Investigating the role of new physics in $b\to c\tau {\bar{\nu }}_{\tau }$ transitions

Abstract

In recent times, the charged-current mediated semileptonic $b\to c\tau {\bar{\nu }}_{\tau }$ processes have attracted a lot of attention after the observation of lepton non-universality ratios, $R_{D^{(\ast )}}$, $R_{J/\psi }$ and the measurements on $D^{\ast }$ and $\tau$ longitudinal polarization fractions in $\bar{B}\to D^{\ast }\tau {\bar{\nu }}_{\tau }$ processes. We present a model-independent analysis of $\bar{B}\to D^{(\ast )}\tau {\bar{\nu }}_{\tau }$, $B_s\to D_s^{(\ast )}\tau {\bar{\nu }}_{\tau }$, $B_c^{+}\to ({\eta }_c,J/\psi ){\tau }^{+}{\nu }_{\tau }$, ${\Lambda }_b\to {\Lambda }_c\tau {\bar{\nu }}_{\tau }$ and $\bar{B}\to D^{\ast \ast }\tau {\bar{\nu }}_{\tau }$ (where $D^{\ast \ast }=\{D_0^{\ast },D_1^{\ast },D_1,D_2^{\ast }\}$ are the four lightest excited charm mesons) processes involving $b\to c\tau \bar{\nu }$ quark level transitions by considering the most general effective Lagrangian in the presence of new physics. We perform a global fit to various set of new coefficients, including the measurements on $R_{D^{(\ast )}}$, $R_{J/\psi }$ and the upper limit on Br($B_c^{+}\to {\tau }^{+}{\bar{\nu }}_{\tau }$). We then show the implications of constrained new couplings on the branching fractions, lepton non-universality ratios and various angular observables of these decay modes in four different bins of $q^2$.

Comments: 40 pages, 24 figures