Looking for signs of new physics in the flavor sector: a determination of the Vcb CKM matrix element from first principles using lattice QCD.*

The CKM matrix relates energy and flavor eigenstates of the quarks in the Standard Model. Its elements are determined by combining experimental measurements of decay rates of heavy mesons with theoretical predictions for the form factors. They are of primary interest because a discrepancy in different determinations can be an indicative of new physics.

The matrix element we deal with in this talk is Vcb, which governs the weak transitions between the charm and the bottom quarks. Inclusive and exclusive semileptonic decays of the B meson to D and D* mesons yield inconsistent values of Vcb.  This tension might be understood with knowledge of the form factor as a function of the recoil parameter (w).  However, experiments can't disentangle the form factor from the differential decay rate, and lattice QCD calculations (a first principle method based on our current theory for the strong interaction) for the D* final state have so far provided results only at w=0. In our current work we are determining the form factor for the semileptonic B¯→D⋆ℓν¯ decay at non-zero w from first principles. The results are expected to have a long-lasting impact on the current |V_cb| values and R(D*).