Abstract
The combined analysis of the BABAR, Belle, and LHCb data on B→Dτν, B→D∗τν and Bc→J/Ψτν decay observables shows evidence of physics beyond the Standard Model (SM). In this article, we study all the one- and two-dimensional scenarios which can be generated by adding a single new particle to the SM. We put special emphasis on the model-discriminating power of FL(D∗) and of the τ polarizations, and especially on the constraint from the branching fraction BR(Bc→τν). We critically review this constraint and do not support the aggressive limit of BR(Bc→τν)<10% used in some analyses. While the impact of FL(D∗) is currently still limited, the BR(Bc→τν) constraint has a significant impact: depending on whether one uses a limit of 60%, 30% or 10%, the pull for new physics (NP) in scalar operators changes drastically. More specifically, for a conservative 60% limit a scenario with scalar operators gives the best fit to data, while for an aggressive 10% limit this scenario is strongly disfavored and the best fit is obtained in a scenario in which only a left-handed vector operator is generated. We find a sum rule for the branching ratios of B→Dτν, B→D∗τν and Λb→Λcτν which holds for any NP contribution to the Wilson coefficients. This sum rule entails an enhancement of BR(Λb→Λcτν) over its SM prediction by (24±6)% for the current R(D(*)) data.
Blanke, Monika