Challenges in Semileptonic B Decays

TL;DR

This paper reviews the challenges and discrepancies in measuring CKM matrix elements from semileptonic B decays, highlighting tensions between different methods and potential signs of new physics, and discusses strategies to resolve these issues with upcoming experiments.

Contribution

It summarizes discussions from a 2018 workshop on improving theoretical and experimental approaches to address tensions in semileptonic B decay measurements.

Findings

Discrepancies between exclusive and inclusive determinations of |V_ub| and |V_cb|.

Potential violation of lepton flavor universality in tau channels.

Upcoming experiments at LHCb and Belle II may resolve current puzzles.

Abstract

Two of the elements of the Cabibbo-Kobayashi-Maskawa quark mixing matrix, $|V_{ub}|$ and $|V_{cb}|$, are extracted from semileptonic B decays. The results of the B factories, analysed in the light of the most recent theoretical calculations, remain puzzling, because for both $|V_{ub}|$ and $|V_{cb}|$ the exclusive and inclusive determinations are in clear tension. Further, measurements in the $\tau$ channels at Belle, Babar, and LHCb show discrepancies with the Standard Model predictions, pointing to a possible violation of lepton flavor universality. LHCb and Belle II have the potential to resolve these issues in the next few years. This article summarizes the discussions and results obtained at the MITP workshop held on April 9--13, 2018, in Mainz, Germany, with the goal to develop a medium-term strategy of analyses and calculations aimed at solving the puzzles. Lattice and continuum theorists working together with experimentalists have discussed how to reshape the semileptonic analyses in view of the much higher luminosity expected at Belle II, searching for ways to systematically validate the theoretical predictions in both exclusive and inclusive B decays, and to exploit the rich possibilities at LHCb.