Investigating the role of new physics in $b \to c \tau \bar \nu_\tau$ transitions

TL;DR

This paper conducts a comprehensive, model-independent analysis of $b o c au ar u$ transitions, exploring potential new physics effects and their impact on decay observables and lepton universality ratios.

Contribution

It performs a global fit to new physics coefficients considering multiple decay modes and measurements, providing insights into possible beyond Standard Model contributions.

Findings

Constraints on new physics couplings from experimental data

Predictions for branching ratios and lepton universality ratios under new physics scenarios

Implications for angular observables in various decay channels

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 $q^2$.