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

Abstract

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 flavor universality is violated in 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$ transitions. 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 sizable 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 standard observables like branching fraction, the lepton flavor 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 the scalar leptoquark $R_2(3,2,7/6)$ and vector leptoquark $U_1(3,1,2/3)$ on these decay modes.