A search for the rare decays Bs>mu+mu and B0>mu+mu is performed using data collected in 2011 and 2012 with the LHCb experiment at the Large Hadron Collider. The data samples comprise 1.1 fb^1 of protonproton collisions at sqrt{s} = 8 TeV and 1.0 fb^1 at sqrt{s}=7 TeV. We observe an excess of Bs > mu+ mu candidates with respect to the background expectation. The probability that the background could produce such an excess or larger is 5.3 x 10^4 corresponding to a signal significance of 3.5 standard deviations. A maximumlikelihood fit gives a branching fraction of BR(Bs > mu+ mu) = (3.2^{+1.5}_{1.2}) x 10^9, where the statistical uncertainty is 95% of the total uncertainty. This result is in agreement with the Standard Model expectation. The observed number of B0 > mu+ mu candidates is consistent with the background expectation, giving an upper limit of BR(B0 > mu+ mu) < 9.4 x 10^10 at 95% confidence level.
$\textrm{CL}_{\textrm{s}}$ as a function of the assumed $ B^0\to\mu^+\mu^$ branching fraction for the combined 2011+2012 dataset. The dashed gray curve is the median of the expected $\textrm{CL}_{\textrm{s}}$ distribution if background and SM signal were observed. The shaded yellow area covers, for each branching fraction value, 34 % of the expected $\textrm{CL}_{\textrm{s}}$ distribution on each side of its median. The solid red curve is the observed $\textrm{CL}_{\textrm{s}}$ . 
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Invariant mass distribution of the selected $ B^0_s \to\mu^+\mu^$ candidates (black dots) with ${\rm BDT}>0.7$ in the combined 2011+2012 dataset. The result of the fit is overlaid (blue solid line) and the different components detailed: $ B^0_s \to\mu^+\mu^$ (red long dashed), $ B^0\to\mu^+\mu^$ (green medium dashed), $ B^0_{(s)}\to h^+{h}^{\prime }$ (pink dotted), $ B^0\to \pi^ \mu^+ \nu_\mu$ (black short dashed) and $ B^{0(+)} \to \pi^{0(+)} \mu^+ \mu^$ (light blue dot dashed), and the combinatorial background (blue medium dashed). 
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Animated gif made out of all figures. 
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Expected and observed limits on the $ B^0\to\mu^+\mu^$ branching fractions for the 2012 and for the combined 2011+2012 datasets. 
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Expected combinatorial background, $ B^0_{(s)}\to h^+{h}^{\prime }$ peaking background, crossfeed, and signal events assuming the SM prediction, together with the number of observed candidates in the $ B^0_s \to\mu^+\mu^$ and $ B^0\to\mu^+\mu^$ mass signal regions, in bins of BDT for the 2011 (top) and for the 2012 (bottom) data samples. The quoted errors include statistical and systematic uncertainties. 
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Created on 15 February 2019.Citation count from INSPIRE on 21 February 2019.