New Physics in $b\to c\tau \nu$: Impact of Polarisation Observables and $B_c\to \tau \nu$

Abstract

The experimental values of the lepton-flavour-universality tests $\mathcal{R}(D)$ and $\mathcal{R}(D^{\ast })$ show a tension of about $3.1\sigma$ with their Standard Model prediction. Motivated by this tension, we perform a fit of the $b\to c\tau \nu$ data. We consider one-particle scenarios imposing consecutive limits on $\text{BR}(B_c\to \tau {\nu }_{\tau })$, and analyse how these limits affect the fits. We include the polarisation observables available to date and predict those that are still to be measured, and conclude that they have a high model-resolving power. For each scenario we also predict $\mathcal{R}({\Lambda }_c)$, observing that an enhancement of $\mathcal{R}(D^{(\ast )})$ implies an enhancement of $\mathcal{R}({\Lambda }_c)$ in any scenario. We trace back this enhancement to a sum-rule valid irrespective of the scenario used to fit $\mathcal{R}(D^{(\ast )})$.

Comments: 8 pages, 4 figures, Proceedings for FPCP 2019, 6-10 May 2019, Victoria BC Canada. v2: references added