Meson

A meson is a composite particle belonging to the hadron family in particle physics. Conventional mesons contain a quark and an antiquark bound through the strong interaction. They have zero baryon number and integer angular momentum, making them bosons, unlike ordinary baryons, which contain three valence quarks. The broader meson spectrum also includes states whose properties require descriptions beyond a simple quark–antiquark pair. (cms.cern)

Historical development

In 1935, Hideki Yukawa proposed a massive exchange particle to explain the short-range force between protons and neutrons in an atomic nucleus. Its predicted mass lay between those of the electron and proton. A particle subsequently identified in cosmic radiation initially appeared to satisfy the prediction, but its weak interaction with matter showed that it was unsuitable as the nuclear-force carrier. This particle is the muon, now classified as a lepton rather than a meson. (nobelprize.org)

The pion, discovered in cosmic-ray investigations by Cecil Powell and collaborators in 1947, had the required strong-interaction properties. Yukawa received the 1949 Nobel Prize in Physics for his prediction. The term meson eventually acquired a structural meaning within the quark model rather than remaining a designation for particles within an intermediate mass range. (nobelprize.org)

Internal structure

The underlying theory of mesons is quantum chromodynamics (QCD), the strong-interaction component of the Standard Model. Quarks interact through gluons, and physical hadrons are color-neutral states. Confinement prevents isolated quarks and gluons from appearing as freely propagating particles under ordinary conditions. A meson’s mass therefore reflects its interacting internal system, not merely an arithmetic sum of constituent quark masses. (home.web.cern.ch)

The quark–antiquark description specifies the minimal, or valence, content of a conventional meson. It does not mean that the particle contains only two permanently identifiable objects: gluonic degrees of freedom and additional quark–antiquark components contribute to its quantum state. The quark model organizes the spectrum effectively, while lattice QCD calculates hadronic properties directly from the underlying theory. States with identical overall quantum numbers can mix, complicating assignments of internal structure. (pdg.lbl.gov)

Classification and major families

Mesons are grouped by quark flavor and by quantum numbers. Important families include light mesons, strange mesons, charmed mesons, bottom mesons, and heavy quark–antiquark systems. The Particle Data Group separates these categories when compiling measured masses, lifetimes, decay modes, and other properties. Different excitations within a family can share the same flavor content while differing substantially in mass and angular momentum. (pdg.lbl.gov)

Representative examples are:

• Pions: the lightest conventional mesons. Their charged states have valence compositions $u\bar d$ and $d\bar u$; the neutral pion involves a combination of light quark–antiquark states.
• Kaons: mesons containing a strange quark or antiquark; for example, $K^+=u\bar s$.
• D mesons: states containing charm together with a lighter antiquark, or the corresponding antiparticle combination.
• B mesons: states containing bottom or antibottom together with lighter constituents.
• Quarkonia: heavy quark–antiquark systems of matching flavor, including charmonium such as $J/\psi$ and bottomonium such as $\Upsilon$. (pdg.lbl.gov)

Classification also uses total angular momentum $J$, parity $P$, and, where applicable, charge-conjugation parity $C$. Quark spins and relative orbital motion combine to produce the observed angular momentum. Pseudoscalar mesons have $J^P=0^-$, whereas vector mesons have $J^P=1^-$. These labels distinguish particles that may otherwise have similar quark compositions. (pdg.lbl.gov)

Decays and lifetimes

Mesons are unstable, but their lifetimes vary widely because different decay mechanisms are available. The charged pion has a mean lifetime of approximately $2.60\times10^{-8}$ seconds and decays predominantly into a muon and a neutrino. The neutral pion decays mainly into two photons. Many heavier mesons can decay strongly into lighter hadrons, producing much shorter-lived resonances. (pdg.lbl.gov)

Decays mediated by the weak interaction are particularly useful for studying quark flavor. Charged pseudoscalar mesons can decay into a charged lepton and neutrino; their rates depend on both weak-interaction parameters and a meson decay constant describing strong-interaction effects. Accurate theoretical predictions consequently require control of QCD contributions, including calculations of decay constants and corrections to measured decay rates. (pdg.lbl.gov)

Nuclear forces and symmetry tests

Meson exchange provides an effective description of interactions between nucleons. Pions and heavier mesons contribute to descriptions of the nuclear force, whereas gluons mediate the fundamental strong interaction between quarks. These are different levels of description: a pion is a composite hadron, not an elementary force carrier in the Standard Model. (repository.cern)

Neutral mesons also provide sensitive tests of fundamental symmetries. Some can mix with their antiparticles, and their decays can violate charge–parity symmetry. CP violation was first observed in neutral-kaon decays in 1964 and has also been measured in B-meson systems. Such measurements test the Standard Model’s account of differences between matter and antimatter. (home.cern)

Exotic mesons

Observed states beyond the conventional spectrum include tetraquarks, with two valence quarks and two valence antiquarks. Their internal organization may resemble a tightly bound four-quark system, a molecule-like association of two mesons, or a mixture of configurations. Establishing which description applies requires more than identifying a peak in a mass distribution. Measurements of quantum numbers, decay channels, and production behavior constrain the interpretation; even then, states with the same quantum numbers can mix. (home.cern)