Baryon

In particle physics, a baryon is a composite hadron with baryon number $B=1$, conventionally described as a bound state of three quarks. Baryons are fermions, with half-integer spin. The proton and neutron are the most familiar examples and form the nuclei of ordinary atoms. The corresponding antiparticles, called antibaryons, have $B=-1$. Baryons are distinguished from mesons, which have baryon number zero and conventionally contain a quark–antiquark pair. More elaborate quark configurations can also constitute baryons. (pdg.lbl.gov)

Quark content and quantum numbers

The three-quark description specifies a conventional baryon’s valence content, rather than a complete inventory of everything inside it. A proton has valence content $uud$, comprising two up quarks and one down quark; a neutron has $udd$. Their internal structure also involves gluons and quark–antiquark pairs. Consequently, a baryon is not simply three isolated particles held in fixed positions. (energy.gov)

Baryon number is an additive quantum number. Each quark contributes $+1/3$, each antiquark contributes $-1/3$, and gluons contribute zero. For a specified quark configuration,

where $N_q$ and $N_{\bar q}$ denote the numbers of quarks and antiquarks. Additional quark–antiquark pairs therefore leave baryon number unchanged. An ordinary antibaryon has three valence antiquarks. Baryon number is distinct from electric charge: both a positively charged proton and an electrically neutral neutron have $B=1$. (pdg.lbl.gov)

Baryons have half-integer spin angular momentum, including values such as $1/2$ and $3/2$. Their classification also uses quark flavor, total angular momentum, and parity. These properties distinguish particles that share the same valence quark content but differ in their internal quantum state. (pdg.lbl.gov)

Strong interaction and mass

The dynamics of baryons are governed by quantum chromodynamics (QCD), the theory of the strong interaction. Quarks and gluons carry color charge, while an observable baryon forms an overall color-singlet state. Color is a quantum property, not a visible color. Color confinement prevents the constituent quarks from appearing as isolated particles under ordinary conditions. (pdg.lbl.gov)

Most of the proton’s mass cannot be explained by adding the small masses of its up and down quarks. Instead, it emerges from the dynamics of quarks and gluons, including their motion and strong interactions. The familiar three-quark picture is therefore useful for identifying quantum numbers but insufficient for explaining the full distribution of mass inside a baryon. (energy.gov)

Baryon structure is investigated through particle collisions and scattering measurements. These probe how quarks and gluons share momentum and contribute to properties such as spin. Theoretical approaches include phenomenological quark models and lattice QCD, which numerically studies QCD on a discrete spacetime lattice. Comparing calculated masses and other properties with measurements tests the description of strongly interacting matter. (energy.gov)

Families and excited states

The proton and neutron together form the nucleon family. Other conventional baryons include the Lambda, Sigma, Xi, and Omega families, as well as baryons containing charm or bottom quarks. Representative valence assignments include $uds$ for the neutral Lambda and $sss$ for the negatively charged Omega. Different arrangements of quark flavor and spin produce a spectrum of related particles. (pdg.lbl.gov)

The approximate flavor symmetry of the up, down, and strange quarks organizes many light baryons into multiplets. Particularly important are the spin-$1/2$ octet and spin-$3/2$ decuplet. This classification relates particles with different quark compositions and helped establish the quark model as an explanation of hadron properties. (pdg.lbl.gov)

Baryons also possess excited states, often observed as short-lived resonances through their decay products. A resonance may share a ground-state baryon’s valence content while differing in angular momentum or internal excitation. Thus, identifying a baryon requires more information than naming its constituent quark flavors. (pdg.lbl.gov)

Decay and conservation

A free neutron undergoes beta decay, producing a proton, an electron, and an electron antineutrino, with a mean lifetime of approximately fifteen minutes. This process is mediated by the weak interaction. It preserves baryon number because one baryon becomes another; the emitted leptons have baryon number zero. A neutron’s stability within a nucleus depends on the energetics of the nuclear system. (energy.gov)

Many heavier baryons decay into lighter baryons accompanied by mesons or other particles. The proton has no experimentally established decay mode. Searches for proton decay and neutron–antineutron conversion test whether baryon-number conservation can fail; experimental limits must be specified for particular processes rather than interpreted as proof of absolute conservation. (pdg.lbl.gov)

Exotic baryons

Baryons need not have only three valence constituents. A pentaquark has a minimal content of four quarks and one antiquark, giving $B=1$. On July 14, 2015, the LHCb collaboration at CERN reported pentaquark structures in the decay of a bottom baryon. Their internal organization can involve tightly correlated quarks or a molecular-like association of a baryon and a meson; distinguishing these descriptions requires further experimental and theoretical analysis. (home.cern)

Baryonic matter in cosmology

In cosmology, baryonic matter conventionally means ordinary matter, including associated electrons, although electrons themselves are not baryons. Analyses of the cosmic microwave background from the Planck mission indicate that ordinary matter accounts for roughly 5% of the universe’s total mass–energy density, distinct from dark matter and dark energy. The observed predominance of baryonic matter over antibaryonic matter is the baryon asymmetry problem; its physical origin remains unresolved. (jpl.nasa.gov)