New predictions for ${\mathrm{\Lambda }}_b\to {\mathrm{\Lambda }}_c$ semileptonic decays and tests of heavy quark symmetry

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