Λb →pℓ-ν ¯ ℓ and Λb → Λcℓ-ν ¯ ℓ form factors from lattice QCD with relativistic heavy quarks

Measurements of the Λb→pℓ-ν¯ℓ and Λb→Λcℓ-ν¯ℓ decay rates can be used to determine the magnitudes of the Cabibbo-Kobayashi-Maskawa matrix elements Vub and Vcb, provided that the relevant hadronic form factors are known. Here we present a precise calculation of these form factors using lattice QCD with 2+1 flavors of dynamical domain-wall fermions. The b and c quarks are implemented with relativistic heavy-quark actions, allowing us to work directly at the physical heavy-quark masses. The lattice computation is performed for six different pion masses and two different lattice spacings, using gauge-field configurations generated by the RBC and UKQCD Collaborations. The b→u and b→c currents are renormalized with a mostly nonperturbative method. We extrapolate the form factor results to the physical pion mass and the continuum limit, parametrizing the q2 dependence using z expansions. The form factors are presented in such a way as to enable the correlated propagation of both statistical and systematic uncertainties into derived quantities such as differential decay rates and asymmetries. Using these form factors, we present predictions for the Λb→pℓ-ν¯ℓ and Λb→Λcℓ-ν¯ℓ differential and integrated decay rates. Combined with experimental data, our results enable determinations of |Vub|, |Vcb|, and |Vub/Vcb| with theory uncertainties of 4.4%, 2.2%, and 4.9%, respectively.