnetic forces that have been studied with a high degree of accuracy.
Quantum chromodynamics (QCD), which describes the processes of strong interactions, is constructed by analogy with QED, but to a greater extent is a semi-empirical model.
The quark-parton model combines theoretical and experimental results from studies of the properties of particles and their interactions.
To date, no deviations from the Standard Model have been discovered.
The main contents of the Standard Model are presented in tables 1, 2, 3. The constituents of matter are three generations of fundamental fermions (I, II, III), the properties of which are listed in table. 1. Fundamental bosons are carriers of interactions (Table 2), which can be represented using the Feynman diagram (Fig. 1).

Table 1: Fermions - (half-integer spin in units of ћ) constituents of matter

	Leptons, spin = 1/2			Quarks, spin = 1/2
	Aroma	Weight, GeV/s 2	Electric charge, e	Aroma	Weight, GeV/s 2	Electric charge, e
I	ν e	< 7·10 -9	0	u, up	0.005	2/3
I	e, electron	0.000511	-1	d, down	0.01	-1/3
II	ν μ	< 0.0003	0	s, charm	1.5	2/3
II	μ, muon	0.106	-1	s, strange	0.2	-1/3
III	ν τ	< 0.03	0	t, top	170	2/3
III	τ, tau	1.7771	-1	b, bottom	4.7	-1/3

Table 2: Bosons - carriers of interactions (spin = 0, 1, 2 ... in units of ћ)

Vectors interaction	Weight, GeV/c2	Electric charge, e
Electroweak interaction
γ, photon, spin = 1	0	0
W - , spin = 1	80.22	-1
W+, spin = 1	80.22	+1
Z 0 , spin = 1	91.187	0
Strong (color) interaction
5, gluons, spin = 1	0	0
Undiscovered bosons
H 0 , Higgs, spin = 0	> 100	0
G, graviton, spin = 2	?	0

Table 3: Comparative characteristics fundamental interactions

The strength of the interaction is indicated relative to the strong one.

Rice. 1: Feynman diagram: A + B = C + D, a is the interaction constant, Q 2 = -t - 4-momentum that particle A transfers to particle B as a result of one of four types of interactions.

1.1 Fundamentals of the Standard Model

Hadrons consist of quarks and gluons (partons). Quarks are fermions with spin 1/2 and mass m 0; gluons are bosons with spin 1 and mass m = 0.
Quarks are classified according to two characteristics: flavor and color. There are 6 known flavors of quarks and 3 colors for each quark.
Aroma is a characteristic that persists in strong interactions.
The gluon is composed of two colors - color and anticolor, and all other quantum numbers are equal to zero. When a gluon is emitted, the quark changes color, but not flavor. There are 8 gluons in total.
Elementary processes in QCD are constructed by analogy with QED: bremsstrahlung emission of a gluon by a quark, production of quark-antiquark pairs by a gluon. The process of gluon production by a gluon has no analogue in QED.
The static gluon field does not tend to zero at infinity, i.e. the total energy of such a field is infinite. Thus, quarks cannot escape from hadrons, confinement takes place.
There are attractive forces between quarks that have two unusual properties: a) asymptotic freedom at very small distances and b) infrared trapping - confinement, due to the fact that the potential interaction energy V(r) increases unlimitedly with increasing distance between the quarks r, V(r ) = -α s /r + ær, α s and æ are constants.
The quark-quark interaction is not additive.
Only color singlets can exist in the form of free particles:
meson singlet, for which the wave function is determined by the relation

and baryon singlet with wave function

where R is red, B is blue, G is green.

There are current and component quarks, which have different masses.
The cross sections of the process A + B = C + X with the exchange of one gluon between the quarks included in the hadrons are written in the form:

ŝ = x a x b s, = x a t/x c .

The symbols a, b, c, d denote quarks and the variables related to them, the symbols A, B, C are hadrons, ŝ, , , are quantities related to quarks, the distribution function of quarks a in hadron A (or, respectively, - quarks b in hadron B), is the fragmentation function of quark c into hadrons C, d/dt is the elementary cross section of qq interaction.

