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.

Valence quark

 0  0

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.

Quark mass ( $L_{m} = - \sum_{f} m_{f} \overset{q}{ˉ}_{f} q_{f}$ )

 0  0

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.

Constituent quark mass ( $m_{q}^{const}$ )

 0  0

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.

Quark field ( $ψ^{i} (x), i = 1, 2, 3$ )

 0  0

A quark field is a Dirac field or a chiral Weyl spinor carrying the colour fundamental representation of

S U (3)

. Its three colour components transform as

ψ^{i} \mapsto U^{i}_{j} ψ^{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.

Quark flavor ( $u, d, s, c, b, t$ )

 0  0

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.

Charm (quantum number, $C = N_{c} - N_{\overset{c}{ˉ}}$ )

 1  0

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.

Net quark flavor number ( $F_{f} = N_{f} - N_{\overset{ˉ}{f}}$ )

 0  0

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}

Heavy quark

 0  0

Top quark ( $t$ )

 1  0

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.

Top-quark decay before hadronization ( $Γ (t \to bW) = G_{F} m_{t}^{3} ∣ V_{t b} ∣^{2} (1 - x)^{2} (1 + 2 x) / (8 π 2)$ )

 0  0

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

1 0^{- 25} s

, shorter than the hadronization time. A fifth-power Sargent's rule extrapolation misses this physical regime.^[ref]

Two-body top-quark decay width ( $Γ (t \to b W^{+}) = \frac{g _{W}^{2} ∣ V _{t b} ∣ ^{2} m _{t}^{3}}{64 π m _{W}^{2}} (1 - x)^{2} (1 + 2 x)$ )

 0  0

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_{t b} ∣^{2} (m_{t}^{2} - m_{W}^{2}) (m_{t}^{2} + 2 m_{W}^{2}) / (4 m_{W}^{2})

. The two-body Lorentz-invariant phase space has

p^{*} = (m_{t}^{2} - m_{W}^{2}) / (2 m_{t})

; multiplying by

p^{*} / (8 π 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} ↓ m_{W}

. The formula requires

m_{t} > m_{W}

and receives finite-mass and radiative corrections.

Spectator quark

 0  0

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.