Eur. Phys. J. C (2020) 80: 966
https://doi.org/10.1140/epjc/s10052-020-08490-x

Review

Challenges in semileptonic $\mathit{B}$ decays

¹ Physics Department, University of Turin and INFN Sezione di Torino, 10125, Turin, Italy
² Theoretical Physics Department, Fermilab, 60510-5011, Batavia, IL, USA
³ Laboratori Nazionali di Frascati, INFN, 00044, Frascati, RM, Italy
⁴ Institut für Hochenergiephysik, Österreichische Akademie der Wissenschaften, 1050, Wien, Austria
⁵ Physikalisches Institut der Rheinischen Friedrich-Wilhelms-Universität Bonn, 53115, Bonn, Germany
⁶ Aix Marseille Univ, Université de Toulon, CNRS, CPT, Marseille, France
⁷ INFN, Sezione di Ferrara, Ferrara, Italy
⁸ University of Milano Bicocca and INFN, Milano Bicocca, Milan, Italy
⁹ European Organization for Nuclear Research (CERN), Geneva, Switzerland
¹⁰ SUPA, School of Physics and Astronomy, University of Glasgow, G12 8QQ, Glasgow, UK
¹¹ Department of Physics, University of Illinois, 61801, Urbana, IL, USA
¹² Theory Center, High Energy Accelerator Research Organization (KEK), School of High Energy Accelerator Science (SOKENDAI), 305-0801, Tsukuba, Japan
¹³ Theoretische Physik 1, Naturwissenschaftlich-Technische Fakultät, Universität Siegen, 57068, Siegen, Germany
¹⁴ Lawrence Berkeley National Laboratory, University of California, 94720, Berkeley, CA, USA
¹⁵ Physics Department, Indiana University, 47405, Bloomington, IN, USA
¹⁶ SLAC National Accelerator Laboratory, 94309, Stanford, CA, USA
¹⁷ Department of Physics, University of Arizona, 85721, Tucson, AZ, USA
¹⁸ Department of Physics and Astronomy, Wayne State University, 48201, Detroit, MI, USA
¹⁹ Department of Physics, LEPP, Cornell University, 14853, Ithaca, NY, USA
²⁰ INFN, Sezione di Roma Tre, Rome, Italy
²¹ Department of Physics and Astronomy, University of Utah, 84112, Salt Lake City, UT, USA

^a gambino@to.infn.it

Received: 11 June 2020
Accepted: 16 September 2020
Published online: 19 October 2020

Abstract

Two of the elements of the Cabibbo–Kobayashi–Maskawa quark mixing matrix, $|V_{\mathit{ub}}|$ and $|V_{\mathit{cb}}|$, 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 $|V_{\mathit{ub}}|$ and $|V_{\mathit{cb}}|$ 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.