Search for the Lepton-Flavor-Violating Decays $B_s^0\to e^{\mathbf{\pm }}{\mu }^{\mathbf{\mp }}$ and $B^0\to e^{\mathbf{\pm }}{\mu }^{\mathbf{\mp }}$

A search for the lepton-flavor-violating decays $B_s^0\to e^{\pm }{\mu }^{\mp }$ and $B^0\to e^{\pm }{\mu }^{\mp }$ is performed with a data sample, corresponding to an integrated luminosity of $1.0{\mathrm{fb}}^{-1}$ of $pp$ collisions at $\sqrt{s}=7\mathrm{TeV}$, collected by the LHCb experiment. The observed number of $B_s^0\to e^{\pm }{\mu }^{\mp }$ and $B^0\to e^{\pm }{\mu }^{\mp }$ candidates is consistent with background expectations. Upper limits on the branching fractions of both decays are determined to be $\mathcal{B}(B_s^0\to e^{\pm }{\mu }^{\mp })<1.1(1.4)\times {10}^{-8}$ and $\mathcal{B}(B^0\to e^{\pm }{\mu }^{\mp })<2.8(3.7)\times {10}^{-9}$ at 90% (95%) confidence level (C.L.). These limits are a factor of 20 lower than those set by previous experiments. Lower bounds on the Pati-Salam leptoquark masses are also calculated, $M_{\mathrm{LQ}}(B_s^0\to e^{\pm }{\mu }^{\mp })>101\mathrm{TeV}/c^2$ and $M_{\mathrm{LQ}}(B^0\to e^{\pm }{\mu }^{\mp })>126\mathrm{TeV}/c^2$ at 95% C.L., and are a factor of 2 higher than the previous bounds.

Figure 1

${\mathrm{CL}}_{\mathrm{s}}$ as a function of the assumed branching fraction for (left) $B_s^0\to e^{\pm }{\mu }^{\mp }$ and (right) $B^0\to e^{\pm }{\mu }^{\mp }$ decays. The dashed lines are the medians of the expected ${\mathrm{CL}}_{\mathrm{s}}$ distributions if background only was observed. The yellow (green) area covers, at a given branching fraction, 34% (47.5%) of the expected ${\mathrm{CL}}_{\mathrm{s}}$ distribution on each side of its median. The solid black curves are the observed ${\mathrm{CL}}_{\mathrm{s}}$. The upper limits at 90% (95%) C.L. are indicated by the dotted (solid) vertical lines in blue for the expectation and in red for the observation.