1.2 Search for deviations from the Standard Model

At the existing energies of accelerated particles, all provisions of QCD and, even more so, QED are well satisfied. In planned experiments with higher particle energies, one of the main tasks is to search for deviations from the Standard Model.
Further development High energy physics is associated with solving the following problems:

Search for exotic particles with a structure different from that accepted in the Standard Model.
Search for neutrino oscillations ν μ ↔ ν τ and the related problem of neutrino mass (ν m ≠ 0).
Search for proton decay, the lifetime of which is estimated to be τ exp > 10 33 years.
Search for the structure of fundamental particles (strings, preons at distances d< 10 -16 см).
Detection of deconfined hadronic matter (quark-gluon plasma).
Study of the violation of CP invariance during the decay of neutral K-mesons, D-mesons and B-particles.
Studying the nature of dark matter.
Study of the composition of vacuum.
Search for the Higgs boson.
Search for supersymmetric particles.

1.3 Unresolved questions of the Standard Model

The fundamental physical theory, the Standard Model of electromagnetic, weak and strong interactions of elementary particles (quarks and leptons) is a generally recognized achievement of physics of the 20th century. It explains all known experimental facts in the physics of the microworld. However, there are a number of questions that are not answered in the Standard Model.

The nature of the mechanism for spontaneous violation of electroweak gauge invariance is unknown.

Explaining the existence of masses for W ± - and Z 0 -bosons requires the introduction into the theory of scalar fields with a ground state - vacuum - that is non-invariant under gauge transformations.
The consequence of this is the emergence of a new scalar particle - the Higgs boson.

The SM does not explain the nature of quantum numbers.

What are charges (electric; baryon; lepton: Le, L μ, L τ: color: blue, red, green) and why are they quantized?
Why are there 3 generations of fundamental fermions (I, II, III)?

The SM does not include gravity, hence the way to include gravity in the SM - New hypothesis about the existence of additional dimensions in the space of the microworld.
There is no explanation why the fundamental Planck scale (M ~ 10 19 GeV) is so far from the fundamental scale of electroweak interactions (M ~ 10 2 GeV).

Currently, a path has been outlined to solve these problems. It consists in developing a new understanding of the structure of fundamental particles. It is assumed that fundamental particles are objects that are commonly called "strings". The properties of strings are addressed in the rapidly developing Superstring Model, which purports to establish connections between phenomena occurring in particle physics and astrophysics. This connection led to the formulation of a new discipline - cosmology of elementary particles.

Today, the Standard Model is one of the most important theoretical constructs in particle physics, describing the electromagnetic, weak and strong interactions of all elementary particles. The main provisions and components of this theory are described by physicist, corresponding member of the Russian Academy of Sciences Mikhail Danilov

1

Now, based on experimental data, a very perfect theory has been created that describes almost all the phenomena that we observe. This theory is modestly called the “Standard Model of Elementary Particles.” It has three generations of fermions: quarks and leptons. This is, so to speak, building material. Everything we see around us is built from the first generation. It includes u- and d-quarks, an electron and an electron neutrino. Protons and neutrons are made up of three quarks: uud and udd, respectively. But there are two more generations of quarks and leptons, which to some extent repeat the first, but are heavier and ultimately decay into particles of the first generation. All particles have antiparticles that have opposite charges.

2

The standard model includes three interactions. Electromagnetic force holds electrons within an atom and atoms within molecules. The carrier of electromagnetic interaction is the photon. The strong interaction holds protons and neutrons inside the atomic nucleus, and quarks inside protons, neutrons and other hadrons (as L. B. Okun proposed to call particles participating in the strong interaction). The strong interaction involves quarks and hadrons built from them, as well as the carriers of the interaction itself - gluons (from the English glue - glue). Hadrons consist either of three quarks, like a proton and a neutron, or of a quark and an antiquark, like, say, a π± meson, consisting of u- and anti-d-quarks. The weak interaction leads to rare decays, such as the decay of a neutron into a proton, an electron, and an electron antineutrino. The carriers of the weak interaction are W- and Z-bosons. Both quarks and leptons take part in the weak interaction, but at our energies it is very small. This, however, is simply explained by the large mass of W and Z bosons, which are two orders of magnitude heavier than protons. At energies greater than the mass of the W- and Z-bosons, the forces of the electromagnetic and weak interactions become comparable, and they combine into a single electroweak interaction. It is assumed that at much b O higher energies and strong interaction will unite with the rest. In addition to the electroweak and strong interactions, there is also a gravitational interaction, which is not included in the Standard Model.

