Testing lepton flavour universality in semileptonic Λ(b) → Λ(c)* decays

Lepton Flavour Universality tests with semileptonic Lambda(b) -> Lambda(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-Lambda 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 alpha(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 Lambda(b) -> Lambda(c)*mu(v) over bar decays. We demonstrate explicitly the need to perform a simultaneous fit to both Lambda(c)* final states. Finally, we provide projections for the uncertainty of R-Lambda c* based on the form factors analysis from semimuonic decays and theoretical relations based on the heavy-quark expansion.