HadronEdit

Hadron is the term for any subatomic particle that is made up of quarks bound together by the strong interaction. Hadrons are the vast majority of visible matter in the universe, from the protons and neutrons that form atomic nuclei to the many resonances and bound states observed in high-energy experiments. They are not elementary particles; their properties arise from the dynamics of quarks and gluons held together by the force described by Quantum chromodynamics.

Hadrons come in a few broad families. The most familiar are baryons, which consist of three quarks (for example, the proton and the neutron). The other large family is mesons, which are composed of a quark and an antiquark pair. Beyond these conventional categories, the experimental program has uncovered more exotic configurations, such as tetraquarks (four-quark states), pentaquarks (five-quark states), and hybrid states where gluonic excitations play a role. These discoveries illuminate the richness of the strong force and the ways quarks can bind together under it. See, for example, discussions of Baryons, Mesons, Exotic hadrons, and Quark-level dynamics.

The theory that governs hadrons is Quantum chromodynamics (QCD), a part of the Standard Model that describes how quarks carry a type of charge called color and interact by exchanging gluons. The key features of this theory are color confinement—the idea that individual quarks and gluons cannot be isolated—and asymptotic freedom, which makes interactions weak at very short distances. These properties explain why hadrons appear as bound states rather than free quarks in detectors. For readers seeking the formal framework, see Quantum chromodynamics and Color charge; for the carriers of the strong force, see Gluon.

By their nature, hadrons are complex composites. Their masses, lifetimes, spin, and internal structure emerge from the interplay of quarks and gluons within the constraints of QCD. The lightest mesons, such as the Pion, play a special role as mediators of the residual strong force that binds nuclei, while heavier hadrons containing charm or bottom quarks—such as charmonium or bottomonium states—probe the behavior of the strong interaction at shorter distances and higher masses. The proton and neutron, collectively known as the nucleons, are the building blocks of atomic nuclei and thus of most of the visible matter in the universe. See Proton, Neutron, Pion, Kaon; for heavier quark bound states see Charmonium and Bottomonium.

Overview of the theoretical framework - Quark model and flavor classification: The arrangement of hadrons follows patterns that were codified by the quark model and flavor symmetries. The early work led to the Eightfold Way and SU(3) flavor symmetry, which organized observed hadrons into multiplets before QCD was fully established. For the historical foundations, see Murray Gell-Mann and Quark model; for symmetry concepts see Eightfold Way and SU(3) flavor symmetry. - The binding mechanism: Quarks interact via the strong force by exchanging gluons, with color charge as the interacting property. The phenomenon of confinement ensures that quarks and gluons are observed only inside color-neutral hadrons. See Color confinement and Gluon. - Computational approaches: Because the strong interaction becomes very strong at typical hadronic scales, nonperturbative methods such as Lattice QCD are essential for predicting hadron spectra and structure from first principles. See Lattice QCD. - Spectroscopy and resonances: The spectrum of hadrons is rich, with ground-state baryons and mesons and numerous excited states. Modern experiments map these states and test QCD-based predictions, including the existence of exotic configurations such as Tetraquarks and Pentaquarks.

Historical context and major milestones - Discovery and interpretation of quarks: The idea that hadrons are built from more fundamental constituents—quarks—was proposed in the 1960s and quickly found support from deep inelastic scattering results at facilities like SLAC and from the systematic patterns of hadron multiplets. See Deep inelastic scattering and Quark model. - Establishing QCD as the theory of the strong interaction: The development of QCD as the correct description of quark-gluon dynamics unified the understanding of hadron structure and the behavior of the strong force across energy scales. See Quantum chromodynamics. - Experimental confirmation of hadronic spectroscopy and exotics: Over the past few decades, experiments at high-energy accelerators identified a wide range of hadrons, including conventional baryons and mesons as well as more unusual configurations such as certain Exotic hadrons. See experiments at LHC and analyses from LHCb and other facilities.

Key concepts and properties - Structure and binding: Hadrons are bound states whose properties depend on the arrangement of quarks and the dynamics of the strong interaction. Baryons are three-quark states; mesons are quark-antiquark pairs; exotic hadrons extend beyond these simple configurations. - Masses and lifetimes: Hadron masses span a broad range—from the light pions to heavy bound states with charm or bottom quarks—and lifetimes range from stable nucleons to very short-lived resonances that decay via the strong or weak interaction. - Form factors and internal structure: Probing hadrons with high-energy processes reveals distributions of charge, magnetization, and partons (the quarks and gluons inside hadrons). See Form factors and Parton model. - Probing through experiments: Hadron properties are studied in scattering experiments, production in particle colliders, and nuclear processes. The results feed into and test the predictions of Quantum chromodynamics and related frameworks.

Controversies and debates within the field - Existence and interpretation of exotic hadrons: For a long period, multiquark configurations were controversial, with initial claims sometimes met by skepticism until multiple experiments confirmed several candidate states. The current program seeks to map the spectrum of tetraquarks and pentaquarks, understand their internal structures (compact multiquark states vs meson–meson or meson–baryon molecules), and test how QCD binds these configurations. See Tetraquark and Pentaquark. - Proton radius and spin puzzles: Precise measurements of the proton’s charge radius and the decomposition of the proton’s spin have challenged simple pictures of nucleon structure. These puzzles drive refinements in both experimental methods and theoretical modeling, including lattice calculations and effective field theories. See Proton radius puzzle and Proton spin decomposition. - Nonperturbative methods and their limits: While lattice QCD provides a powerful first-principles tool, solving the full theory at low energies remains computationally intensive, leading to ongoing development of complementary approaches like effective field theories and phenomenological models. See Lattice QCD and Effective field theory (particle physics).

Impact and relevance - Nuclear physics and the structure of matter: The properties of hadrons underpin the forces that hold atomic nuclei together and determine the behavior of matter under a wide range of conditions, including those found in neutron stars and high-energy environments. See Neutron star. - Particle physics and the Standard Model: Hadrons are central to tests of the Standard Model, providing stringent checks on QCD and on the interplay between the strong force and the electroweak interactions. See Standard Model and Strong interaction. - Technology and discovery culture: The experimental techniques, data analysis methods, and large-scale collaborations developed to study hadrons drive advances in detectors, computing, and international scientific cooperation.

See also - Baryon - Meson - Quark - Quantum chromodynamics - Color charge - Gluon - Proton - Neutron - Pion - Kaon - Charmonium - Bottomonium - Lattice QCD - Exotic hadron - Tetraquark - Pentaquark - Deep inelastic scattering - Eightfold Way - SU(3) flavor symmetry - Hadronization