Quark

A quark is a spin one-half fermion carrying color charge and participating in quantum chromodynamics. There are six quark flavors: up quark, down quark, strange quark, charm quark, bottom quark and top quark. Up-type flavors have electric charge $+2e/3$ and down-type flavors $-e/3$; every quark has baryon number $1/3$.^[ref]

Confinement prevents ordinary isolated quark final states. A proton has net valence content $uud$, and a neutron $udd$; their quark and gluon fields also contain additional fluctuations. A meson has minimal valence content consisting of one quark and one antiquark. Deep inelastic scattering tests quark structure through short-distance interactions, while hadron spectroscopy tests the allowed collective quantum states.

A quark or antiquark in the nonvalence component of a hadron's parton distribution functions. Such pairs do not change the net valence flavor numbers. Approximately equal light-flavor sea distributions can dominate small-x deep inelastic scattering, but exact equality is not required by their definition.

A constituent used to specify the net quark flavor content of a hadron. The quark model uses three valence quarks for an ordinary baryon and a quark-antiquark pair for an ordinary meson; this does not exclude additional sea pairs and gluons in the full state.

A quark mass is the coefficient of its Dirac mass term. This term couples left- and right-handed quarks, so nonzero quark masses explicitly break independent rotations of the two chiralities. For two equal masses, the vector flavor symmetry survives, while the nonsinglet axial current obeys the flavor-nonsinglet axial Ward identity with equal quark masses. In the renormalized theory the mass parameter depends on the subtraction scale and scheme, while measurable hadron masses are physical observables.

A constituent quark mass is an effective mass parameter in a low-energy constituent-quark model. It accounts approximately for strong-interaction dressing and some spin-independent bound-state dynamics; it is not identical to a renormalized quark mass in the Standard Model Lagrangian.

A quark field is a Dirac field or a chiral Weyl spinor carrying the colour fundamental representation of $SU(3)$. Its three colour components transform as ${\psi }^i\mapsto U^i{}_j{\psi }^j$ under local gauge transformations. The colour index is independent of the Lorentz spinor index, and the weak representations of its left and right chiral components differ in the Standard Model.

A quark flavor is one of the six Standard Model quark species: up, down, strange, charm, bottom and top. Each flavor occurs in three colour states and is counted as one Dirac species when both chiralities are included. The active flavor number in a low-energy QCD description excludes quarks heavier than the relevant scale, with matching of the running coupling when a heavy threshold is crossed.

The net number of charm quarks minus charm antiquarks. It is conserved by the strong interaction and electromagnetic interaction, but can change through weak charged currents.

The number of quarks of quark flavor $f$ minus the number of corresponding antiquarks. The strong interaction and electromagnetic interaction conserve each such difference even when they create or annihilate pairs. Weak charged currents can change these differences. For the strange quark, strangeness is $S=-F_s$.

A heavy quark has a mass large compared with the characteristic low-energy scale of quantum chromodynamics. Charm quarks, bottom quarks and top quarks are usually called heavy; treating charm as asymptotically heavy is a less accurate approximation than the corresponding treatment of bottom.

The separation of scales permits an expansion in inverse quark mass for many properties of a hadron containing one heavy quark. In the leading limit, the light degrees of freedom respond chiefly to the heavy quark's color field, with reduced sensitivity to its spin and precise flavor. This helps relate patterns in heavy-hadron spectroscopy and decays. The top quark is a special case: it decays before forming ordinary long-lived hadrons, so not every bound-state argument for charm and bottom applies to it.

The top quark is the up-type member of the third quark generation. Its mass exceeds the W boson mass, so a real W boson cannot decay into an on-shell top quark. A massless approximation for accessible decay products must not silently remove this channel threshold.

Here $x=m_W^2/m_t^2$, and the tree-level decay width neglects the bottom quark mass. The open two-body W channel gives a top quark lifetime of a few $10^{-25}\mathrm{s}$, shorter than the hadronization time. A fifth-power Sargent's rule extrapolation misses this physical regime.^[ref]

Here $x=m_W^2/m_t^2$, the bottom quark mass is neglected, and the W boson is on shell. The weak charged current gives the spin-averaged squared scattering amplitude $g_W^2|V_{tb}{|}^2(m_t^2-m_W^2)(m_t^2+2m_W^2)/(4m_W^2)$. The two-body Lorentz-invariant phase space has $p^{\ast }=(m_t^2-m_W^2)/(2m_t)$; multiplying by $p^{\ast }/(8\pi m_t^2)$ gives the displayed decay width. Its longitudinal-vector contribution explains the large-$m_t$ cubic growth, and its threshold factor makes the width vanish as $m_t\downarrow m_W$. The formula requires $m_t>m_W$ and receives finite-mass and radiative corrections.

