Testing lepton flavour universality in semileptonic $Λ_b \to Λ_c^*$ decays

Abstract

Lepton Flavour Universality tests with semileptonic $Λ_b\toΛ_c^*$ decays are important to corroborate the present anomalies in the similar ratios $R_{D^{(*)}}$, and can provide complementary constraints on possible origins of these anomalies beyond the Standard Model. In this paper we provide - for the first time - all the necessary theoretical ingredients to perform and interpret measurements of $R_{Λ_c^*}$ at the LHCb experiment. For this, we revisit the heavy-quark expansion of the relevant hadronic matrix elements, and provide their expressions to order $α_s$ and $1/m$ accuracy. Moreover, we study the sensitivity to the form factor parameters given the projected size and purity of upcoming and future LHCb datasets of $Λ_b\to Λ_c^*μ\barν$ decays. We demonstrate explicitly the need to perform a simultaneous fit to both $Λ_c^*$ final states. Finally, we provide projections for the uncertainty of $R_{Λ_c^*}$ based on the form factor analysis from semimuonic decays and theoretical relations based on the heavy-quark expansion.

27 pages, 6 figures. v2: Fixed error in subleading IW function, added supplementary information; conclusions unchanged