New Predictions for Λb → Λc Semileptonic Decays and Tests of Heavy Quark Symmetry

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