New Predictions for ${\mathrm{{\Lambda}}}_{b}{\rightarrow}{\mathrm{{\Lambda}}}_{c}$ Semileptonic Decays and Tests of Heavy Quark Symmetry

The heavy quark effective theory makes model independent predictions for semileptonic ${\mathrm{{\Lambda}}}_{b}{\rightarrow}{\mathrm{{\Lambda}}}_{c}$ decays in terms of a small set of parameters. No subleading Isgur-Wise function occurs at order ${\mathrm{{\Lambda}}}_{\mathrm{QCD}}/{m}_{c,b}$, and only two subsubleading functions enter at order ${\mathrm{{\Lambda}}}_{\mathrm{QCD}}^{2}/{m}_{c}^{2}$. These features allow us to fit the form factors and decay rates calculated up to order ${\mathrm{{\Lambda}}}_{\mathrm{QCD}}^{2}/{m}_{c}^{2}$ to LHCb data and lattice QCD calculations. We derive a significantly more precise standard model prediction for the ratio $\mathcal{B}({\mathrm{{\Lambda}}}_{b}{\rightarrow}{\mathrm{{\Lambda}}}_{c}{\tau}\overline{{\nu}})/\mathcal{B}({\mathrm{{\Lambda}}}_{b}{\rightarrow}{\mathrm{{\Lambda}}}_{c}{\mu}\overline{{\nu}})$ than prior results, and find the expansion in ${\mathrm{{\Lambda}}}_{\mathrm{QCD}}/{m}_{c}$ well behaved, addressing a long-standing question. Our results allow more precise and reliable calculations of ${\mathrm{{\Lambda}}}_{b}{\rightarrow}{\mathrm{{\Lambda}}}_{c}{\ell}\overline{{\nu}}$ rates, and are systematically improvable with better data on the ${\mu}$ (or $e$) modes.