Bc decays from highly improved staggered quarks and NRQCD

Abstract

We calculate semileptonic form factors for the decays Bc→ηc lν and Bcrarr;J/ψ lν over the entire q2 range, using a highly improved lattice quark action for charm at several lattice spacings down to a = 0:045 fm. We have two ways of treating the b quark: either with an O(αs) improved NRQCD formalism or by extrapolating a heavy mass mh to mb in the relativistic formalism. Comparison of the two approaches provides an important cross-check of methodologies in lattice QCD. Nonperturbative renormalisation of the currents in the relativistic theory also allows us then to fix NRQCD-charm normalisation for b to c decays such as B→D and B→D∗.