Precise predictions for $Lambda_b o Lambda_c$ semileptonic decays.

We calculate the $Lambda_b Lambda_c ell nu$ form factors and decay rates for all possible $bto c ellbarnu$ four-Fermi interactions beyond the Standard Model, including nonzero charged lepton masses and terms up to order $alpha_s, Lambda_text{QCD}/m_{c,b}$ and $Lambda_text{QCD}^2/m_c^2$ in the heavy quark effective theory. At this order, we obtain model independent predictions for semileptonic $Lambda_b Lambda_c$ decays in terms of only two unknown sub-subleading Isgur-Wise functions, which can be determined from fitting LHCb and lattice QCD data. We thus obtain model independent results for $Lambda_bto Lambda_cellbarnu$ decays, including predictions for the ratio $R(Lambda_c) = {cal B}(Lambda_bto Lambda_c taubarnu) / {cal B}(Lambda_bto Lambda_c mubarnu)$ in the presence of new physics, that are more precise than prior results in the literature, and systematically improvable with better data on the decays with $mu$ (or $e$) in the final state. We also explore tests of factorization in $Lambda_b Lambda_cpi$ decays, and emphasize the importance of measuring at LHCb the double differential rate $d^2Gamma(Lambda_btoLambda_cellbarnu) / (d q^2, dcostheta)$, in addition to the $q^2$ spectrum.