Exploring the role of new physics in $b \to u \tau \bar \nu$ decays

TL;DR

This paper investigates how new physics could influence rare $b o u au ar u$ decays, revealing potential deviations in observables that current experiments like LHCb and Belle-II can test, especially considering scalar and vector leptoquarks.

Contribution

It provides a model-independent analysis of new physics effects in $b o u au ar u$ decays, highlighting significant deviations in key observables and discussing implications of specific leptoquark models.

Findings

Significant deviations in branching ratios and asymmetries due to new physics.

Current experiments can potentially observe these deviations.

Scalar and vector leptoquarks can impact decay modes.

Abstract

The recent measurements on $R_D$, $R_{D^*}$ and $R_{J/\psi}$ by three pioneering experiments, BaBar, Belle and LHCb, indicate that the notion of lepton flavour universality is violated in the weak charged-current processes, mediated through $b \to c \ell \bar \nu_\ell$ transitions. These intriguing results, which delineate a tension with their standard model predictions at the level of $(2-3)\sigma$ have triggered many new physics propositions in recent times, and are generally attributed to the possible implication of new physics in $ b \to c \tau \bar \nu$ transition. This, in turn, opens up another avenue, i.e., $ b \to u \tau \bar \nu$ processes, to look for new physics. Since these processes are doubly Cabibbo suppressed, the impact of new physics could be significant enough, leading to sizeable effects in some of the observables. In this work, we investigate in detail the role of new physics in $B \to (\pi,\rho,\omega)\tau \bar \nu$ and $B_s \to (K,K^*) \tau \bar \nu$ processes considering a model independent approach. In particular, we focus on the standard observables like branching fraction, lepton flavour non-universality (LNU) parameter, forward-backward asymmetry and polarization asymmetries. We find significant deviations in some of these observables, which can be explored by the currently running experiments LHCb and Belle-II. We also briefly comment on the impact of scalar leptoquark $R_2(3,2,7/6)$ and vector leptoquark $U_1(3,1,2/3)$ on these decay modes.