Form factor counting and HQET matching for new physics in Λ_b → Λ_c*lν

Abstract

We calculate the Λ_b → Λ∗_c(2595)lν and Λ_b → Λ∗_c(2625)lν form factors and decay rates for all possible b→clν̅ four-Fermi interactions in and beyond the Standard Model (SM), including nonzero charged lepton masses and terms up to order O(α_s,1/m_(c,b)) in the heavy quark effective theory (HQET). We point out a subtlety involving the overcompleteness of the representation of the spin-parity 1/2⁺ → 3/2⁻ antisymmetric tensor form factors, relevant also to other higher excited-state transitions, and present a general method for the counting of the physical form factors for any hadronic transition matrix element and their matching onto HQET. We perform a preliminary fit of a simple HQET-based parametrization of the Λ_b → Λ∗_c form factors at O(α_s,1/m_(c,b)) to an existing quark model, providing preliminary predictions for the lepton universality ratios R(Λ∗_c) beyond the SM. Finally, we examine the putative incompatibility of recent lattice QCD results with expectations from the heavy-quark expansion and available experimental data.