By the middle of the 20th century, the concept of a “zoo of particles” appeared in physics, which means many different elementary components of matter that scientists encountered after they created sufficiently powerful accelerators. One of the most numerous inhabitants of the "zoo" were objects, called mesons. This family of particles, along with baryons, is included in a large group of hadrons. Studying them made it possible to penetrate into a deeper level of the structure of matter and contributed to streamlining knowledge of it in the modern theory of fundamental particles and interactions - the Standard Model.
Discovery story
In the early 1930s, after clarifying the composition of the atomic nucleus, the question arose about the nature of the forces that ensure its existence. It was clear that the interaction that binds the nucleons should be extremely intense and carried out through the exchange of certain particles. Calculations performed in 1934 by the Japanese theoretician H. Yukawa showed that these objects are 200-300 times larger than the electron in mass and, therefore, several times smaller than the proton. Later they received the name of the mesons, which means “average” in Greek. However, their first direct detection turned out to be a “misfire” associated with the proximity of the mass values of very different particles.
In 1936, objects were discovered in cosmic rays (they were called muons) with a mass corresponding to Yukawa's calculations. The sought-after quantum of nuclear forces seemed to be found. But then it turned out that muons are particles that are not related to exchange interactions between nucleons. They, together with the electron and neutrino, belong to another class of microcosm objects - leptons. Particles were renamed to muons, and searches continued.
Yukawa quanta were discovered only in 1947 and received the name "pi-mesons", or peonies. It turned out that an electrically charged or neutral p-meson is indeed the particle whose exchange allows nucleons to coexist in the nucleus.
Meson structure
Almost immediately it became clear: the peonies came to the “particle zoo” not alone, but with numerous relatives. However, it was thanks 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.
A meson is a bound state of a quark and an antiquark (communication is carried out by means of quanta of strong interaction - gluons). A “strong” quark charge is a quantum number, conventionally called a “color”. However, all hadrons, and mesons among them, are colorless. What does it mean? A meson can be formed by different types of quarks and antiquarks (or, as they say, aromas, “flavors”), but it always combines color and anticolor. For example, a 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 quarks, while the meson remains colorless.
The quarks of the older generations, such as s, c and b, tell the mesons formed by them the corresponding aromas - strangeness, charm and charm, expressed by their own quantum numbers. The integer electric charge of the meson consists of fractional charges of the particles and antiparticles forming 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, their quarks, 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, with the conservation of certain values.
Weak processes are the least intense, they conserve energy, electric charge, momentum, angular momentum (spin) - in other words, only universal symmetries act. In electromagnetic interaction, parity and flavor quantum numbers of mesons are also preserved. These are processes that play an important role in decay reactions.
Strong interaction is most symmetrical, while retaining other quantities, in particular, isospin. It is responsible for the retention of nucleons in the nucleus through ion exchange. 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 quark level is illustrated in the figure below.
Strong interaction also controls the scattering of mesons by nucleons, their production in hadron collisions, and other processes.
What is quarkonium
The combination of quark and antiquark of one flavor is commonly called quarkonium. This term, as a rule, applies to mesons, which contain massive c- and b-quarks. The extremely heavy t-quark does not manage to enter into a bound state at all, instantly breaking up into lighter ones. The combination cc̄ is called charmonium, or a particle with hidden charm (J / ψ-meson); the combination of bb̄ - bottomonium, which is inherent in latent 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 aromas, since the masses of these quarks are close in value. So, the neutral π 0 meson is a superposition of the states uū and dd̄ that have the same set of quantum numbers.
Meson instability
The combination of particles and antiparticles leads to the fact that the life of any meson ends with their annihilation. The lifetime depends on which interaction controls decay.
- Mesons that decay along the channel of “strong” annihilation, say, into gluons, followed by the birth of new mesons, do not live very long - 10 -20 - 10 -21 s. An example of such particles is quarkonia.
- Electromagnetic annihilation is also quite intense: the lifetime of the π 0 meson, the quark-antiquark pair of which annihilates with a probability of almost 99% in two photons, is about 8 ∙ 10 -17 s.
- Weak annihilation (decay into leptons) proceeds with much lower intensity. So, a charged pion (π + - ud̄ - or π - - dū) lives for a rather long time - on average 2.6 ∙ 10 -8 s and usually decays into a muon and neutrino (or corresponding antiparticles).
Most mesons are the so-called hadron resonances, short-lived (10 -22 - 10 -24 s) phenomena that occur in certain high-energy ranges, similar to the excited states of an atom. They are not recorded at the 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 a half-integer spin ½. If the angular momenta of the quark and antiquark are parallel, then their sum — the spin of the meson — is equal to 1; if they are antiparallel, it will be equal to zero.
Due to the mutual inversion 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 (associated with the particle’s own frame of reference) P-parity, the following types of mesons are distinguished:
- pseudoscalar - the most light (S = 0, P = -1);
- vector (S = 1, P = -1);
- scalar (S = 0, P = 1);
- pseudovector (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 - the isotopic spin. In strong processes, particles with the same value of isospin participate symmetrically, regardless of their electric charge, and can be represented as different charge states (projections of isospin) of one object. The totality of such particles, very close in mass, is called an isomultiplet. For example, the pion isotriplet includes three states: π + , π 0 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. So, the isospin of a pion is equal to 1, and its projections I z in a special charge space are equal to +1, 0 and -1, respectively. The four strange mesons - kaons - form two isodoublets: K + and K 0 with isospin + ½ and strangeness +1 and a doublet of antiparticles K - and K̄ 0 , for which these values are negative.
The electric charge of hadrons (including mesons) Q is associated with the projection of isospin I z 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 = I z + Y / 2. It is clear that all members of the same 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 - nonet (nine), and so on. This is a manifestation of a higher level of symmetry called unitary.
Mesons and the search for New Physics
At present, physicists are actively searching for phenomena whose description would lead to the expansion of the Standard Model and to go 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 go into 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 having a structure that does not fit into the framework of a conventional model. So, at the Large Hadron Collider in 2014, tetraquark Z (4430) was confirmed — the bound state of two quark – antiquark ud̄cc̄ pairs, an intermediate decay product of the attractive 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 at the same time have zero baryon number - for example, glueballs formed only by gluons without quarks. All such objects can significantly replenish our knowledge about the nature of fundamental interactions and contribute to the further development of microworld physics.