Tests of Lepton Universality at LHCb

In this report, measurements of the ratios of branching fractions $R_K = \mathcal{B}(B^+\to K^+\mu^+\mu^-)/\mathcal{B}(B^+\to K^+e^+e^-)$ and $R(D^*) = \mathcal{B}(\bar B^0\to D^{*+}\tau^-\bar \nu_{\tau})/\mathcal{B}(\bar B^0\to D^{*+}\mu^-\bar \nu_{\mu})$, as well as preliminary upper limits on the $B_{(s)}^0\to\tau^+\tau^-$ branching fractions are presented, using proton-proton collision data corresponding to an integrated luminosity of $3.0 \text{fb}^{-1}$ recorded by the LHCb experiment during 2011 and 2012. The value of $R_K$ is measured to be $0.745^{+0.090}_{-0.074}\:(\mathrm{stat}) \pm 0.036\:(\mathrm{syst})$, $R(D^*)$ equals $0.336 \pm 0.027\:(\mathrm{stat}) \pm 0.030\:(\mathrm{syst})$, and at $95\%$ confidence level $\mathcal{B}(B_s^0\to\tau^+\tau^-) < 3.0\times 10^{-3}$ and $\mathcal{B}(B^0\to\tau^+\tau^-) < 1.3\times 10^{-3}$. The results for $R_K$ and $R(D^*)$ are respectively $-2.6$ and $+2.1$ standard deviations away from the values expected from lepton universality of the Standard Model.