uud

Uud is the shorthand notation for the valence quark content of the proton, denoting two up quarks (u) and one down quark (d). In the Standard Model, up quarks carry an electric charge of +2/3 and down quarks carry -1/3, so the combination u-u-d yields a total electric charge of +1, matching the proton’s observed charge. The proton is a baryon with baryon number +1, and its primary valence structure is uud, though it also contains sea quarks and gluons arising from quantum chromodynamics (QCD) interactions.

The proton’s mass, about 938 MeV/c^2, cannot be understood simply as the sum of the constituent quark masses. Most of the mass comes from the dynamics of the strong force, the energy of the gluon field, and contributions from sea quarks. The uud configuration determines key quantum numbers such as isospin (I = 1/2) and spin (the proton has spin 1/2). The internal structure is described by QCD, with valence quarks accompanied by a fluctuating sea of quark–antiquark pairs and gluons.

In experiments, uud is used to denote the valence-quark composition of the proton when discussing scattering processes, parton distribution functions, and lattice QCD calculations. While the three valence quarks define the proton’s basic properties, its full internal structure is dynamic, with gluons and sea quarks contributing to its observable behavior at different energy scales. The neutron, in contrast, has the udd valence composition.