Hadron

Hadron is a composite particle held together by the strong interaction, formed from quarks and gluons, and overall color neutral. The concept arose in 1964 with the quark model introduced by Murray Gell-Mann and George Zweig to classify strongly interacting particles.

Hadrons are categorized into baryons, consisting of three quarks, and mesons, consisting of a quark and an antiquark. Baryons are fermions; examples include the proton (uud) and the neutron (udd). Mesons are bosons; examples include pions (such as π+) and kaons (K+).

In addition to conventional hadrons, experiments have found exotic hadrons such as tetraquarks (four quarks) and pentaquarks (four quarks and an antiquark). Heavier quarkonia, like charmonium and bottomonium, are bound states of a heavy quark with its antiquark.

Hadrons are governed by quantum chromodynamics (QCD), the theory of the strong interaction. They are color singlets, and color confinement prevents isolated quarks from being observed. The lightest hadrons, such as pions, arise from the dynamics of QCD in low-energy regimes and chiral symmetry breaking.

Most hadrons are unstable and decay via strong, electromagnetic, or weak interactions. Strong decays occur extremely fast (about 10^-23 seconds), while weak decays can last much longer. Protons are effectively stable on cosmological timescales; neutrons decay outside atomic nuclei.

Top quarks, due to their large mass and rapid decay, do not hadronize, so no hadrons contain a top quark.