downquark

The down quark is one of the six flavors of quarks that make up matter in the Standard Model. It is a fundamental fermion with spin 1/2 and carries a color charge, so it participates in the strong interaction. Its electric charge is −1/3 of the elementary charge, and its current quark mass is typically quoted as a few MeV/c^2 (about 4 to 5 MeV/c^2, depending on the renormalization scale).

In hadrons, down quarks appear in combinations with other quarks. The lightest baryons include the neutron (udd) and the proton (uud). In mesons, down quarks appear paired with antiquarks, such as the negatively charged pion, π− (d anti-u). The down quark is part of the isospin doublet that includes the up quark, and up and down quarks have nearly equal masses, which underpins many features of low-energy hadron structure.

The weak interaction can change a down quark into an up quark by emitting a W− boson, a process that underlies beta decay. For example, neutron beta decay (n → p + e− + ν̄e) involves a down quark transforming into an up quark within a neutron. This flavor-changing mechanism is a central aspect of weak processes in nucleons and nuclei.

History and naming: the quark model was proposed in 1964 by Murray Gell-Mann and George Zweig. The terms up and down arise from the isospin symmetry that treats these two light quarks as a related pair. Today, the down quark is understood as a fundamental component of matter, participating in strong, weak, and electromagnetic interactions.