Mind the (semileptonic) gap: Understanding flavor anomalies at Belle II

by Dr Philipp Horak (University of Victoria)

Wednesday 7 Oct 2026, 12:30 → 14:30 America/Los_Angeles

48/2-224 - Madrone (SLAC)

Several measurements in flavor physics currently deviate from Standard Model expectations. Measurements of the lepton universality ratios R(D(*)) and newer results such as the B -> K nu nu excess seen at Belle II show tensions on the 3-4 sigma level, while inclusive and exclusive determinations of |Vcb| and |Vub| have disagreed with each other for more than two decades. These tensions are often discussed as possible signs of new physics, but perhaps there is an equally plausible explanation. Both the R(D(*)) and the |Vcb|, |Vub| puzzles rely on our description of the hadronic system in semileptonic B decays, and an incomplete description might result in tensions. In this talk I will discuss how Belle II is addressing these questions. I will start by recapping the most precise |Vcb|, |Vub| determinations to date, as well as the most recent measurements of R(D(*)). I will then turn to the inputs that are currently limiting. The sum of measured exclusive b -> c l nu modes falls short of the inclusive rate, and this gap enters background models for R(D(*)) as well as inclusive |Vcb| extractions. I will present a first search for B-> D eta l nu as a candidate gap mode, and a model-independent approach that reconstructs the gap shape from measured inclusive moments through the Hausdorff moment problem. Finally, I will present a new hybrid model for B -> Xu l nu based on optimal transport, which aims to combine nonresonant modelling with hadronization in a novel and improved way. I will close with the open questions where input from theory would help most.