New Physics effect on $B_c \to J/\psi \tau\bar\nu$ in relation to the $R_{D^{(*)}}$ anomaly

TL;DR

This paper explores how new physics could explain anomalies in B meson decays involving tau leptons, analyzing recent measurements and their implications for physics beyond the Standard Model.

Contribution

It identifies potential new physics solutions to B decay anomalies and assesses their consistency with recent experimental data and indirect constraints.

Findings

NP scenarios improve fit over SM despite no direct fit improvement with R_{J/psi}

Adding R_{J/psi} does not significantly refine NP solutions

Predictions for tau polarization provide additional constraints

Abstract

We study possible new physics (NP) effects on $B_c \to J/\psi \tau\bar\nu$, which has been recently measured at LHCb as the ratio of $R_{J/\psi} = \mathcal B(B_c \to J/\psi \tau\bar\nu)/\mathcal B(B_c \to J/\psi \mu\bar\nu)$. Combining it with the long-standing $R_{D^{(*)}}$ measurements, in which the discrepancy with the prediction of the standard model is present, we find possible solutions to the anomaly by several NP types. Then, we see that adding the $R_{J/\psi}$ measurement does not improve NP fit to data, but the NP scenarios still give better $\chi^2$ than the SM. We also investigate indirect NP constraints from the lifetime of $B_c$ and NP predictions on the $\tau$ longitudinal polarization in $\bar B \to D^* \tau\bar\nu$.