Hadrons. Elementary particles

HADRONS (from the Greek αδρ? Σ - large, strong), particles participating in strong interactions. Hadrons include mesons and baryons (including protons and neutrons). Hadrons should be distinguished from atomic nuclei, which consist of two or more nucleons.

Hadrons are not elementary, they are composed of quarks. The best-studied baryons are made up of three quarks, and mesons are made up of a quark and an antiquark, glued together by gluons. All known hadrons consist of six types (or, as they often say, flavors) of quarks, denoted by the letters u, d, s, c, b, t. Nucleons consist of the lightest quarks: u and d (for example, the proton p and neutron n are represented as p \u003d uud, n \u003d ddu). Baryons containing heavier quarks (s, c, b) are called hyperons. The interaction of gluons with quarks and gluons with gluons is due to the presence of specific charges in quarks, antiquarks and gluons, called color charges (or color). The theory describing these interactions is called quantum chromodynamics (QCD).

The quark of each fragrance comes in three color varieties (red, yellow, blue). Antiquark colors are complementary (orange, green, purple). Each of the eight gluons carries a double color charge, for example, red-orange, yellow-blue, and so on. The names of the colors are arbitrary, but the above choice in accordance with the terminology accepted in optics is convenient because it is natural to call hadrons (which do not have color charges) as colorless or white particles. Colored particles - quarks, antiquarks, gluons - are, as it were, enclosed inside white hadrons. This phenomenon is called confinement. A consistent theory of confinement within QCD has not yet been developed. The consequence of confinement is that in collisions of high-energy hadrons with each other or with other particles - photons or leptons - hadrons are born, but not free quarks and gluons.

High-energy particle accelerators are looking for so-called exotic hadrons, the structure of which is more complex than three quarks in the case of baryons and a quark-antiquark in the case of mesons. Exotic mesons, consisting only of gluons, are called glueballs.

Hadrons containing, in addition to the minimum number of quarks, also a gluon are called hybrids. Since the electric charge of gluons is zero and they have no aroma, the glueballs must be electrically neutral, and the hybrids must have the same aroma as the corresponding hadron, without additional gluon. Instead of an extra gluon, an exotic hadron can contain a quark-antiquark pair (for example, uu or ds, where the dash above the quark symbol means an antiquark). In the first case, the flavor of the exotic hadron coincides with the flavor of the main one, in the second it differs from it.

Historically, the first hadrons studied were nucleons (proton and neutron) and the lightest of the mesons - pi-mesons, discovered in 1947. Strange particles were discovered in the 1950s. Their study and systematization led in 1964 to the creation of a quark model of hadrons, and the s-quark, which is part of strange particles, was called a strange quark. In 1974, the first meson was discovered containing charmed quark c and antiquark c (see Charmed particles). Such mesons are called mesons with hidden charm (charm). This was followed by the discovery of mesons with a clear charm, such as eu or cd. In 1976, the first mesons of the bb type were discovered, and then the mesons of the bu, bd, bs, etc. types. In 1984, pairs of the heaviest quarks t and t were born at the proton-antiproton collider. The mass of the t-quark is about 175 GeV, its lifetime is so short (of the order of 10 -24 s) that it has no time to form the corresponding hadrons either with the t-quark or with lighter quarks accompanying its creation.

Lit .: Okun LB Physics of elementary particles. 2nd ed. M., 1988.

ἁδρός "large; massive ") - a class of compound particles subject to strong interaction. The term was proposed by the Soviet physicist LB Okun 'in 1962, during the transition from Sakata's model of strongly interacting particles to the quark theory. For elementary particles that do not participate in strong interactions, LB Okun 'then proposed the name adenones .

Hadrons possess quantum numbers conserved in the processes of strong interaction: strangeness, charm, charm, etc.

Hadrons are divided into two main groups according to their quark composition:

AT recent times the so-called exotic hadrons, which are also strongly interacting particles, but which do not fit into the framework of the quark-antiquark or three-quark classification of hadrons. Some hadrons are still only suspected of being exotic. Exotic hadrons are divided into:

exotic baryons, in particular pentaquarks, the minimum quark composition of which is 4 quarks and 1 antiquark.
exotic mesons - in particular, hadronic molecules, glueballs and hybrid mesons.

Baryons (fermions)

See a more detailed list of baryons.

Ordinary baryons (fermions) each contain three valence quarks or three valence antiquarks.

Nucleons are the fermionic constituents of an ordinary atomic nucleus:
Hyperons, such as Λ-, Σ-, Ξ- and Ω-particles, contain one or more s-quarks, decay rapidly and are heavier than nucleons. Although there are usually no hyperons in the atomic nucleus (it contains only an admixture of virtual hyperons), there are related systems one or more hyperons with nucleons, called hypernuclei.
Charmed and adorable baryons were also discovered.

