$Λ_b\toΛ_cτ\barν_τ$ decay in scalar and vector leptoquark scenarios

Abstract

It has been shown that the anomalies observed in $\bar B\to D^{(\ast)}τ\barν_τ$ and $\bar B\to \bar K\ell^+\ell^-$ decays can be resolved by adding a single scalar or vector leptoquark to the Standard Model, while constraints from other precision measurements in the flavour sector can be satisfied without fine-tuning. To further explore these two interesting scenarios, in this paper, we study their effects in the semi-leptonic $Λ_b\toΛ_cτ\barν_τ$ decay. Using the best-fit solutions for the operator coefficients allowed by the current data of mesonic decays, we find that (i) the two scenarios give similar amounts of enhancements to the branching fraction $\mathcal B(Λ_b\toΛ_cτ\barν_τ)$ and the ratio $R_{Λ_c}=\mathcal B(Λ_b\toΛ_c τ\barν_τ)/\mathcal B(Λ_b\toΛ_c\ell\barν_\ell)$, (ii) the two best-fit solutions in each of these two scenarios are also indistinguishable from each other, (iii) both scenarios give nearly the same predictions as those of the Standard Model for the longitudinal polarizations of $Λ_c$ and $τ$ as well as the lepton-side forward-backward asymmetry. With future measurements of these observables in $Λ_b\toΛ_cτ\barν_τ$ decay at the LHCb, the two leptoquark scenarios could be further tested, and even differentiated from the other NP explanations for the $R_{D^{(\ast)}}$ anomalies. We also discuss the feasibility for the measurements of these observables at the LHC and the future $e^+e^-$ colliders.