$\Lambda_b \to \Lambda_c \tau \bar{\nu}_{\tau}$ Decay in the Standard Model and with New Physics

TL;DR

This paper investigates the $Lambda_b o Lambda_c au ar{ u}_ au$ decay within the Standard Model and with potential new physics, providing detailed angular distributions and predictions for decay rates to test lepton flavor universality.

Contribution

It offers the first comprehensive analysis of the $Lambda_b o Lambda_c au ar{ u}_ au$ decay including general new physics operators and detailed angular distributions.

Findings

Standard Model predicts $R_{Lambda_b} o 0.3$.

New physics can significantly alter the decay rate.

Angular distributions can help distinguish new physics effects.

Abstract

Recently hints of lepton flavor non-universality emerged when the BaBar Collaboration observed deviations from the standard model predictions in $R(D^{(*)}) \equiv {\cal B}({\bar B} \to D^{(*)+} \tau^- {\bar\nu}_\tau) / {\cal B}({\bar B} \to D^{(*)+} \ell^- {\bar\nu}_\ell)$ ($\ell =e,\mu$). Another test of this non-universality can be in the semi leptonic $\Lambda_{b}\to\Lambda_{c}\tau\bar{\nu}_{\tau}$ decay. In this work we present predictions for this decay in the standard model and in the presence of new-physics operators with different Lorentz structures. We present the most general four-fold angular distribution for this decay including new physics. For phenomenology, we focus on predictions for the decay rate and the differential distribution in the momentum transfer squared $q^2$. In particular, we calculate $R_{\Lambda_{b}} = \frac{BR[\Lambda_b \to \Lambda_c \tau \bar{\nu}_{\tau}]}{BR[\Lambda_b \to \Lambda_c \ell \bar{\nu}_{\ell}]}$ where $\ell$ represents $\mu$ or $e$, and find the standard model prediction to be around $0.3$ while the new physics operators can increase or slightly decrease this value.