# Measurement of the $B^0_s \to \mu^+ \mu^-$ branching fraction and search for $B^0 \to \mu^+ \mu^-$ decays at the LHCb experiment

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## Abstract

A search for the rare decays $B^0_s \to\mu^+\mu^-$ and $B^0 \to\mu^+\mu^-$ is performed at the LHCb experiment. The data analysed correspond to an integrated luminosity of 1 fb$^{-1}$ of $pp$ collisions at a centre-of-mass energy of 7 TeV and 2 fb$^{-1}$ at 8 TeV. An excess of $B^0_s \to\mu^+\mu^-$ signal candidates with respect to the background expectation is seen with a significance of 4.0 standard deviations. A time-integrated branching fraction of ${\cal B}(B^0_s \to\mu^+\mu^-) = (2.9^{+1.1}_{-1.0})\times 10^{-9}$ is obtained and an upper limit of ${\cal B}(B^0 \to\mu^+\mu^-) < 7.4\times 10^{-10}$ at 95% confidence level is set. These results are consistent with the Standard Model expectations.

## Figures and captions

 Expected distribution of the BDT output for the $B^0_s \to\mu^+\mu^-$ signal (black squares), obtained from $B^0_{(s)}\to h^+{h}^{\prime -}$ control channels, and the combinatorial background (blue circles). Fig1.pdf [9 KiB] HiDef png [109 KiB] Thumbnail [110 KiB] *.C file Invariant mass distribution of the selected $B^0_{(s)}\to \mu^+\mu^-$ candidates (black dots) with ${\rm BDT }>0.7$. The result of the fit is overlaid (blue solid line) and the different components detailed: $B^0_s \to\mu^+\mu^-$ (red long dashed line), $B^0\to\mu^+\mu^-$ (green medium dashed line), combinatorial background (blue medium dashed line), $B^0_{(s)}\to h^+{h}^{\prime -}$ (magenta dotted line), $B^{0(+)} \to \pi^{0(+)} \mu^+ \mu^-$ (light blue dot-dashed line), $B^0\to \pi^- \mu^+ \nu_\mu$ and $B^0_s \to K^- \mu^+ \nu_{\mu}$ (black dot-dashed line). Fig2.pdf [14 KiB] HiDef png [222 KiB] Thumbnail [171 KiB] *.C file $\textrm{CL}_{\textrm{s}}$ as a function of the assumed $B^0\to\mu^+\mu^-$ branching fraction. The dashed curve is the median of the expected $\textrm{CL}_{\textrm{s}}$ distribution for background-only hypothesis. The green area covers, for each branching fraction value, $34.1 \%$ of the expected $\textrm{CL}_{\textrm{s}}$ distribution on each side of its median. The solid red curve is the observed $\textrm{CL}_{\textrm{s}}$ . Fig3.pdf [10 KiB] HiDef png [220 KiB] Thumbnail [169 KiB] *.C file Animated gif made out of all figures. PAPER-2013-046.gif Thumbnail

## Tables and captions

 Expected background yields from $b$-hadron decays, with dimuon mass $m_{\mu\mu}\in[4900,6000] {\mathrm{ Me V /}c^2}$ and the relative fraction with $\rm{BDT }> 0.7$. Table_1.pdf [55 KiB] HiDef png [71 KiB] Thumbnail [36 KiB] tex code Expected limits for the background only (bkg) and background plus SM signal (bkg+SM) hypotheses, and observed limits on the $B^0\to\mu^+\mu^-$ branching fraction. Table_2.pdf [43 KiB] HiDef png [60 KiB] Thumbnail [29 KiB] tex code