Delving into new physics in semileptonic $b \to c \tau \bar \nu$ transitions

Abstract

Recently, several indications of lepton non-universality observables have been perceived in semileptonic $B$ meson decay processes, both in the neutral-current ($b \to s ll $) and charged-current ($b \to c l \bar \nu_l$) transitions. Influenced by these fascinating quotients, we examine the semileptonic decays involving the $b \to c l \bar \nu_l$ quark level transitions. We execute it through a model independent analysis in order to probe the nature of new physics. Taking into consideration the most general effective Hamiltonian, we scrutinize $\Lambda_b \to \Lambda_c \tau \bar \nu_\tau$, $ B_c^+ \to \eta_c \tau^+ \nu_\tau$, and $ 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, in the presence of new physics. We perform a global fit to different sets of new coefficients, making use of the measurements on $R_{D}$, $R_{D^{*}}$, $R_{J/\psi}$, $P_{\tau}^{D^*}$ and the upper limit on Br($B_c^+ \to \tau^+ \nu_\tau$). We then inspect the effect of constrained new couplings on the branching ratios, forward-backward asymmetry parameters, lepton non-universality ratios (LNU), lepton and hadron polarization asymmetries of these decay modes.