Pentaquarks are composed of five valence quarks (more precisely, four quarks and one antiquark).

Signs of the existence of exotic baryons containing five valence quarks have recently been found; however, there have been reports of negative results. The question of their existence remains open.

Mesons (bosons)

See a more detailed list of mesons.

Ordinary mesons contain a valence quark and a valence antiquark. These include pion, kaon, J / ψ -meson, and many other types of mesons. In nuclear force models, interactions between nucleons are transported by mesons.

There may also be exotic mesons (their existence is still in question):

Tetraquarks are composed of two valence quarks and two valence antiquarks.

Hadrons are divided into two groups: mesons (s \u003d 0, 1, participates in strong interaction) and baryons (s \u003d 1/2, 3/2, participates in strong interaction). Baryons are divided into nucleons (s \u003d 1/2) and hyperons (s \u003d 1/2, 3/2).

2. What was the quark hypothesis of M. Gellman and D. Zweig? What experiments have confirmed the existence of three point charges in nucleons? Why is the spin of these charges (quarks) half-integer?

It consisted in the fact that hadrons are constituent particles. The existence of three point charges in nucleons was confirmed by an experiment in which the scattering of electrons with an energy of 20 GeV by protons and neutrons was studied.

Because nucleons have half-integer spin and consist of three quarks, and if we assume that all quarks have the same spin, then it must be half-integer.

3. Give the calculation of possible electric charges of quarks. What were these quarks named?

Let's denote by Q and q the possible electric charges of quarks.

If we solve this system of two equations in two variables we get

(such a quark was called a u-quark); q \u003d -1 / 3e (d-quark).

4. What conservation laws reflected the conservation of charge and mass numbers in nuclear reactions? Formulate the law of conservation of baryonic charge. How does he confirm the impossibility of decay of a baryon into smaller particles?

The law of conservation of electric charge reflects the conservation of the charge number, and the law of conservation of mass reflects the conservation of the mass number.

Baryon charge conservation law: The baryon charge is conserved in all interactions. The impossibility of decay of a proton into more fine particles is explained by the conservation of the baryonic charge. The baryon charge of quarks is 1/3, for baryons (protons and neutrons) B \u003d 1 (baryon charge of the nucleus). In β-decay, the baryon charge conservation law has the form

Hadronami elementary particles that can participate and actually participate in strong interactions are called. All of them are also subject to electromagnetic, weak and gravitational interactions. The class of hadrons is the most numerous: it has more than 300 particles (including antiparticles). , according to modern views, are constituent particles. The first indirect indication of this can be at least the fact that there are a lot of them - several hundred. Further, most hadrons are resonances — extremely unstable particles. But, most importantly, an internal structure was discovered in hadrons. Already from the results of experiments on elastic scattering of electrons by nucleons, carried out in the 50-60s, it followed that the radii of the proton and neutron are nonzero. Of course, it is not possible to directly measure these radii, we mean the root-mean-square radii of the distribution of the electric charge and magnetic moment in these particles.

R N \u003d 0.8 · 10 -15 m.

In this case, the electric charge and magnetic moment are unevenly distributed in them: they fall off from the center to the periphery according to an exponential law (the neutron has no electric charge distribution). So, the density of the electric charge of a proton is well described by the following experimentally found formula:

g (r) \u003d e 3.06exp (-4.25r).

Moreover, experiments on inelastic scattering of electrons by nucleons, carried out in the 60s and 70s, revealed the granular ("parton") structure of the proton and neutron.

consist of quarks. are combined in such a way that their fractional charges add up to the whole hadron charge, i.e. hadrons have no color charge, although quarks do. All hadrons, with the exception of the proton, are unstable, i.e. decay into other particles. For example, a neutron decays into a proton, an electron and an antineutrino; The K meson decays into two pi-mesons, which further decay into a muon and neutrino, etc.

Distinguish between stable (more precisely, meta-stable) hadrons with average lifetimes T\u003e 10 -23 s and resonances whose lifetimes T ~ 10 -24 -10 -23 from. The most characteristic feature of resonances is that they decay as a result of strong interactions, while decays of "stable" hadrons are due to much less intense interactions, mainly weak, sometimes electromagnetic. This property of resonances can serve as their most adequate definition.

Hadrons are classified into baryons and mesons.

Hadrons with half-integral spins are called baryons; hadrons with integer spins are called mesons.

