$Λ_b \to Λ_c^*(2595,2625)\ell^-\barν$ form factors from lattice QCD

Abstract

We present the first lattice-QCD determination of the form factors describing the semileptonic decays $Λ_b \to Λ_c^*(2595)\ell^-\barν$ and $Λ_b \to Λ_c^*(2625)\ell^-\barν$, where the $Λ_c^*(2595)$ and $Λ_c^*(2625)$ are the lightest charm baryons with $J^P=\frac12^-$ and $J^P=\frac32^-$, respectively. These decay modes provide new opportunities to test lepton flavor universality and also play an important role in global analyses of the strong interactions in $b\to c$ semileptonic decays. We determine the full set of vector, axial vector, and tensor form factors for both decays, but only in a small kinematic region near the zero-recoil point. The lattice calculation uses three different ensembles of gauge-field configurations with $2+1$ flavors of domain-wall fermions, and we perform extrapolations of the form factors to the continuum limit and physical pion mass. We present Standard-Model predictions for the differential decay rates and angular observables. In the kinematic region considered, the differential decay rate for the $\frac12^-$ final state is found to be approximately 2.5 times larger than the rate for the $\frac32^-$ final state. We also test the compatibility of our form-factor results with zero-recoil sum rules.