Probing Lepton Flavour Universality with ${\Lambda }_b$ decays to ${\tau }^{+}{\tau }^{-}$ final states

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.

Comments: 17 pages, 3 figures, 4 tables