Mesons (from the Greek. m esos - medium) consist of one quark and one antiquark. For example, a meson like a peony ( π + ), has the structure ud (i.e. consists of one u-quark and one d-antiquark). Similarly, antipion ( p -) has a completely different structure du (i.e. consists of one u-antiquark and one d-quark). Mesons are bosons, i.e. to vectors of interactions (see the standard model).

Since a meson consists of a particle and an antiparticle, it is very unstable. However, such a meson as kaon ( K), has a much longer life span than the rest of the mesons, and therefore the quark included in it was called strange.

Thus, there are stable mesons and stable baryons, as well as meson resonances and baryon resonances. To characterize this difference, introduce physical quantity, which is analogous to the lepton charge - the baryon charge.

Baryon charge.

By definition, all baryons have B \u003d + 1, all antibaryons have B \u003d - 1, and all other particles (including mesons) have B \u003d 0. So far, it is assumed that the baryon charge is conserved in all interactions. This is due, in particular, to the stability of the proton - the lightest baryon. All mesons and baryons are subdivided into "ordinary", "strange", "charmed" and "adorable". Note that "pretty" baryons have not yet been recorded, although there is no doubt that they exist. In addition, a new class of "true" particles with very large masses is predicted.

In addition to space-time symmetries, symmetries of a different nature, called internal or dynamic symmetries, operate in the world of particles. They allow, for example, to classify the entire variety of hadrons.

To explain what internal symmetry is, it is best to use a specific example. Consider the two lightest baryons - a proton and a neutron. Particle masses are very close: m p \u003d 938.2 MeV, m n \u003d 939.5 MeV... Proton charge Q \u003d +1, neutron charge Q \u003d 0... Experiments show that in strong interaction reactions, both particles behave in the same way. If you do not take into account the electromagnetic charge, then in all other respects they are indistinguishable. AT nuclear world a proton and a neutron act as a single particle - a nucleon, which can be in two different states, proton and neutron.

The "interchangeability" of a proton and a neutron in reactions of strong interactions can be mathematically described as symmetry with respect to rotations in some hypothetical (having nothing to do with ordinary) three-dimensional space, called isotopic space. Just as an electron with spin ½ is able to be in two states with spin projections on a chosen direction (quantization axis) ½ or -½, so a proton with a neutron can be considered the states of one particle (nucleon N), endowed with a new quantum number - isotopic spin T \u003d ½but with different projections: Tz \u003d ½ and Tz \u003d -½ (the concept of isotopic spin was introduced in 1940 by W. Heisenberg). The indistinguishability of a proton and a neutron is now expressed as the invariance of the equations of the theory of strong interactions with respect to "rotations" in isotopic space. Or, in other words, the identity of the proton and neutron with respect to the strong interaction finds its concrete expression in the property of the charge independence of nuclear forces: they are the same for systems p - p, p - p, p - p.

All hadrons are distributed over small families - isomultiplets. The strong interaction of their individual members is the same, but they differ only in their attitude to electromagnetic and weak interactions. If the last two interactions could be ignored, then the members of one isomultiplet would become identical, indistinguishable particles. Characteristic outward sign the belonging of particles to one isomultiplet is the approximate equality of their masses at different values \u200b\u200bof the electric charge. It is believed that small differences in masses arise precisely due to the electromagnetic interaction. The most famous example of an isomultiplet is the nucleon isodoublet Ncontaining a proton r and neutron p, on the example of which we carried out the initial explanation.

The mathematical apparatus used to describe different isomultiplets and their individual terms is almost identical to the apparatus created to describe the ordinary spin and different spin states of a given particle. Isomultiplet is generally attributed to isospin T, which determines the number of its members by the formula:

N \u003d 2T + 1. ()

A particle with ordinary spin J there is 2J + 1 spin states with different spin projections J 3... By analogy with this, the isospin projection is introduced T s, the values \u200b\u200bof which differ between the individual members of the isomultiplet (although here no visual geometric images are suitable). The quantity T s runs values \u200b\u200bfrom -T before T through one in ascending order of electric charge. Here are two simple examples. For nucleon N \u003d 2 (p, n)and therefore T \u003d ½, for the neutron T s \u003d -½ proton T z \u003d + ½... For peony N \u003d 3 (l +, l 0, l -), and therefore T \u003d 1; at l --meson T s \u003d - 1, at l 0-meson T s \u003d 0, at l + -meson T z \u003d + 1.

Isospin is conserved in strong interaction. We only note that the property of charge independence of nuclear forces is a particular consequence of the isospin conservation law. Electromagnetic interaction makes the members of a given isomultiplet different, and therefore, isospin is not conserved in the processes caused by it. It does not persist even in weak interaction.

