We calculate the Λb→Λc?ν form factors and decay rates for all possible b→c?ν four-Fermi interactions beyond the Standard Model, including nonzero charged lepton masses and terms up to order αsΛQCD/mc,b and ΛQCD2/mc2 in the heavy quark effective theory. At this order, we obtain model independent predictions for semileptonic Λb→Λc 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 Λb→Λc?ν decays, including predictions for the ratio R(Λc)=B(Λb→Λcτν)/B(Λb→Λcμν) 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 μ (or e) in the final state. We also explore tests of factorization in Λb→Λcπ decays, and we emphasize the importance of measuring at LHCb the double differential rate d2Γ(Λb→Λc?ν)/(dq2dcosθ), in addition to the q2 spectrum.

The heavy quark effective theory makes model independent predictions for semileptonic Λ_{b}→Λ_{c} decays in terms of a small set of parameters. No subleading Isgur-Wise function occurs at order Λ_{QCD}/m_{c,b}, and only two subsubleading functions enter at order Λ_{QCD}^{2}/m_{c}^{2}. These features allow us to fit the form factors and decay rates calculated up to order Λ_{QCD}^{2}/m_{c}^{2} to LHCb data and lattice QCD calculations. We derive a significantly more precise standard model prediction for the ratio B(Λ_{b}→Λ_{c}τν[over ¯])/B(Λ_{b}→Λ_{c}μν[over ¯]) than prior results, and find the expansion in Λ_{QCD}/m_{c} well behaved, addressing a long-standing question. Our results allow more precise and reliable calculations of Λ_{b}→Λ_{c}ℓν[over ¯] rates, and are systematically improvable with better data on the μ (or e) modes.