Challenges in Semileptonic Decays

The study of flavor physics probes the structure of the Standard Model (SM) through the interactions and transitions among different generations of quarks and leptons. Despite its success, the SM cannot explain key features of our Universe, such as the dominance of matter over antimatter or the nature of dark matter. Semileptonic decays of beauty quarks play a central role in testing the SM and searching for new physics. They provide access to the Cabibbo–Kobayashi–Maskawa (CKM) matrix elements V_cb and V_ub and enable stringent tests of lepton flavor universality (LFU). However, persistent discrepancies between inclusive and exclusive determinations of V_cb and V_ub, as well as tensions in LFU observables such as R(D/D*), highlight unresolved challenges in our understanding of quark mixing and heavy-quark dynamics. 

The Belle II and LHCb experiments are now entering an era of percent-level precision, placing new demands on theoretical inputs, including lattice QCD form factors, heavy-quark expansions, and the modeling of excited charm contributions. Progress requires close coordination between experiment and theory to reduce hadronic and systematic uncertainties and to develop consistent global interpretations. 

The 856. WE-Heraeus-Seminar “Challenges in Semileptonic Decays” will gather leading experts to assess the current status of V_cb and V_ub determinations, review LFU measurements, identify strategies to reduce uncertainties, and foster synergies between Belle II, LHCb, lattice QCD, and effective field theory communities toward a coherent roadmap for the next decade.