Strange.

First of all, it should be recalled that hadrons, which include a particular flavor, become the owners of the corresponding quantum number, in other words, the composition of the strange particles considered below contains a strange quark.

Initially, of the hadrons, only particles N and n were known. The electric charges of these "ordinary" particles can be calculated by the formula q \u003d T 3 + ½B,(1) .

But for "strange" particles discovered in the 50s, this formula is no longer valid. So, at K +-meson q \u003d +1, while Tz \u003d + ½, B \u003d 0... All these particles are assigned a new quantum number - the strangeness S. It is introduced in such a way that the Gell-Mann-Nishijima relation holds for strange particles q \u003d T 3 + ½ (B + S), (2) generalizing the previous formula. In fact, the ratio is now considered simply as a definition of strangeness, allowing you to find its values \u200b\u200bfor specific particles. So, for "ordinary" particles S \u003d 0, and from the last example it is immediately clear that K +-meson should be attributed to strangeness S \u003d + 1.

Strangeness is believed to persist in strong (and electromagnetic) interactions but not in weak interactions. In a stricter sense in the physics of elementary particles, there are so-called. selection rules, one of them in relation to the property of strangeness is formulated as follows: the sums of strangeness of particles at the beginning and end of the strong interaction reaction are the same.

This immediately explains the very an unusual property of strange particles, due to which they mainly got their name: these particles are always born in pairs, and quickly - in a time t ~ 10 -23 s, and decay one by one and slowly - in a time t ~ 10 -10 -t ~ 10 -8 s. The point is that in cosmic radiation "strange" particles are generated by the collision of "ordinary" hadrons N and n with S \u003d 0 and as a result of strong interaction (hence the short times). Since in the initial state S \u003d 0, then in the final state the total strangeness is equal to zero. And this means that if any one particle with S not equal ABOUT, then another particle with the opposite value must also form S... “Strange” particles cannot decay due to strong interaction, since they ultimately turn into “ordinary” particles. Their decays are due to weak interactions that do not preserve strangeness, hence the relatively long lifetimes.

In the 70s, charmed particles were discovered, for which the ratio (2) ... They were credited with a new quantum number - charm C, the introduction of which generalizes the Gell-Mann-Nishijima relationship q \u003d T 3 + ½ (B + S + C), (3) .

Charm obeys the same conservation laws as strangeness. After the discovery of lovely particles, it became necessary to introduce the charm b. q \u003d T 3 + ½ (B + S + C-b), (4) ... (the minus sign was introduced for reasons of a rather random nature).

For "true" particles, if they are discovered, it is necessary to introduce one more quantum number - truth (?).

So, we see that to describe the entire variety of hadrons, we have to use a large number of very unusual physical quantities (and we have not listed all of them). Their deep meaning is that all these quantities obey certain conservation laws, which make it possible to establish selection rules that prohibit or permit the occurrence of certain transformations of particles. It is clear from what has been said that the fundamental interactions differ, along with other characteristics, also their inherent conservation laws.

Every person has heard about atoms and that these small particles of matter make up the matter around us. However, not all people know that the atom is not an elementary "brick" of the universe. What is it? There is no definite answer yet. Nevertheless, considering the question that it is a hadron will help clarify the problem.

The surrounding matter and its structure

Let us begin to consider the question of what it is - a hadron "from above". All substance that a person encounters every day, which can touch, evaluate its color and other properties, consists of a set of molecules and atoms. The latter, in turn, are formed by electrons and nuclei. This fact was established about a century ago thanks to the work of Ernest Rutherford.

Now let's ignore the electron and consider the atomic nucleus. As you know, it is formed by two types of particles: neutrons and protons. And here we finally got to the bottom of it, since the neutron and proton are hadrons.

The concept of a hadron

In general, a hadron is a particle that is formed by quarks and can take part in strong interactions. This definition does not sound entirely clear, since you need to know what quarks and strong fields are, which will be discussed below. What is the meaning of the word "hadron"? It has a Greek root and translates as "massive, dense". That is, we are talking about a dense particle of matter with a large mass.

As mentioned above, hadrons are a proton and a neutron, each of them consists of three quarks.

What is a quark?

Closer to the middle of the 20th century, physicists from all over the world in various experiments began to observe more and more "elementary" particles. At first, experiments were limited to the study of the natural radioactivity of some chemical elementsand then the first particle accelerators were built, which made it possible to collide their high-energy beams, which significantly increased the number of particles. The latter had different charge, spin, mass, life time and behaved differently in various interactions (weak, strong, electromagnetic).

