Search for Rare $B_{(s)}^0\mathbf{\to }{\mu }^{\mathbf{+}}{\mu }^{\mathbf{-}}{\mu }^{\mathbf{+}}{\mu }^{\mathbf{-}}$ Decays

A search for the decays $B_s^0\to {\mu }^{+}{\mu }^{-}{\mu }^{+}{\mu }^{-}$ and $B^0\to {\mu }^{+}{\mu }^{-}{\mu }^{+}{\mu }^{-}$ is performed using data, corresponding to an integrated luminosity of $1.0{\mathrm{fb}}^{-1}$, collected with the LHCb detector in 2011. The number of candidates observed is consistent with the expected background and, assuming phase-space models of the decays, limits on the branching fractions are set: $\mathcal{B}(B_s^0\to {\mu }^{+}{\mu }^{-}{\mu }^{+}{\mu }^{-})<1.6(1.2)\times {10}^{-8}$ and $\mathcal{B}(B^0\to {\mu }^{+}{\mu }^{-}{\mu }^{+}{\mu }^{-})<6.6(5.3)\times {10}^{-9}$ at 95% (90%) confidence level. In addition, limits are set in the context of a supersymmetric model which allows for the $B_{(s)}^0$ meson to decay into a scalar ($S$) and pseudoscalar particle ($P$), where $S$ and $P$ have masses of $2.5\mathrm{GeV}/c$ and $214.3\mathrm{MeV}/c$, respectively, both resonances decay into ${\mu }^{+}{\mu }^{-}$. The branching fraction limits for these decays are $\mathcal{B}(B_s^0\to SP)<1.6(1.2)\times {10}^{-8}$ and $\mathcal{B}(B^0\to SP)<6.3(5.1)\times {10}^{-9}$ at 95% (90%) confidence level.

Figure 1

Feynman diagrams for the $B_s^0\to {\mu }^{+}{\mu }^{-}{\mu }^{+}{\mu }^{-}$ and $B^0\to {\mu }^{+}{\mu }^{-}{\mu }^{+}{\mu }^{-}$ decays, via (a) the resonant $B_s^0\to J/\psi \varphi$ SM channel, (b) the nonresonant SM channel, and (c) the supersymmetric channel. The latter is propagated by scalar $S$ and pseudoscalar $P$ sgoldstino sfermions.

Figure 2

Invariant mass distribution of $K^{+}{\pi }^{-}{\mu }^{+}{\mu }^{-}$ candidates. The $B^0$ and ${\overline{B}}_s^0$ signal distributions are shown by short-dashed black and long-dashed red (gray) lines, respectively. The background shape is denoted by a long-dashed black line. The total fit result is shown as a solid blue (black) line. The inset shows the mass distribution centred around the ${\overline{B}}_s^0$ mass.

Figure 3

Invariant mass distribution of nonresonant $B_{(s)}^0\to {\mu }^{+}{\mu }^{-}{\mu }^{+}{\mu }^{-}$ candidates. The solid (dashed) black lines indicate the boundaries of the $B_s^0(B^0)$ signal window. The blue curve shows the model used to fit the mass sidebands and extract the expected number of combinatorial background events in the $B_s^0$ and $B^0$ signal regions. Only events in the region in which the line is solid have been considered in the fit.