Impact of $b \to c$ measurements on $\Lambda_b \to p \tau \bar{\nu}$ decay in $U_1$ leptoquark model

Abstract

The measurements of several lepton flavor universality (LFU) violating observables in the decays induced by the quark level transition $b \to c \tau \bar{\nu}$ provide an inkling of plausible physics beyond the standard model of electroweak interactions. Such new physics would also impact other sectors. In this work, we estimate the leverage of new physics in $b \to c \tau \bar{\nu}$ on $\Lambda_b \to p \tau \bar{\nu}$ decay in the context of $U_1$ leptoquark model. In this model, the new physics couplings in $b \to u \tau \bar{\nu}$ transition can be written in terms of $b \to c \tau \bar{\nu}$ couplings and hence the extent of allowed new physics in $\Lambda_b \to p \tau \bar{\nu}$ would be determined by $b \to c \tau \bar{\nu}$ transition. Using the new physics parameter space obtained by performing a fit to all $b \to c \tau \bar{\nu}$ data, we obtain predictions of several $\Lambda_b \to p \tau \bar{\nu}$ observables. We find that the current $b \to c \tau \bar{\nu}$ data allows an order of magnitude enhancement in the branching ratio as well as in the LFU ratio. The other observables such as convexity parameter, lepton forward-backward asymmetry, longitudinal polarization of final state baryon and tau lepton are consistent with the SM value.