All this huge layer of information led to the fact that a theory was needed that would bring together all the particles. The quark became such a theoretical guess. This name was first used by Murray Gell-Man, american physicist, in 1963. It is curious to note that he spied the word "quark" in one of the literary works, it meant an imitation of the cry of seagulls.

Thanks to the introduction of a new "brick" into the physics of elementary particles, all the discovered clots of matter fit neatly into the framework of the new concept. Note that only hadrons are formed by quarks, particles such as neutrinos or electrons belong to the class of leptons, they are considered elementary, and quarks have nothing to do with them.

How many quarks are there and what characteristics are they described?

Hadrons are made of quarks. But what is a quark? This is a kind of real object, the size of which is within 10 -18 -10 -15 meters. There are 3 generations of quarks that differ from each other in taste. In fact, only the first generation of quarks is involved in the formation of stable hadrons. The other two generations have a large mass (energy), so they quickly pass into the "base" quarks.

The first generation includes only two particles: u or up and d or down quarks. They differ in isospin (u has +1/2, d has -1/2), charge and mass. We will give the spin specially to show that we are talking about fermions, the behavior of which at high matter densities differs from bosons (integer spin). An example of the latter can be photons, gluons and any other "carriers" of interaction.

Let's say a couple of words about the taste and color of quarks so as not to confuse readers. Taste is a set of properties (isospin, "strangeness", "wonder", "bottom", "top") of a quark, which determines the type of its interaction with the bosons Z and W, that is, determines the nature of the transition between quarks (weak interactions). The taste of particles u and d is determined exclusively by isospin.

As for color, this is a completely different property of quarks, such as their electric charge or mass. Naturally, it has no physical connection with the word "color" familiar to all of us, but was named so because it can take one of 3 meanings ("blue", "red", "green"). Color is associated with the three-dimensionality of space. Roughly we can say that color is a vector directed in one of 3 directions (x, y, z). The introduction of color for quarks made it possible to explain why they can be in the same state (the Pauli exclusion principle, which all fermions follow).

If we take into account the above two quarks (u, d), as well as the fact that each of them can have one of 3 colors, then we get 6 different "bricks" for building hadrons. This number needs to be multiplied by 2, since each of them has its antiparticle.

Hadron classification

When the reader has become acquainted with the meaning of the word "hadron" and with the concept of quarks, one can give a generally accepted classification of elementary particles. So, they are all divided into two large classes: hadrons and leptons.

Hadrons are represented by baryons and mesons. The former are formed by three quarks or three aniquarks, the latter are a collection of only 2 particles: a quark-antiquark, therefore all mesons (pions, kaons) have a short lifetime and annihilate quickly. Baryons are stable hadron particles with the resulting spin (fermions). The proton and neutron are bright representatives of baryons, they are often called nucleons, since they form atomic nuclei.

Thus, the importance of hadrons in the Universe is great, because all the matter around us is baryonic-lepton (an electron is a lepton). However, modern science has come to the threshold of the discovery of another type of substance, that is, not baryonic-lepton (dark matter, the substance of black holes).

Nucleons: proton and neutron

These elementary hadron particles are formed by 2 types of quarks: u and d. The composition of the proton is described as u-u-d, of the neutron - u-d-d. In them, quarks are bound by strong interactions, which are carried by gluons. The farther the quarks are from each other, the stronger their attraction forces increase. This fact explains that a separate quark cannot be found in nature.

As for the mass of a proton and a neutron, it is impossible to determine it by a simple summation of three quarks, since it is much larger than this sum. The point is that the contribution to the mass of these hadrons is made not only by the quark at rest, but also in motion (kinetic energy).

A proton and a neutron can transform into each other as a result of weak interactions leading to a transformation between the u and d quarks.

Note that both quarks in hadrons and hadrons interact with each other through the same mechanism - the gluon field.

The current state of physics of elementary particles

Quarks appeared in physical theory in the early 1960s, and already in the 1970s it was suggested that they, too, are not elementary “building blocks” and consist of so-called preons. The latter have not yet been discovered, however, if this happens, then this should significantly simplify the existing theory of the elementary world.

In addition to the problem above, there are still a number of unresolved issues:

the description of gravity and dark matter does not fit into the standard model of the Universe;
why three quarks in a proton give an exact modulo charge of an elementary particle of a completely different class - an electron (lepton);
there was evidence of the existence of hadrons, consisting not of 2, like mesons, or 3, like baryons, but of 5 quarks.

All the problems mentioned are not simple. Suffice it to say that Albert Einstein devoted the last 30 years of his life to solving some of them and did not come to any result. He had an IQ of 160!