Search for the rare decays $B_s^0 \rightarrow \mu^+ \mu^- \mu^+ \mu^-$ and $B_d^0 \rightarrow \mu^+ \mu^- \mu^+ \mu^-$

A search for the decays $B_s^0 \rightarrow \mu^+ \mu^- \mu^+ \mu^-$ and $B_d^0 \rightarrow \mu^+ \mu^- \mu^+ \mu^-$ in a data sample of 1.0 fb$^{-1}$ collected with the LHCb detector in 2011 is discussed. In the SM, the non-resonant $B_s^0 \rightarrow \mu^+ \mu^- \mu^+ \mu^-$ branching fraction is expected to be $<10^{-10}$. One signal candidate is observed in the $B_d$ channel, and no signal candidates are observed in the $B_s$ channel. These observed events are consistent with the expected backgrounds and upper limits on the branching fractions are set at $\mathcal{B}(B_s^0 \rightarrow \mu^+ \mu^- \mu^+ \mu^-) < 1.3 \times 10^{-8}$ and $\mathcal{B}(B_d^0 \rightarrow \mu^+ \mu^- \mu^+ \mu^-) < 5.4 \times 10^{-9}$ at 95 % CL.