Probing lepton flavour universality with $$\Lambda _b$$ decays to $$\tau ^+\tau ^-$$ final states

Abstract We present a study of the rare baryonic decay $$\Lambda _b \rightarrow \Lambda \tau ^+ \tau ^-$$ Λ b → Λ τ + τ - 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 $$R_{\Lambda }^{\tau /\mu },$$ R Λ τ / μ , 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 $$b \rightarrow c\tau \nu $$ b → c τ ν and $$b \rightarrow s\mu ^+\mu ^-$$ b → s μ + μ - transitions. In this context, $$R_{\Lambda }^{\tau /\mu }$$ R Λ τ / μ can be enhanced by several orders of magnitude, offering a clear target for upcoming searches. The implications for the related mode $$\Lambda _b \rightarrow pK\tau ^+\tau ^-$$ Λ b → p K τ + τ - are also briefly discussed.