W, Z bosons

g - gluons

H0 is the Higgs boson.

3

The Standard Model can only be formulated for massless fundamental particles, i.e. quarks, leptons, W and Z bosons. In order for them to acquire mass, the Higgs field, named after one of the scientists who proposed this mechanism, is usually introduced. In this case, there should be another fundamental particle in the Standard Model - the Higgs boson. The search for this last brick in the slender building of the Standard Model is actively underway at the largest collider in the world - the Large Hadron Collider (LHC). Indications have already been received of the existence of the Higgs boson with a mass of about 133 proton masses. However, the statistical reliability of these indications is still insufficient. It is expected that by the end of 2012 the situation will become clearer.

4

The Standard Model perfectly describes almost all experiments in elementary particle physics, although the search for phenomena beyond the framework of the Standard Model is persistently conducted. The latest hint at physics beyond the SM was the discovery in 2011 of an unexpectedly large difference in the properties of so-called charmed mesons and their antiparticles in the LHCb experiment at the LHC. However, apparently, even such a large difference can be explained within the framework of the SM. On the other hand, in 2011, another confirmation of the SM, which had been sought for several decades, was obtained, which predicts the existence of exotic hadrons. Physicists from the Institute of Theoretical and Experimental Physics (Moscow) and the Institute nuclear physics(Novosibirsk), as part of the international BELLE experiment, hadrons consisting of two quarks and two antiquarks were discovered. Most likely, these are molecules made of mesons, predicted by ITEP theorists M. B. Voloshin and L. B. Okun.

5

Despite all the successes of the Standard Model, it has many shortcomings. The number of free parameters of the theory exceeds 20, and it is completely unclear where their hierarchy comes from. Why is the mass of the t-quark 100 thousand times greater than the mass of the u-quark? Why is the coupling constant of t- and d-quarks, first measured in the international ARGUS experiment with the active participation of ITEP physicists, 40 times less than the coupling constant of c- and d-quarks? The SM does not answer these questions. Finally, why are 3 generations of quarks and leptons needed? Japanese theorists M. Kobayashi and T. Maskawa showed in 1973 that the existence of 3 generations of quarks makes it possible to explain the difference in the properties of matter and antimatter. The hypothesis of M. Kobayashi and T. Maskawa was confirmed in the BELLE and BaBar experiments with the active participation of physicists from BINP and ITEP. In 2008, M. Kobayashi and T. Maskawa were awarded the Nobel Prize for their theory

6

There are also more fundamental problems with the Standard Model. We already know that the SM is not complete. It is known from astrophysical research that there is matter that is not in the SM. This is the so-called dark matter. It is about 5 times more than the ordinary matter that we are made of. Perhaps the main drawback of the Standard Model is its lack of internal self-consistency. For example, the natural mass of the Higgs boson, which arises in the Standard Model due to the exchange of virtual particles, is many orders of magnitude greater than the mass necessary to explain the observed phenomena. One of the solutions, the most popular at the moment, is the supersymmetry hypothesis - the assumption that there is symmetry between fermions and bosons. This idea was first expressed in 1971 by Yu. A. Golfand and E. P. Likhtman at the Lebedev Physical Institute, and now it is extremely popular.

7

The existence of supersymmetric particles not only makes it possible to stabilize the behavior of the SM, but also provides a very natural candidate for the role of dark matter - the lightest supersymmetric particle. Although there is currently no reliable experimental evidence for this theory, it is so beautiful and such an elegant solution to the problems of the Standard Model that many people believe in it. The LHC is actively searching for supersymmetric particles and other alternatives to the SM. For example, they are looking for additional dimensions of space. If they exist, then many problems can be solved. Perhaps gravity becomes strong at relatively large distances, which would also be a big surprise. Other, alternative Higgs models and mechanisms for the emergence of mass in fundamental particles are possible. The search for effects beyond the Standard Model is very active, but so far unsuccessful. A lot should become clearer in the coming years.

