# Phenomenology of $b\to c\tau\bar\nu$ decays in a scalar leptoquark model

## TL;DR

This paper investigates how scalar leptoquark models can explain anomalies in B meson decays involving tau leptons, predicting significant effects on decay distributions and ratios that can be tested in future experiments.

## Contribution

It introduces a scalar leptoquark model with two types of leptoquarks to explain B decay anomalies and provides detailed predictions for various decay observables under experimental constraints.

## Key findings

- LQ effects significantly enhance differential branching fractions.
- LFU ratios are notably increased by LQ contributions.
- Predictions suggest observable signatures at future colliders.

## Abstract

During the past few years, hints of Lepton Flavour Universality (LFU) violation have been observed in $b \to c \tau \bar\nu$ and $b \to s \ell^+ \ell^-$ transitions. Recently, the $D^*$ and $\tau$ polarization fractions $P_L^{D^*}$ and $P_L^\tau$ in $B \to D^* \tau \bar\nu$ decay have also been measured by the Belle collaboration. Motivated by these intriguing results, we revisit the $R_{D^{(*)}}$ and $R_{K^{(*)}}$ anomalies in a scalar leptoquark (LQ) model, in which two scalar LQs, one being $SU(2)_L$ singlet and the other $SU(2)_L$ triplet, are introduced simultaneously. We consider five $b \to c \tau \bar\nu$ mediated decays, $B \to D^{(*)}\tau \bar\nu$, $B_c \to \eta_c \tau \bar\nu$, $B_c \to J/\psi \tau \bar\nu$, and $\Lambda_b \to \Lambda_c \tau \bar\nu$, and focus on the LQ effects on the $q^2$ distributions of the branching fractions, the LFU ratios, and the various angular observables in these decays. Under the combined constraints by the available data on $R_{D^{(*)}}$, $R_{J/\psi}$, $P_L^\tau(D^*)$, and $P_L^{D^*}$, we perform scans for the LQ couplings and make predictions for a number of observables. It is found numerically that both the differential branching fractions and the LFU ratios are largely enhanced by the LQ effects, with the latter being expected to provide testable signatures at the SuperKEKB and High-Luminosity LHC experiments.