Valentiequarks

Valence quarks are the quarks that define the basic flavor content and quantum numbers of a hadron. In the simplest quark model, hadrons are described by a set of valence quarks that determine properties such as baryon number and electric charge: baryons are composed of three valence quarks (qqq) and mesons are made of a quark and an antiquark (q q̄). Examples include the proton (uud), the neutron (udd), and the positively charged pion (u d̄).

In addition to valence quarks, hadrons contain sea quarks — quark-antiquark pairs that momentarily pop in and out of existence due to gluon fluctuations. Sea quarks contribute to the internal structure and dynamics of hadrons, influencing properties like mass, spin, and the detailed momentum distribution of partons, but they do not define the hadron’s basic quantum numbers in the same way as valence quarks.

The valence-quark content largely determines the hadron’s flavor structure and its classification in the quark model. However, the sea and gluons participate in binding and dynamics, so a full description requires quantum chromodynamics (QCD). The concept extends to exotic hadrons as well: tetraquarks (qq q̄ q̄) and pentaquarks (qqqq q̄) are described by valence quarks that go beyond the simple qqq or q q̄ pictures, while still carrying the corresponding quantum numbers.

Overall, valence quarks provide a practical, widely used picture for understanding hadron properties, with sea quarks and gluons shaping the detailed structure beyond the basic quantum numbers.