What is the structure of the Standard Model? What properties do the particles in the Standard Model have? Is the existence of a fourth generation of elementary particles possible? Doctor of Physical and Mathematical Sciences Dmitry Kazakov answers these and other questions.

The last third of the 20th century was marked by the creation, experimental confirmation, acceptance and award of the Nobel Prize for the Standard Model of Fundamental Interactions. What is it?

First of all, it is a model that describes the fundamental particles of matter and all their interactions. This model is a model of quantum field theory and is formulated as Lagrangian quantum field theory. This is a theory that is described as quantum mechanics of fields, the quanta of which are elementary particles, and includes all fundamental particles of matter. There are not so many such particles - they are six quarks and six leptons. They are involved in three types: strong, weak and electromagnetic. In this case, we ignore the gravitational interaction due to its smallness, and it is not included in the Standard Model. So, three types of interactions and six types of particles.

The Standard Model has a structure, this structure is usually associated with symmetry groups. Three types of interactions - three symmetry groups. All these groups belong to the same class - these are the so-called unitary groups. Electromagnetic interactions are described by the symmetry group SU (1), unitary groups with one parameter, and, accordingly, one particle carrier of electromagnetic interactions is a photon. Weak interactions have a symmetry group SU (2), there are already three parameters, and, accordingly, there are three particles that carry weak interactions - these are W- and Z-bosons. Strong interactions are described by the group SU (3), there are already eight parameters and, accordingly, eight fields that carry interactions - they are called gluons. This concerns carriers of interactions.

The particles of matter themselves also belong to the representations of symmetry groups. From the point of view of the group of strong interactions - and only quarks participate in them - quarks appear in the Standard Model in the form of triplets, that is, they have quantum numbers that take on three values, often called the word “color”: blue, red, green. In weak interactions, all particles appear in the form of doublets - this is the lowest representation of the symmetry group of weak interactions. We have up and down quarks, an electron and a neutrino - these are examples of two doublets.

Interestingly, quarks and leptons repeat each other, this is called generations. There are the first generation, second generation and third generation Standard Model. Generally speaking, it is not very clear why nature chose three generations. There is a first generation of particles that make up the entire observable world, there is a copy - the second generation, and there is a third copy - this is the third generation. The Standard Model includes . These particles are fundamental in the sense that we do not see any structure in these particles.

Generally speaking, an absolute statement cannot be made, since previously the proton also seemed to be a particle without structure, and then this structure was discovered. Therefore, it cannot be said that those particles that we now consider structureless are always so.

Perhaps in the future something will be revealed to us that is not known now. But today, those particles that make up the Standard Model are structureless point particles - these are quarks and leptons, they are represented as point particles of the Standard Model. If we want to describe some process occurring in nature, as a rule, it is not the quarks themselves that participate in it, but particles composed of quarks, that is, hadrons. Leptons - electron, muon, taon - are still observed in the form of free or interacting particles in nature. Therefore, the processes that are described with leptons are directly described by the Standard Model, and with hadrons - indirectly.

One way or another, any interactions and any transformations that we observe in nature, both at small and large distances, are described by the Standard Model.

In this sense, the Standard Model crowns the entire edifice of particle physics and, in a sense, the entire edifice of fundamental physics, since it describes the most fundamental laws of nature that are known today.

What properties do the particles included in the Standard Model have? First of all, we are used to describing the quantum world using so-called quantum numbers. An example of a quantum number is electric charge. Electric charge is a characteristic of a particle that we understand. Particles can be positively charged, negatively charged, or not charged at all, and electric charge is actually a quantum number that is conserved in nature. The conservation of electric charge in the Standard Model is described by the corresponding symmetry group; symmetry theory implies the conservation of electric charge.

