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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.

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 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

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.

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.

Two light quarks in a symmetric spatial state and the antisymmetric colour pair inside a three-quark colour singlet require symmetric spin-flavour symmetry. Their isospin singlet and spin singlet are both antisymmetric, while their respective triplets are symmetric. Thus the allowed pairs have $(I,S_{ud})=(0,0)$ or $(1,1)$; opposite exchange parities are excluded. Coupling an isospin-zero spin-one-half quark gives an $I=0,J=1/2$ baryon or $I=1,J=1/2,3/2$ baryons.

Exotic flavour weights can decay into a baryon and a meson while preserving every strong quantum number: $uudd\bar{s}\to p{K}^0$ or $n{K}^{+}$, $ddss\bar{u}\to {\Xi }^{-}{\pi }^{-}$, and $uuss\bar{d}\to {\Xi }^0{\pi }^{+}$. These are quark rearrangements. If above threshold with unsuppressed couplings, such states are short-lived strong resonances. Flavour symmetry alone fixes neither masses nor widths.

A compact astronomical object proposed as a constituent of a galactic dark matter halo. It can be sought through gravitational microlensing even if its emitted light is undetectable. Stellar objects and remnants consume the cosmological baryon budget; primordial black holes do not obey that particular formation-budget argument. Exclusion bounds depend on the object's mass, the halo model and survey selection.

Scattering with one meson and one baryon in the initial and final states. The strong interaction conserves electric charge, baryon number, net strangeness, four-momentum, total angular momentum, and parity. Approximate isospin symmetry imposes additional relations among charge-channel amplitudes.

The neutron is the electrically neutral spin-one-half baryon found with protons in most atomic nuclei.

A neutron is a nucleon: it contributes to an atomic nucleus without adding electric charge. Its spin one-half makes it a fermion, and it belongs to the baryons. Electrical neutrality does not mean an absence of interactions: a neutron can transfer momentum and kinetic energy to an atomic nucleus, producing a charged nuclear recoil that a particle detector can register. This indirect route is central to neutron detection by proton recoil.

The beautiful feature of the Chadwick neutron discovery experiment is that an unseen projectile acquired a measurable mass through the motion of things it struck. Penetration alone left competing explanations open; combining nuclear recoils from different target masses supplied a much stronger test.

The December 20, 2012 preprint was available before the January 2013 examination. Its abstract reports combined cosmic microwave background, baryon acoustic oscillation and Hubble-constant model fits, including spatial curvature and cosmic age. These combined-data constraints should not be attributed to the cosmic microwave background alone.

A weak interaction decay of a baryon into final hadrons without final leptons. A weak charged current changes quark flavour, while spectator quarks and the produced quarks undergo hadronization. A quark-flow Feynman diagram specifies a possible weak topology; it does not calculate the full hadronic decay amplitude.