By the middle of the 20th century, the concept of “particle zoo” appeared in physics, meaning a variety of elementary constituents of matter, which scientists encountered after sufficiently powerful accelerators were created. One of the most numerous inhabitants of the "zoo" were objects called mesons. This family of particles, along with baryons, is included in the large group of hadrons. Their study made it possible to penetrate to a deeper level of the structure of matter and contributed to the ordering of knowledge about it into the modern theory of fundamental particles and interactions - the Standard Model.
Discovery history
In the early 1930s, after the composition of the atomic nucleus was clarified, the question arose about the nature of the forces that ensured its existence. It was clear that the interaction that binds nucleons must be extremely intense and carried out through the exchange of certain particles. Calculations performed in 1934 by the Japanese theorist H. Yukawa showed that these objects are 200–300 times larger than the electron in mass and,respectively, several times inferior to the proton. Later they received the name of mesons, which in Greek means "middle". However, their first direct detection turned out to be a "misfire" due to the proximity of the masses of very different particles.
In 1936, objects (they were called mu-mesons) with a mass corresponding to Yukawa's calculations were discovered in cosmic rays. It seemed that the sought-for quantum of nuclear forces had been found. But then it turned out that mu-mesons are particles that are not related to the exchange interactions between nucleons. They, together with the electron and neutrino, belong to another class of objects in the microcosm - leptons. The particles were renamed muons and the search continued.
Yukawa quanta were discovered only in 1947 and were called "pi-mesons", or pions. It turned out that an electrically charged or neutral pi-meson is indeed the particle whose exchange allows nucleons to coexist in the nucleus.
Meson structure
It became clear almost immediately: the peonies came to the “particle zoo” not alone, but with numerous relatives. However, it was due to the number and variety of these particles that it was possible to establish that they are combinations of a small number of fundamental objects. Quarks turned out to be such structural elements.
Meson is a bound state of a quark and an antiquark (the connection is carried out by means of quanta of strong interaction - gluons). The "strong" charge of a quark is a quantum number, conventionally called "color". However, all hadronsand mesons among them, are colorless. What does it mean? A meson can be formed by a quark and an antiquark of different types (or, as they say, flavors, “flavors”), but it always combines color and anticolor. For example, π+-meson is formed by a pair of u-quark - anti-d-quark (ud̄), and the combination of their color charges can be "blue - anti-blue", "red - anti-red" or green-anti-green. The exchange of gluons changes the color of the quarks, while the meson remains colorless.
Quarks of older generations, such as s, c and b, give the corresponding flavors to the mesons they form - strangeness, charm and charm, expressed by their own quantum numbers. The integer electric charge of the meson is made up of the fractional charges of the particles and antiparticles that form it. In addition to this pair, called valence quarks, the meson includes many ("sea") virtual pairs and gluons.
Mesons and fundamental forces
Mesons, or rather, the quarks that make them up, participate in all types of interactions described by the Standard Model. The intensity of the interaction is directly related to the symmetry of the reactions caused by it, that is, to the conservation of certain quantities.
Weak processes are the least intense, they conserve energy, electric charge, momentum, angular momentum (spin) – in other words, only universal symmetries act. In the electromagnetic interaction, the parity and flavor quantum numbers of mesons are also conserved. These are the processes that play an important role in the reactionsdecay.
The strong interaction is the most symmetrical, preserving other quantities, in particular, isospin. It is responsible for the retention of nucleons in the nucleus through ion exchange. By emitting and absorbing charged pi-mesons, the proton and neutron undergo mutual transformations, and during the exchange of a neutral particle, each of the nucleons remains itself. How this can be represented at the level of quarks is shown in the figure below.
The strong interaction also governs the scattering of mesons by nucleons, their production in hadron collisions and other processes.
What is quarkonium
The combination of a quark and an antiquark of the same flavor is called quarkonia. This term is usually applied to mesons that contain massive c- and b-quarks. An extremely heavy t-quark does not have time to enter a bound state at all, instantly decaying into lighter ones. The combination cc̄ is called charmonium, or a particle with hidden charm (J/ψ-meson); the combination bb̄ is bottomonium, which has a hidden charm (Υ-meson). Both are characterized by the presence of many resonant - excited - states.
Particles formed by light components - uū, dd̄ or ss̄ - are a superposition (superposition) of flavors, since the masses of these quarks are close in value. Thus, the neutral π0-meson is a superposition of the states uū and dd̄, which have the same set of quantum numbers.