A constituent quark that does not participate in the elementary weak vertex of a meson decay. It combines with a produced quark or antiquark in the final hadron.

The bottom quark $b$ is the third-generation down-type quark, with electric charge $-e/3$, spin $1/2$ and baryon number $1/3$. It is a heavy quark; its weak interaction decays make bottom-containing hadrons unstable. The charge assignment is listed alongside the other flavors in ^[ref].

A bottom quark combined with a light antiquark forms the minimal valence content of a bottom meson. Measuring the decay products and displaced decay point of such a hadron helps identify bottom production in a particle detector. The observed particles are hadrons and their descendants, rather than an isolated bottom quark propagating freely through the detector.

The charm quark $c$ is the second-generation up-type quark, with electric charge $+2e/3$, spin $1/2$ and baryon number $1/3$. It is heavier than the strange quark and is commonly treated as a heavy quark.^[ref]

A charm–charm antiquark pair supplies the minimal valence content of charmonium, while a charm quark bound with a light antiquark gives an open-charm meson. These systems probe different aspects of quantum chromodynamics: internal heavy-pair motion in the former and heavy–light dynamics in the latter. The approximate flavor classification that includes charm does not make charm and the light quarks equal in mass.

The charm antiquark $\bar{c}$ is the antiparticle of the charm quark. It has electric charge $-2e/3$, baryon number $-1/3$ and the opposite charm flavor number, while retaining spin $1/2$ and the same mass. Its color transformation is the conjugate of a quark's.^[ref]

A charm–anticharm pair has zero net charm and can form charmonium. Zero net charm does not imply absence of charm constituents: it states that quark and antiquark flavor numbers cancel. This distinction is useful when balancing conserved flavor numbers at a strong interaction vertex or classifying a meson's minimal valence content.

The strange quark $s$ is the second-generation down-type quark, with electric charge $-e/3$, spin $1/2$, baryon number $1/3$ and strangeness $-1$. Its strange antiquark has the opposite additive quantum numbers.^[ref]

The strong interaction can create a strange–antistrange pair without changing total strangeness. A later weak interaction can change net strangeness, allowing a strange hadron to decay into nonstrange products. This separation of production and decay mechanisms makes the strangeness label useful in reconstructing particle reactions. Its sign is a convention for the flavor quantum number, rather than the sign of an additional physical electric charge.

The antiparticle of the strange quark has electric charge $+1/3$ and baryon number $-1/3$.

The down quark $d$ is the first-generation down-type quark, with electric charge $-e/3$, spin $1/2$ and baryon number $1/3$. Together with the up quark, it supplies the net valence flavor content of ordinary protons and neutrons.^[ref]

The minimal neutron content $udd$ has total electric charge zero and baryon number one. In beta decay, a down-to-up weak charged current can turn a neutron into a proton, accompanied by the required leptons. This is a flavor change, rather than an electron previously stored inside the neutron escaping. Additional quark pairs and gluons are part of the full hadron state.

The antiparticle of the down quark has electric charge $+1/3$ and baryon number $-1/3$.

The up quark $u$ is the first-generation up-type quark, with electric charge $+2e/3$, spin $1/2$ and baryon number $1/3$. It is a light quark, and the up quark and down quark together account for the net valence flavor content of ordinary nuclei.^[ref]

The minimal proton content $uud$ has total charge $2(2e/3)-e/3=e$ and baryon number one. The proton's mass is not simply the sum of three free-quark masses: its interacting quark and gluon fields carry most of its energy. The quark model organizes the flavor content, while quantum chromodynamics describes the dynamics needed to compute hadron properties.

An antiquark is the antiparticle of a quark, with opposite electric charge, baryon number and additive flavor numbers, the same mass and spin, and the conjugate color representation. Thus an anti-up quark has charge $-2e/3$, whereas an anti-down quark has charge $+e/3$.

A quark–antiquark pair can have zero net baryon number and combine into a color singlet, supplying the minimal valence content of a meson. Flavor cancellation is separate from electric-charge cancellation: $u\bar{d}$ has baryon number zero but electric charge $+e$. Antiquarks also occur in the nonvalence partonic content of hadrons, so their presence does not imply that the entire hadron is an antimatter counterpart of another one.