https://doi.org/10.1140/epjc/s10052-020-08490-x
Review
Challenges in semileptonic 
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
Received:
11
June
2020
Accepted:
16
September
2020
Published online:
19
October
2020
Two of the elements of the Cabibbo–Kobayashi–Maskawa quark mixing matrix, 
and 
, 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 and the exclusive and inclusive determinations are in clear tension. Further, measurements in the 
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.
© The Author(s) 2020
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