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An antibaryon is the antiparticle of a baryon. In the valence quark model it consists of three antiquarks in a colour singlet. Charge conjugation reverses its additive charges and third component of isospin, preserving total isospin and spin.

The antiproton is the antiparticle of the proton: it has the same rest mass and spin $1/2$, with opposite electric charge and baryon number. A negative electric charge alone cannot identify an antiproton, because a negative pion also bends in that direction in a magnetic field. Combining magnetic rigidity with velocity determines a rest mass; agreement with the proton rest mass then makes the identification much more discriminating.

The mode expansion of a Dirac field explains how independent particle and antiparticle excitations enter a relativistic field description. It supplies a theoretical language for the antiproton, while the Chamberlain-Segrè antiproton identification supplies an experimental test. The proton is a composite baryon, so a free Dirac field is not a complete account of its internal dynamics or of antiproton production in a target.

A baryon is a hadron with half-integer spin and baryon number one. Ordinary valence baryons contain three quarks, whereas an antibaryon has opposite baryon number and conjugate valence content. The proton and neutron are the most familiar examples.

Three valence quarks do not fix every property: their flavor, spin coupling and orbital angular momentum distinguish different states. For example, a proton and a Delta-plus baryon both have $uud$ valence content but different spin and excitation energy. Gluonic and additional quark–antiquark contributions also belong to the interacting state, so adding three free-quark masses is not a complete baryon-mass calculation.

The baryon asymmetry is an excess of baryons over antibaryons. Here $n_B-n_{\bar{B}}$ counts net baryon number per unit volume, including the baryons bound in atomic nuclei. It is dynamically generated by baryogenesis. Once the antibaryon abundance is negligible, its photon-normalized value agrees with the usual baryon-to-photon ratio.

The epoch at which momentum exchange with photons ceases to drag baryons efficiently. The sound horizon at this epoch calibrates the late-time baryon acoustic oscillation ruler. It is related to, but not identical with, photon last scattering.

This effective baryon mass model adds a colour-magnetic spin-spin interaction to a sum of constituent quark masses. The common coefficient $A^{\prime }$ absorbs a spatial contact density, colour factors, and the strong coupling. $A^{\prime }$ has dimensions of energy cubed in natural units. A shared coefficient is an approximation because different flavour configurations can have different spatial wavefunctions.

The ordinary matter component whose mass is predominantly in baryons, including stars and gas. Cosmological baryonic-matter budgets conventionally include the associated electrons. Its cosmic fraction among total matter is ${\Omega }_b/{\Omega }_m$, rather than ${\Omega }_b$ alone.

In the additive ground-state quark model, a baryon magnetic moment is the expectation of ${\sum }_i{Q}_{i}e{\mathbf{\sigma }}_i/(2m_i)$ in its spin-and-flavor wavefunction, with constituent quark masses $m_i$. For a spin-up proton and neutron this gives ${\mu }_p=(4{\mu }_u-{\mu }_d)/3$ and ${\mu }_n=(4{\mu }_d-{\mu }_u)/3$. Approximately equal up and down constituent masses imply ${\mu }_u\simeq -2{\mu }_d$ and ${\mu }_p/{\mu }_n\simeq -3/2$. This is evidence for the valence quark model, not proof that sea quark-antiquark pairs are absent.

The difference between baryon number and lepton number. Standard electroweak sphalerons violate $B+L$ while conserving $B-L$. Consequently an early baryon asymmetry with zero $B-L$ need not survive equilibration; a surviving nonzero $B-L$ can be redistributed between baryons and leptons. Additional lepton-number-violating interactions can change this conclusion.

A baryon containing a charm quark. For one charm and two up/down quarks in the spatial ground state, light-diquark spin-isospin correlation gives the isosinglet Lambda c baryon and the isotriplet Sigma c baryon families.

For $Q^{\alpha \beta \gamma }$, put $S=Q^{(\alpha \beta \gamma )}$, $d_1^{\alpha }={ϵ}_{\beta \gamma }{Q}^{\alpha \beta \gamma }$, $d_2^{\alpha }={ϵ}_{\beta \gamma }{Q}^{\beta \alpha \gamma }$ and $d_3^{\alpha }={ϵ}_{\beta \gamma }{Q}^{\beta \gamma \alpha }$. With ${ϵ}_{12}=1$ and inverse ${ϵ}^{12}=-1$, the two-dimensional alternating identity gives $d_1-d_2+d_3=0$. The inverse decomposition is $Q^{\alpha \beta \gamma }=S^{\alpha \beta \gamma }+{ϵ}^{\alpha \beta }(2d_1^{\gamma }-d_2^{\gamma })/3-{ϵ}^{\alpha \gamma }(d_1^{\beta }+d_2^{\beta })/3$. The symmetric part is the spin-$3/2$ SU(2) representation, and the two independent contractions are spin-$1/2$ representations. This is the spin coupling underlying ground-state three-quark baryons, before the Pauli constraint on three-quark flavour multiplets is imposed.

