A hadron is a composite subatomic particle whose constituents are bound through the strong interaction. In quantum chromodynamics (QCD), hadrons are color-neutral states of quarks, antiquarks, and gluons. Familiar examples include the proton and neutron, together with many unstable particles produced in high-energy collisions. Hadrons are not elementary particles: they have internal structure and can occur in excited states. Their properties connect the fundamental interactions of quarks and gluons with the behavior of observable matter. (pdg.lbl.gov)
Structure and classification
The conventional quark model divides hadrons into two main families. A baryon has three valence quarks; an antibaryon has three valence antiquarks. The proton has the valence composition (uud), while the neutron has (udd), where (u) and (d) denote up and down quarks. Baryons have half-integer spin and are fermions. A meson conventionally contains a quark–antiquark pair. Mesons have integer spin and are bosons. Pions, kaons, and particles containing charm or bottom quarks belong to this family. (pdg.lbl.gov)
These constituent counts describe valence structure, not everything inside a hadron. A proton also contains gluons and a continually changing population of quark–antiquark pairs, commonly called the sea. Its internal structure is therefore not adequately represented by three small objects orbiting one another. Different experiments probe different aspects of this many-particle system, including how its constituents share momentum and contribute to spin. (cms.cern)
Within the Standard Model, quarks occur in six flavors: up, down, strange, charm, bottom, and top. Unlike the other flavors, the top quark normally decays before hadron formation can occur. Consequently, ordinary hadron spectroscopy concerns combinations of the other five flavors and their antiquarks. (home.web.cern.ch)
Color neutrality and confinement
Quarks and gluons carry color charge, the charge associated with QCD; this terminology does not refer to visible colors. An observable hadron is a color singlet, meaning that its overall state is neutral under the color symmetry. Color neutrality does not imply neutrality in electric charge: the proton is electrically charged, whereas the neutron is not. (pdg.lbl.gov)
Color confinement explains why isolated quarks and gluons are not observed as freely propagating particles. At the comparatively low energy scales characteristic of hadronic structure, QCD is strongly coupled, making calculations based on small corrections difficult. At large momentum transfers, the interaction becomes weaker, a property called asymptotic freedom. These two regimes require different theoretical tools; high-energy scattering can probe constituents even though the final observable particles are color-neutral. (pdg.lbl.gov)
Mass and measurable properties
A hadron’s mass is not simply the sum of its valence quarks’ masses. For the proton, the quark masses account for only a small fraction of the total. Most arises from quark and gluon dynamics, including motion and interactions. Through mass–energy equivalence, this internal energy contributes to the mass of the composite particle. This distinguishes the origin of most proton mass from the mechanism that supplies elementary quarks with mass. (energy.gov)
Experiments characterize hadrons through their masses, charges, spins, lifetimes, and decay channels. Measurements also investigate their spatial structure. A charge radius describes the distribution inferred from an electromagnetic response, but it need not coincide with a radius associated with the distribution of mass or gluonic interactions. Measurements using electrons and the production of particular mesons provide complementary information about these distributions. (pdg.lbl.gov)
Hadrons need not be stable. A free neutron undergoes beta decay into a proton, an electron, and an electron antineutrino, with a mean lifetime of approximately fifteen minutes. Other hadrons exhibit widely differing decay channels and lifetimes, which are catalogued alongside their other particle properties. (energy.gov)
Exotic hadrons
The three-quark and quark–antiquark descriptions do not exhaust the possible structures permitted by QCD. Experiments have identified exotic states whose properties require more complicated explanations. Tetraquark configurations contain two quarks and two antiquarks; pentaquark configurations contain four quarks and an antiquark. Their observation extends hadron spectroscopy beyond the simplest conventional configurations. (home.cern)
The internal arrangement of an exotic state can be difficult to establish. Proposed descriptions include compact multiquark structures and hadronic molecules, in which two color-neutral hadrons interact to form a larger state. QCD also permits gluonic bound states, called glueballs, and states involving excited gluonic degrees of freedom. States with identical overall quantum numbers can mix, so assigning a simple constituent label does not necessarily determine a unique physical structure. (cern-courier.web.cern.ch)
Production and investigation
In energetic collisions, quarks and gluons can produce showers of further constituents before forming observable hadrons. This transition is called hadronization. The resulting particles often form a jet, whose measured properties provide information about the initiating high-energy process. Hadronization is an essential connection between calculations involving quarks and gluons and the particles recorded by detectors. (home.web.cern.ch)
Collisions of heavy nuclei at the Large Hadron Collider can produce quark–gluon plasma, a medium in which quarks and gluons are not confined inside individual hadrons. As this medium cools, hadrons form again. Their distributions, together with the energy lost by jets traversing the plasma, reveal properties of the hot medium. (home.cern)
On the theoretical side, lattice QCD calculates hadronic properties by numerically treating QCD on a discretized spacetime lattice. It provides a route from the underlying theory to quantities such as hadron masses and interaction matrix elements, allowing direct comparison with experimental measurements rather than relying solely on constituent models. (pdg.lbl.gov)