But this is not the only characteristic of particles, since, as is known, there are three symmetry groups in the Standard Model. Strong interactions describe colored objects. Color, of course, is a conditional concept, just a quantum number that takes on three meanings, conveniently denoted by color for clarity. So, the color charge also has a symmetry group and is also a conserved quantity, the color charge of quarks is conserved. Weak interactions have their own charge, it is called left-handed because of the spin - a slightly complicated name that has a historical reason, but this is also a characteristic of weak interactions, this is also a charge that is conserved. Thus, all particles have quantum numbers, quantum charges, which are conserved, as follows from the symmetry of the Standard Model.

There are properties in the Standard Model that are not very clear at first glance. For example, when we talk about quarks, we say that quarks cannot be observed in a free state. That is, we are so sure that quarks exist inside hadrons that the fact that we cannot directly observe them no longer seems strange to us. But the properties that these particles possess are very well demonstrated in experiment, and therefore in experiment we confirm all the properties of the Standard Model.

There are characteristics that are not obvious. For example, the Standard Model describes the masses of particles and the transitions of one type of particle to another, while maintaining the necessary symmetries. An interesting example of weak interaction, in which a number of symmetries are violated, in particular spatial parity violation or charge conjugation violation, when particles are replaced by antiparticles.

What else is included in the Standard Model? In addition to quarks and leptons, the Standard Model includes the Higgs boson. arose in theory for the reason that it was necessary to find a mechanism that would give mass to all particles of the Standard Model. This was achieved by spontaneously discovering symmetry, by introducing into the theory an additional scalar field, that is, one with spin zero, which was called the Higgs boson.

Thus, the complete composition of the fields of the Standard Model consists of six quarks, six leptons, one Higgs boson and carriers of all three types of interactions. All these particles have been experimentally discovered. The last particle to be discovered was the Higgs boson - it was discovered in 2012. All the others were discovered back in the 20th century, the last neutrino to be discovered was called the taun neutrino, the third neutrino, and it was discovered in 2000. Thus, the 20th century completed the Standard Model with the exception of the Higgs boson, and all particles were experimentally confirmed.

The question arises: does the story end here, or maybe there are some other particles that have not yet entered the Standard Model, but will have to enter there? Or maybe there is something completely different that is not described by the Standard Model? There are different answers to all these questions; we don’t know the truth yet.

First of all, if we talk about new particles like new quarks and new leptons that have not yet been discovered, as I said, there are three generations of these particles in the Standard Model. The question is: is there a fourth generation? Experimentally, the fourth generation is not visible. Moreover, there is indirect evidence related to both particle physics experiments and cosmology that there may not be a fourth generation. The fact is that in the Standard Model there is a so-called: as many quarks as there are leptons. But for leptons (more precisely, for neutrinos), the number of independent neutrino fields is equal to three. There is a small loophole for a fourth, but in all likelihood that will also be closed soon.

If the number of neutrinos is three and there is quark-lepton symmetry, then the number of generations of all other particles is equal to three, and thus we complete the Standard Model.

There is only one Higgs boson. Could there be two, or four, or more? The answer is the same: maybe. There may be other Higgs bosons; perhaps we have only discovered one so far. But theory allows the presence large quantity Higgs bosons. Whether they exist or not is a question for experiment. In this sense, it may turn out that the Standard Model is not yet complete, and new particles will still be discovered. But maybe not - one boson is enough to give mass to all particles.

New interactions - we talked about three types of interactions that are included in the Standard Model, all of them are implemented as an exchange of carriers, gauge fields with spin one. In a sense, the Higgs boson can be considered as a carrier of the fourth interaction, when it acts as a carrier of the interaction with spin zero. But is there more? Are there any new interactions or some new symmetry groups broader than the Standard Model? Isn't the Standard Model part of some more general theory? This question is also open. It is possible that this is so, it is possible that it is part of a more general theory, but this is not yet visible.

It must be said that when we talk about the triumphant completion of the Standard Model, we are talking about the fact that all, without exception, experiments that are carried out at accelerators, in underground physics, in space - all of them are brilliant, absolutely with enviable accuracy, with an accuracy sometimes up to ten ten-thousandths digits, are described by the Standard Model. In this sense, this is a completely unique model that allows you to describe a huge part of inanimate nature using very simple universal mathematical formulas.