Global fit to b → cτν transitions

Abstract

We perform a general model-independent analysis of $b\to c\tau {\overline{\nu}}_{\tau }$ transitions, including measurements of ℛ_D, ℛ_D∗, their q² differential distributions, the recently measured longitudinal D* polarization ${F}_L^{D\ast }$, and constraints from the ${B}_c\to \tau {\overline{\nu}}_{\tau }$ lifetime, each of which has significant impact on the fit. A global fit to a general set of Wilson coefficients of an effective low-energy Hamiltonian is presented, the solutions of which are interpreted in terms of hypothetical new-physics mediators. From the obtained results we predict selected $b\to c\tau {\overline{\nu}}_{\tau }$ observables, such as the baryonic transition ${\Lambda}_b\to {\Lambda}_c\tau {\overline{\nu}}_{\tau }$, the ratio ℛ_J/ψ, the forward-backward asymmetries ${\mathcal{A}}_{\mathrm{FB}}^{D\left(\ast \right)}$, the τ polarization asymmetries ${\mathcal{P}}_{\tau}^{D\left(\ast \right)}$, and the longitudinal D* polarization fraction ${F}_L^{D\ast }$. The latter shows presently a slight tension with any new-physics model, such that an improved measurement could have an important impact. We also discuss the potential change due the very recently announced preliminary ${\mathrm{\mathcal{R}}}_{\mathcal{D}\left(\ast \right)}$ measurement by the Belle collaboration.