Meson

Meson is a hadron consisting of a quark and an antiquark bound together by the strong interaction. Mesons are color-neutral and are bosons, with integer spin. They form a major class of hadrons along with baryons, which are three-quark states. In the quark model, mesons are bound states of a quark q and an antiquark q̄; their properties reflect the flavors of their constituents and the dynamics of quantum chromodynamics (QCD). The lightest mesons include the pions (π±, π0) and the kaons (K±, K0, anti-K0); heavier families include D mesons (c q̄), B mesons (b q̄), and quarkonia such as charmonium (c c̄) and bottomonium (b b̄). Neutral mesons can mix with their antiparticles, leading to oscillations and, in some systems, CP violation.

Mesons are categorized by quantum numbers and mass: pseudoscalar mesons with J^P = 0− (for example π, K, η) and vector mesons with J^P = 1− (such as ρ, ω, φ, J/ψ, Υ). They may appear as resonances in scattering experiments and decay via strong, electromagnetic, or weak interactions. Many mesons have lifetimes ranging from fractions of a nanosecond to longer, depending on the available decay channels.

Historically, the term meson derives from the Greek mesōn, meaning middle, reflecting the particle's role between nucleons and heavier particles. The concept was introduced to explain the range of the nuclear force before the advent of QCD. Today, mesons remain essential for understanding hadron structure, quark dynamics, and flavor physics.