Λ_b→pℓ-ν_̄ℓ form factors from lattice QCD with static b quarks

We present a lattice QCD calculation of form factors for the decay Λ_b→pμ^-ν_̄μ, which is a promising channel for determining the Cabibbo-Kobayashi-Maskawa matrix element |V_ub| at the Large Hadron Collider. In this initial study we work in the limit of static b quarks, where the number of independent form factors reduces to two. We use dynamical domain-wall fermions for the light quarks, and perform the calculation at two different lattice spacings and at multiple values of the light-quark masses in a single large volume. Using our form factor results, we calculate the Λ_b→pμ ^-ν_̄μ differential decay rate in the range 14 GeV2≤q2≤qmaxâ¡2, and obtain the integral ∫14 GeV2qmaxâ¡2[dΓ/dq2]dq2/|V_ub|2=15.3±4.2 ps^-1. Combined with future experimental data, this will give a novel determination of |V_ub| with about 15% theoretical uncertainty. The uncertainty is dominated by the use of the static approximation for the b quark, and can be reduced further by performing the lattice calculation with a more sophisticated heavy-quark action.