$B\to D^\ast\ell\nu$ at non-zero recoil

Abstract

The current status of the lattice-QCD calculations of the form factors of the $B\to D^\ast\ell\nu$ semileptonic decay is reviewed. Particular emphasis is given to the most mature calculation at non-zero recoil coming from the Fermilab Lattice and MILC collaborations. Blinded, preliminary results for the form factors are shown, including a preliminary, but detailed error budget. The lattice results seem to favor a large slope at small recoil, in contrast to the latest untagged results coming from the Belle collaboration. A comprehensive comparison between the latest BGL $z$ expansions of Belle, Babar, the lattice and a joint BGL fit including lattice and Belle data is presented, and a roadmap to improve the current calculation is discussed. The current implications for $V_{cb}$ and $R(D^\ast)$ are discussed.