// Target: quantum-field-theory.bigb

For electrically neutral, fully ionized stellar matter with electrons and Positrons as its charged leptons, the electron fraction in stellar matter is the net electron number per baryon:Here $n_i$ is the number density of nuclear species $i$, $Z_i$ its atomic number, $A_i$ its mass number, and $n_b$ the total baryon number density. Charge neutrality equates the net negative lepton charge to the total proton charge. The subtraction of Positrons matters in hot matter containing abundant thermally produced Positrons; ordinary electron number alone need not equal nuclear charge.^[ref]

Neglecting small nuclear mass defect corrections, isotope mass fractions $X_i$ give $Y_e={\sum }_i{X}_i{Z}_i/A_i$. Pure fully ionized helium-4 therefore has $Y_e=1/2$. Strong interactions and electromagnetic reactions preserve this proton fraction, whereas electron capture and beta-plus decay reduce it and beta-minus decay can increase it. The total proton-to-neutron number ratio is $Y_e/(1-Y_e)$; electron-positron pair production changes neither of those baryon counts. If other charged leptons such as muons are appreciably populated, their charge must also be included in the neutrality equation.

In a hydrogen–helium gas with negligible doubly ionized helium, define the nuclear number densities $n_H=n_{H^0}+n_{H^{+}}$ and $n_{He}=n_{H{e}^0}+n_{H{e}^{+}}$. Neglecting nuclear binding energy in the abundance conversion, the baryon number density and helium mass fraction satisfyThe ionization fractions $X_{H^{+}}=n_{H^{+}}/n_H$ and $X_{H{e}^{+}}=n_{H{e}^{+}}/n_{He}$ count ionized nuclei within each species. Charge neutrality gives $n_e=n_H{X}_{H^{+}}+n_{He}{X}_{H{e}^{+}}$, henceThus complete single ionization gives $\mathcal{F}=1-3Y_p/4$, rather than one electron per baryon. This fraction uses a different denominator from $n_e/n_H$. At sufficiently high temperature to doubly ionize helium, the helium contribution must instead count two electrons per such nucleus.

In his 1964 paper, Murray Gell-Mann proposed fractional-charge quarks, constructing baryons and mesons from them. He also distinguished a mathematical constituent model from the hypothesis of finite-mass physical particles. The successful bookkeeping did not itself settle that distinction.^[ref]

George Zweig's 1964 CERN model used three constituents called aces, with fractional electric charge and baryon number $1/3$. Its abstract also proposed an experimental search.^[ref] The theoretical construction and the experiment that might discriminate between interpretations are separate parts of the argument. The Omega baryon discovery experiment illustrates the power of testing an incomplete particle pattern against new observations.

George Zweig proposed a constituent organization of hadrons in 1964, calling the constituents aces. Combining their quantum numbers accounts for patterns among mesons and baryons, connecting the larger observed particle families to a smaller set of building blocks. His model supplied a physical way to interpret flavor symmetry and the relative suppression of some reactions, rather than treating the multiplets only as numerical classifications. The constituent proposal was independent of the later experimental identification of quark substructure.^[ref]

A physicist whose research includes mathematical models of wave propagation and experimental tests of those models.^[ref]

A hadron is a strongly interacting composite particle described by quantum chromodynamics. Familiar hadrons contain quarks and gluons in an overall color-neutral state. The proton and neutron are baryons; the pion is a meson.

The quark model labels ordinary hadrons by their valence content, but this does not list every dynamical constituent. Gluons and additional quark–antiquark contributions affect masses and interactions. Distinguishing a composite hadron from an elementary particle is useful when comparing low-energy models with scattering experiments that resolve internal structure.

A baryon containing one or more strange quarks and no heavier quark. The light hyperon multiplets include lambda, sigma, xi and omega baryons. Strong and electromagnetic interactions conserve strangeness, whereas a weak interaction can change it; a photon in the final state does not by itself make a hyperon decay electromagnetic.

For an identical-flavour pair in the symmetric spatial ground state of a baryon, the flavour factor is symmetric and the three-quark colour singlet is antisymmetric under their exchange. The Pauli exclusion principle therefore requires a symmetric pair spin factor. Two spin-one-half quarks then form a triplet state, $S_{qq}=1$, rather than a spin singlet. The conclusion applies to the identical pair in both spin-one-half and spin-three-half ground-state baryons.

In the large-color limit of QCD with $g_s^2{N}_c$ fixed, baryon masses scale as $N_c$ while meson masses remain of order one. A Skyrme model action with coefficients of order $N_c$ has soliton mass and rotational moment of inertia of order $N_c$, and rotor splittings of order $1/N_c$. This motivates a semiclassical mesonic description of baryons; it does not uniquely determine every term in a finite-color effective theory.

A mechanism in which an initial lepton asymmetry is generated and then partly converted into a baryon asymmetry, commonly through electroweak anomalous processes. Heavy Majorana neutrino decays offer one possible realization; a seesaw mechanism by itself does not guarantee the required asymmetry.