Investigating the effect of $U_1$ vector leptoquark on $b\to u \tau\bar{\nu}$ mediated $B$ decays

Abstract

The recent measurements of lepton flavor university (LFU) violating observables in semileptonic $b \to c \ell \bar{\nu} $ and $b\to s\ell^+ \ell^-$ transitions by various experiments exhibit $(2-3)\sigma$ deviations from their corresponding Standard Model (SM) predictions. These tantalizing signals hint towards the possible role of new physics (NP) in $b \to c \tau \bar{\nu}$ and $b\to s\mu^+\mu^-$ decay channels. This in turn indicates that the same class of NP as appeared in $b \to c \tau \bar{\nu}$, might also show up in other tree level processes involving $b \to u \tau \bar{\nu}$ transition. Since these charged current transitions are doubly Cabibbo suppressed, the NP contributions could be significant enough leading to sizeable effects in some of the observables. In this paper, we study the implications of the vector leptoquark $U_1(3,1,2/3)$, which is one of the few scenarios that can simultaneously explain the LFU violation signals both in the charged-current as well as neutral-current sectors, on the semileptonic decays $B \to (\pi, \rho, \omega)\tau \bar{\nu}$ and $B_s \to (K, K^*) \tau \bar{\nu}$. In particular, we pay our attention to the branching fraction, lepton flavor non-universality (LNU) observable, forward-backward asymmetry and the polarization asymmetry parameters of these modes. We find substantial deviations in the branching fractions as well as LNU observables of these decay modes due to the $U_1$ contributions, which can be probed by the currently running experiments LHCb and Belle-II.