$\Lambda_b \to \Lambda_c^*(2595,2625)\ell^-\bar{\nu}$ form factors from lattice QCD

TL;DR

This paper presents the first lattice QCD calculations of form factors for semileptonic decays of Lambda_b to excited Lambda_c states, providing essential data for testing lepton flavor universality and analyzing strong interactions.

Contribution

It introduces the first lattice-QCD determination of form factors for Lambda_b decays to specific excited Lambda_c states, including continuum and physical pion mass extrapolations.

Findings

Differential decay rate for J^P=1/2^- is about 2.5 times larger than for J^P=3/2^-

Form factors are computed near zero recoil, enabling Standard-Model predictions

Results are consistent with zero-recoil sum rules

Abstract

We present the first lattice-QCD determination of the form factors describing the semileptonic decays $\Lambda_b \to \Lambda_c^*(2595)\ell^-\bar{\nu}$ and $\Lambda_b \to \Lambda_c^*(2625)\ell^-\bar{\nu}$, where the $\Lambda_c^*(2595)$ and $\Lambda_c^*(2625)$ 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.