The long standing discrepancy between determinations of the CKM matrix element Vub obtained from exclusive and inclusive semileptonic B decays are at the level of 2-3 standard deviations. This discrepancy continues to challenge our understanding of the semileptonic decays on both the theoretical and experimental sides. Exclusive semileptonic Bs decays are in principle under good theoretical control and provide complementary information with respect to the B-factories in this sector. The first observation of the decay Bs->K mu nu is reported. Using the measurement of the Bs->K mu nu branching fraction relative to the well known Bs->Ds mu nu decay, and the most recent theoretical knowledge of the relevant Bs decays form factors, the ratio of CKM matrix elements Vub/Vcb is extracted, with a precision competitive with the existing measurements. However, the regions of high and low momentum transfer, show inconsistent results. Further investigations are needed to solve the observed discrepancy.
|Collaboration / Activity||LHCb|