valon

Valon is a concept used in particle physics to describe a constituent component of hadrons within the valon model. In this framework, a valon is a dressed valence quark that carries the same flavor and electric charge as the valence quark it represents, but is surrounded by a cloud of gluons and sea quarks. A valon is not a free, observable particle; instead, it is an effective degree of freedom used to organize the internal structure of hadrons.

In the valon model, the structure of a hadron is described by two levels. First, there are distributions of valons inside the hadron, which encode the nonperturbative, long-distance physics. Second, within each valon, the partons (quarks and gluons) evolve perturbatively according to quantum chromodynamics (QCD) as the momentum transfer Q^2 increases. The hadron’s parton distribution functions can be modeled as a convolution of the valon distributions with the parton distributions inside a valon: f_h(x, Q^2) = sum_v ∫_x^1 dy G_h^v(y) f_v^h(x/y, Q^2), where G_h^v(y) describes the probability of finding a valon v carrying momentum fraction y in the hadron h, and f_v^h is the parton distribution inside that valon.

The valon concept has been used to describe deep inelastic scattering data and other processes by separating nonperturbative hadron structure from perturbative parton evolution. It is one of several phenomenological approaches to modeling hadron structure and is complemented by other frameworks such as the standard parton model and lattice QCD.