Probing the Standard Model with flavor physics: an exclusive determination of $|V_{cb}|$ from the $B\to D^\ast\ell\nu$ semileptonic decay at non-zero recoil

  • May 20, 2021, 2:30 pm US/Central
  • Alex Vaquero, Utah U.
  • Will Jay
  • slides
  • video

A very rich place to look for phenomena to challenge our current understanding of physics is the flavor sector of the Standard Model (SM). In particular, the $V_{cb}$ matrix element of the CKM matrix is the subject of a long standing tension, depending on whether it is determined using inclusive or exclusive methods. On top of that, the current average of $R(D^\ast)$ shows a 3$\sigma$ tension between experiment and the SM calculations. Theoretical support is urgently needed, and in this work I present the first unquenched lattice QCD calculation of the form factors for the decay $B\to D^\ast\ell\nu$ at non-zero recoil. After the form factors are extrapolated from lattice data to the physical point and the continuum limit in the small recoil range, these are extended to the whole kinematic range by using a model-independent parametrization. Comparison with current experimental results coming from Belle and BaBar follows, as well as a joint fit to extract the matrix element $|V_{cb}|$. The lattice prediction for the differential decay rate can be integrated to determine $R(D^\ast)$ using only lattice data.