Meson instability
The combination of particle and antiparticle results inthat the life of any meson ends in their annihilation. The lifetime depends on which interaction controls the decay.
- Mesons that decay through the channel of "strong" annihilation, say, into gluons with the subsequent birth of new mesons, do not live very long - 10-20 - 10- 21 p. An example of such particles is quarkonia.
- Electromagnetic annihilation is also quite intense: the lifetime of the π0-meson, whose quark-antiquark pair annihilates into two photons with a probability of almost 99%, is about 8 ∙ 10 -17 s.
- Weak annihilation (decay into leptons) proceeds with much less intensity. Thus, a charged pion (π+ – ud̄ – or π- – dū) lives quite a long time – on average 2.6 ∙ 10 -8 s and usually decays into a muon and a neutrino (or into the corresponding antiparticles).
Most mesons are the so-called hadron resonances, short-lived (10-22 – 10-24 c) phenomena that occur in certain high energy ranges, similar to the excited states of the atom. They are not recorded on detectors, but are calculated based on the energy balance of the reaction.
Spin, orbital momentum and parity
Unlike baryons, mesons are elementary particles with an integer value of the spin number (0 or 1), that is, they are bosons. Quarks are fermions and have half-integer spin ½. If the moments of momentum of a quark and an antiquark are parallel, then theirthe sum - meson spin - is equal to 1, if antiparallel, it will be equal to zero.
Due to the mutual circulation of a pair of components, the meson also has an orbital quantum number, which contributes to its mass. The orbital momentum and spin determine the total angular momentum of the particle, associated with the concept of spatial, or P-parity (a certain symmetry of the wave function with respect to mirror inversion). In accordance with the combination of spin S and internal (related to the particle's own frame of reference) P-parity, the following types of mesons are distinguished:
- pseudoscalar - the lightest (S=0, P=-1);
- vector (S=1, P=-1);
- scalar (S=0, P=1);
- pseudo-vector (S=1, P=1).
The last three types are very massive mesons, which are high-energy states.
Isotopic and unitary symmetries
For the classification of mesons it is convenient to use a special quantum number - isotopic spin. In strong processes, particles with the same isospin value participate symmetrically, regardless of their electric charge, and can be represented as different charge states (isospin projections) of one object. A set of such particles, which are very close in mass, is called an isomultiplet. For example, the pion isotriplet includes three states: π+, π0 and π--meson.
The value of isospin is calculated by the formula I=(N–1)/2, where N is the number of particles in the multiplet. Thus, the isospin of a pion is equal to 1, and its projections Iz in a special chargespace are respectively +1, 0 and -1. The four strange mesons - kaons - form two isodoublets: K+ and K0 with isospin +½ and strangeness +1 and the doublet of antiparticles K - and K̄0, for which these values are negative.
The electric charge of hadrons (including mesons) Q is related to the isospin projection Iz and the so-called hypercharge Y (the sum of the baryon number and all flavor numbers). This relationship is expressed by the Nishijima–Gell-Mann formula: Q=Iz + Y/2. It is clear that all members of one multiplet have the same hypercharge. The baryon number of mesons is zero.
Then, the mesons are grouped with additional spin and parity into supermultiplets. Eight pseudoscalar mesons form an octet, vector particles form a nonet (nine), and so on. This is a manifestation of a higher level symmetry called unitary.
Mesons and the search for New Physics
Currently, physicists are actively searching for phenomena, the description of which would lead to the expansion of the Standard Model and to going beyond it with the construction of a deeper and more general theory of the microworld - New Physics. It is assumed that the Standard Model will enter it as a limiting, low-energy case. In this search, the study of mesons plays an important role.
Of particular interest are exotic mesons - particles with a structure that does not fit into the framework of the usual model. So, at the Large HadronCollider in 2014 confirmed the Z(4430) tetraquark, a bound state of two ud̄cc̄ quark-antiquark pairs, an intermediate decay product of the beautiful B meson. These decays are also interesting in terms of the possible discovery of a hypothetical new class of particles - leptoquarks.
Models also predict other exotic states that should be classified as mesons, since they participate in strong processes, but have zero baryon number, such as glueballs, formed only by gluons without quarks. All such objects can significantly replenish our knowledge of the nature of fundamental interactions and contribute to the further development of the physics of the microworld.
