Semileptonic decays provide an excellent environment for testing the Standard Model (SM). Violation of lepton universality would be a smoking gun for physics beyond the SM. Using semi-tauonic $B$ decays, LHCb finds a value of $\mathcal{ R}(D^*) = \mathcal{ B}(\bar{ B}^0 \to D^{ *+} \tau^{ -} \bar{ \nu}_{ \tau})/\mathcal{ B}(\bar{ B}^0 \to D^{ *+} \mu^{ -} \bar{ \nu}_{ \mu}) = 0.336 \pm 0.027 \rm{ (stat)} \pm 0.030 \rm{ (syst)}$, which is 2.1 standard deviations larger than the value expected from the SM. Moreover, the measurement of the $CP$ asymmetry in mixing of $B_s^0$ mesons is highly sensitive to physics beyond the SM. This article presents the latest result on semileptonic asymmetries; using the full Run 1 dataset, it is found that $a_{ \rm{ sl}}^s = (0.39 \pm 0.26 \rm{ (stat)} \pm 0.20 \rm{ (syst)})\%$, which is consistent with the Standard Model.
Categories: hep-ex