We present the first lattice-QCD determination of the form factors describing the semileptonic decays ${\mathrm{\Lambda }}_b\to {\mathrm{\Lambda }}_c^{*}(2595){\ell }^{-}\overline{\nu }$ and ${\mathrm{\Lambda }}_b\to {\mathrm{\Lambda }}_c^{*}(2625){\ell }^{-}\overline{\nu }$, where the ${\mathrm{\Lambda }}_c^{*}(2595)$ and ${\mathrm{\Lambda }}_c^{*}(2625)$ are the lightest charm baryons with $J^P={\frac{1}{2}}^{-}$ and $J^P={\frac{3}{2}}^{-}$, 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 ${\frac{1}{2}}^{-}$ final state is found to be approximately 2.5 times larger than the rate for the ${\frac{3}{2}}^{-}$ final state. We also test the compatibility of our form-factor results with zero-recoil sum rules.

• Received 22 March 2021
• Accepted 9 April 2021

DOI:https://doi.org/10.1103/PhysRevD.103.094516

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Published by the American Physical Society