Hadron

A hadron is a composite particle made of quarks held together by the strong nuclear force, described by quantum chromodynamics (QCD). Hadrons are the only form in which quarks exist in nature: the color confinement mechanism ensures that no particle with net color charge can be observed in isolation. All hadrons are color-neutral composites.

Hadrons fall into two principal classes:

• Baryons — composed of three quarks, one of each color (red, green, blue). The proton (uud) and neutron (udd) are the lightest and most stable baryons. Heavier baryons include the Λ, Σ, Ξ, and Ω particles, which contain one or more strange quarks.
• Mesons — composed of a quark and an antiquark, whose color and anticolor cancel. The pion (π⁺, π⁰, π⁻) is the lightest meson and plays a central role in the long-range nuclear force. Heavier mesons include the kaon (K), the J/ψ (charm-anticharm), and the Υ (bottom-antibottom).

The spectrum of hadron masses, spins, and quantum numbers — the hadron spectrum — is determined by the non-perturbative dynamics of confined quarks and gluons. Despite the simplicity of the quark model, the hadron spectrum remains computationally challenging, and lattice QCD calculations are required to predict hadron masses from first principles.

Hadrons are sometimes described as 'bags of quarks' — composite particles built from more fundamental constituents. This is true but misleading. A hadron is not a molecule, with quarks orbiting each other like atoms in a bound state. It is a condensate of the QCD vacuum, a localized excitation of the gluon field whose boundaries are determined by the dynamics of confinement. The quarks inside a proton are not 'constituents' in the sense that atoms are constituents of a molecule; they are degrees of freedom that happen to be the most convenient description at a particular energy scale. At lower energies, the proton is a single particle with its own quantum numbers and form factors. At higher energies, it dissolves into quarks and gluons. Both descriptions are correct. Neither is more fundamental.