Search for Lepton-Flavor Violating Decays $B^{+}\to K^{+}{\mu }^{\pm }{e}^{\mp }$

A search for the lepton-flavor violating decays $B^{+}\to K^{+}{\mu }^{\pm }{e}^{\mp }$ is performed using a sample of proton-proton collision data, collected with the LHCb experiment at center-of-mass energies of 7 and 8 TeV and corresponding to an integrated luminosity of $3{\mathrm{fb}}^{-1}$. No significant signal is observed, and upper limits on the branching fractions are set as $\mathcal{B}(B^{+}\to K^{+}{\mu }^{-}{e}^{+})<7.0(9.5)\times {10}^{-9}$ and $\mathcal{B}(B^{+}\to K^{+}{\mu }^{+}{e}^{-})<6.4(8.8)\times {10}^{-9}$ at 90% (95)% confidence level. The results improve the current best limits on these decays by more than one order of magnitude.

Figure 1

Efficiency of (left) $B^{+}\to K^{+}{\mu }^{-}{e}^{+}$ and (right) $B^{+}\to K^{+}{\mu }^{+}{e}^{-}$ as a function of the squared invariant masses $m_{K^{+}{e}^{\pm }}^2$ and $m_{K^{+}{\mu }^{\mp }}^2$. The variation of efficiency across the Dalitz plane is due to applied vetoes. The efficiencies are given in per mille.

Figure 2

Invariant-mass distributions of the (left) $B^{+}\to K^{+}{\mu }^{-}{e}^{+}$ and (right) $B^{+}\to K^{+}{\mu }^{+}{e}^{-}$ candidates obtained on the combined datasets recorded in 2011 and 2012 with background only fit functions (blue continuous line) and the signal model normalized to 10 candidates (red dashed line) superimposed. The signal window is indicated with gray dotted lines. Difference between the two distributions arises from the effect of the $m(K^{+}{\ell }^{-})$ requirement.

Figure 3

Upper limits on the branching fractions of (left) $B^{+}\to K^{+}{\mu }^{-}{e}^{+}$ and (right) $B^{+}\to K^{+}{\mu }^{+}{e}^{-}$ decays obtained on the combined datasets recorded in 2011 and 2012. The red solid line (black solid line with data points) corresponds to the distribution of the expected (observed) upper limits, and the light blue (dark blue) band contains the $1\sigma$ ($2\sigma$) uncertainties.