semileptonic

Semileptonic describes processes in which a hadron decays or scatters via the weak force into a final state that includes a lepton and its neutrino, together with hadronic remnants. In these transitions, a charged current mediated by a W boson induces a flavor-changing quark transition q -> q', while the leptonic part is governed by electroweak interactions. The hadronic transition is encoded in nonperturbative QCD effects, typically summarized by form factors or structure functions.

Semileptonic decays of mesons such as kaons (K -> pi l nu), D mesons (D -> K l nu, D -> pi l nu), and B mesons (B -> D l nu, B -> pi l nu) are widely studied. Neutron beta decay (neutron -> proton + e- + anti-nu_e) and other baryon semileptonic decays also provide important data. These processes are favored experimentally because the leptons offer clean signatures, while the hadronic system reflects strong interaction dynamics that can be constrained by theory.

The decay rate depends on hadronic form factors that parameterize the nonperturbative QCD transition, and on CKM matrix elements such as V_cb or V_ub. By combining experimental measurements with theoretical calculations of form factors, one can extract CKM elements and test the consistency of the Standard Model.

Semileptonic decays thus play a central role in flavor physics, offering relatively clean probes of weak interactions, tests of lepton universality, and constraints on new physics. They complement fully hadronic decays and purely leptonic processes in the broader effort to understand quark mixing and fundamental interactions.