https://doi.org/10.1140/epjc/s10052-021-09724-2
Regular Article - Theoretical Physics
Lepton-flavour non-universality of 
angular distributions in and beyond the Standard Model
1
Technische Universität München, James-Franck-Straße 1, 85748, Garching, Germany
2
Dipartimento di Fisica, Università di Torino & INFN, Sezione di Torino, 10125, Turin, Italy
3
Technische Universität München, James-Franck-Straße 1, 85748, Garching, Germany
4
Theoretische Physik 1, Naturwissenschaftlich-Technische Fakultät, Universität Siegen, Walter-Flex-Straße 3, 57068, Siegen, Germany
Received:
15
April
2021
Accepted:
7
October
2021
Published online:
10
November
2021
We analyze in detail the angular distributions in decays, with a focus on lepton-flavour non-universality. We investigate the minimal number of angular observables that fully describes current and upcoming datasets, and explore their sensitivity to physics beyond the Standard Model (BSM) in the most general weak effective theory. We apply our findings to the current datasets, extract the non-redundant set of angular observables from the data, and compare to precise SM predictions that include lepton-flavour universality violating mass effects. Our analysis shows that the number of independent angular observables that can be inferred from current experimental data is limited to only four. These are insufficient to extract the full set of relevant BSM parameters. We uncover a 
tension between data and predictions that is hidden in the redundant presentation of the Belle 2018 data on decays. This tension specifically involves observables that probe 
lepton-flavour universality. However, we find inconsistencies in these data, which renders results based on it suspicious. Nevertheless, we discuss which generic BSM scenarios could explain the tension, in the case that the inconsistencies do not affect the data materially. Our findings highlight that non-universality in the SM, introduced by the finite muon mass, is already significant in a subset of angular observables with respect to the experimental precision.
Supplementary Information The online version contains supplementary material available at https://doi.org/10.1140/epjc/s10052-021-09724-2.
© The Author(s) 2021
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Funded by SCOAP3
