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

Abstract Lepton Flavour Universality tests with semileptonic Λ b → Λ c * decays are important to corroborate the present anomalies in the similar ratios RD∗ $$ {R_D}_{{}^{\left(\ast \right)}} $$, 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 RΛc* $$ {R}_{\Lambda_c^{*}} $$ at the LHCb experiment. For this, we revisit the heavy-quark expansion of the relevant hadronic matrix elements, and provide their expressions to order 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 Λb→Λc*μν¯ $$ {\Lambda}_b\to {\Lambda}_c^{*}\mu \overline{\nu} $$ 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 RΛc* $$ {R}_{\Lambda_c^{*}} $$ based on the form factors analysis from semimuonic decays and theoretical relations based on the heavy-quark expansion