Probing Lepton Flavour Universality with $\Lambda_b$ decays to $\tau^+\tau^-$ final states

TL;DR

This paper investigates the decay $ ext{Lambda}_b o ext{Lambda} au^+ au^-$ as a new physics probe, predicting the LFU ratio with high precision and exploring potential enhancements due to lepton non-universal effects.

Contribution

It provides the first precise Standard Model prediction of the LFU ratio for this decay and analyzes possible new physics effects motivated by current anomalies.

Findings

LFU ratio $R_{ ext{Lambda}}^{ au/ ext{mu}}$ can be predicted with less than 10% uncertainty.

New physics can enhance $R_{ ext{Lambda}}^{ au/ ext{mu}}$ by several orders of magnitude.

The study discusses implications for related decay modes such as $ ext{Lambda}_b o pK au^+ au^-$.

Abstract

We present a study of the rare baryonic decay $\Lambda_b \to \Lambda \tau^+ \tau^-$ as a probe of new physics (NP) coupled preferentially to third-generation fermions. Within the Standard Model, we evaluate the branching ratio and the lepton-flavour-universality (LFU) ratio $R_{\Lambda}^{\tau/\mu}$, including both perturbative and long-distance charm contributions. We show that the LFU ratio can be predicted with an uncertainty below 10%. Possible NP effects arising from lepton non-universal dynamics are analysed within an effective field theory framework motivated by the current anomalies in $b \to c\tau\nu$ and $b \to s\mu^+\mu^-$ transitions. In this context, $R_{\Lambda}^{\tau/\mu}$ can be enhanced by several orders of magnitude, offering a clear target for upcoming searches. The implications for the related mode $\Lambda_b \to pK\tau^+\tau^-$ are also briefly discussed.