Form Factor Counting and HQET Matching for New Physics in $Λ_b \to Λ_c^*lν$

Abstract

We calculate the $Λ_b \to Λ_c^*(2595) l ν$ and $Λ_b \to Λ_c^*(2625) l ν$ form factors and decay rates for all possible $b \to c l \barν$ four-Fermi interactions in and beyond the Standard Model (SM), including nonzero charged lepton masses and terms up to order $\mathcal{O}(α_s, 1/m_{c,b})$ in the heavy quark effective theory (HQET). We point out a subtlety involving the overcompleteness of the representation of the spin-parity $1/2^+ \to 3/2^-$ 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 $Λ_b \to Λ_c^*$ form factors at $\mathcal{O}(α_s, 1/m_{c,b})$ to an existing quark model, providing preliminary predictions for the lepton universality ratios $R(Λ_c^*)$ beyond the SM. Finally, we examine the possible incompatibility of recent lattice QCD results with expectations from the heavy-quark expansion and available experimental data.