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, $R_{D^{(*)}}$, $R_{J/\psi}$ and the measurements on $D^*$ and $\tau$ longitudinal polarization fractions in $\bar B\to D^* \tau \bar \nu_\tau$ processes. We present a model-independent analysis of $ \bar B\to D^{(*)} \tau \bar \nu_\tau$, $ B_s\to D_s^{(*)} \tau \bar \nu_\tau$, $ B_c^+ \to (\eta_c, J/\psi) \tau^+ \nu_\tau$, $\Lambda_b \to \Lambda_c \tau \bar \nu_\tau$ and $\bar B \to D^{**} \tau \bar \nu_\tau$ (where $D^{**} = \{D^*_0, D_1^*, D_1, D_2^*\}$ 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 $R_{D^{(*)}}$, $R_{J/\psi}$ 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 $q^2$.