Abstract
We analyze in detail the angular distributions in {bar{B}}rightarrow D^*ell {{bar{nu }}} 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 sim 4sigma tension between data and predictions that is hidden in the redundant presentation of the Belle 2018 data on {bar{B}}rightarrow D^*ell {{bar{nu }}} decays. This tension specifically involves observables that probe e-mu 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 e-mu 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.
Highlights
Exclusive B → D(∗) νdecays have become precision probes of the semileptonic parton-level transitions b → c ν
This endeavour is important for the determination of Vcb in the Standard Model, a complete understanding of the weak effective theory (WET) beyond the SM (BSM), and to gain new insights into the persistent b → cτ νanomaly
We focus on the angular distribution in B → D∗ νwith light leptons = e, μ and highlight strategies for improved experimental analyses
Summary
Exclusive B → D(∗) νdecays have become precision probes of the semileptonic parton-level transitions b → c ν. The angular observables Ji( ) depend on the momentum transfer q2, or equivalently the hadronic recoil w Their calculation involves the lepton-flavour-universal hadronic form factors, as well as the short-distance coefficients of the lowenergy effective theory. The two recent Belle analyses for instance [12,13] provide binned CP-averaged measurements of the four single-differential distributions longitudinal polarization fraction FL( ) , the lepton forwardbackward asymmetry A(FB) , and two further angular observables FL( ) and S3( ) as independent observables in the distributions. CP-averaged single-angular distributions depend on only five out of the 12 angular observables defined in Eq (1) Out of these five observables, the CP-averaged S9( ) vanishes independently of the BSM scenario, as discussed above Eq (2), and is not relevant for our analysis.
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