A search for the decays Bs > mu+ mu and B0 > mu+ mu is performed with 0.37 fb^1 of pp collisions at sqrt{s} = 7 TeV collected by the LHCb experiment in 2011. The upper limits on the branching fractions are BR (Bs > mu+ mu) < 1.6 x 10^8 and BR(B0 > mu+ mu) < 3.6 x 10^9 at 95 confidence level. A combination of these results with the LHCb limits obtained with the 2010 dataset leads to BR (Bs > mu+ mu) < 1.4 x 10^8 and BR (B0 > mu+ mu) < 3.2 x 10^9 at 95 confidence level.
Distribution of the $\mu^+\mu^$ invariant mass for events in each BDT output bin. The curve shows the model used to fit the sidebands and extract the expected number of combinatorial background events in the $ B ^0_s$ and $ B ^0$ signal regions, delimited by the vertical dotted orange and dashed green lines respectively. Only events in the region in which the line is solid have been considered in the fit. 
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Invariant mass distributions of $ B^0_{(s)}\rightarrow h^+h^{'}$ candidates in the $\mu^+\mu^$ mass hypothesis for the whole sample (top left) and for the samples in the three highest bins of the BDT output (top right, bottom left, bottom right). The $ B^0_{(s)}\rightarrow h^+h^{'}$ exclusive decays, the combinatorial background and the physical background components are drawn under the fit to the data (solid blue line). 
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BDT probability distribution functions of signal events (solid squares) and combinatorial background (open circles): the PDF for the signal is obtained from the inclusive sample of TIS $ B^0_{(s)}\rightarrow h^+h^{'}$ events, the PDF for the combinatorial background is obtained from the events in the mass sidebands. 
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Dimuon invariant mass spectrum in the ranges (2.9  3.9) $ {\mathrm{ Ge V /}c^2}$ (left) and (911) $ {\mathrm{ Me V /}c^2}$ (right). 
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Invariant mass distributions of the $ B^+\rightarrow J/\psi K^+$ (left) and $ B^0_s\rightarrow J/\psi \phi$ (right) candidates used in the normalization procedure. 
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Distribution of selected dimuon events in the invariant massBDT plane. The orange shortdashed (green longdashed) lines indicate the $\pm 60 {\mathrm{ Me V /}c^2} $ search window around the mean $ B ^0_s$ ( $ B ^0$ ) mass. 
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Distribution of selected dimuon events in the $ B ^0_s \rightarrow \mu^+\mu^$ mass window for the four BDT output bins. The black dots are data, the light grey histogram shows the contribution of the combinatorial background, the black filled histogram shows the contribution of the $ B^0_{(s)}\rightarrow h^+h^{'}$ background and the dark grey filled histogram the contribution of $ B ^0_s \rightarrow \mu^+\mu^$ signal events according to the SM rate. The hatched area depicts the uncertainty on the sum of the expected contributions. 
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Distribution of selected dimuon events in the $ B ^0 \rightarrow \mu^+\mu^$ mass window for the four BDT output bins. The black dots are data, the light grey histogram shows the contribution of the combinatorial background, the black filled histogram shows the contribution of the $ B^0_{(s)}\rightarrow h^+h^{'}$ background and the dark grey filled histogram shows the crossfeed of $ B ^0_s \rightarrow \mu^+\mu^$ events in the $ B ^0$ mass window assuming the the SM rate. The hatched area depicts the uncertainty on the sum of the expected contributions. 
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$\textrm{CL}_{\textrm{s}}$ as a function of the assumed $\cal B$ . Expected (observed) values are shown by dashed black (dotted blue) lines. The expected $\textrm{CL}_{\textrm{s}}$ values have been computed assuming a signal yield corresponding to the SM branching fractions. The green (grey) shaded areas cover the region of $\pm 1 \sigma$ of compatible observations. The measured upper limits at 90% and 95% CL are also shown. Left: $ B ^0_s \rightarrow \mu^+\mu^$ , right: $ B ^0 \rightarrow \mu^+\mu^$ . 
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Animated gif made out of all figures. 
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Summary of the quantities and their uncertainties required to calculate the normalization factors ($\alpha^{\rm norm}_{ B^0_{(s)}\rightarrow \mu^+\mu^ }$) for the three normalization channels considered. The branching fractions are taken from Refs. [19,24]. The trigger efficiency and the number of $ B ^0 \rightarrow K^+\pi^$ candidates correspond to TIS events. 
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Expected combinatorial background events, expected peaking ( $ B^0_{(s)}\rightarrow h^+h^{'}$ ) background events, expected signal events assuming the SM branching fraction prediction, and observed events in the $ B ^0_s \rightarrow \mu^+\mu^$ search window. 
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Expected combinatorial background events, expected peaking ( $ B^0_{(s)}\rightarrow h^+h^{'}$ ) background events, expected $ B ^0 \rightarrow \mu ^+ \mu ^ $ signal events assuming the SM branching fraction , expected crossfeed events from $ B ^0_s \rightarrow \mu^+\mu^$ assuming the SM branching fraction and observed events in the $ B ^0 \rightarrow \mu^+\mu^$ search window. 
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Expected and observed limits on the $ B ^0_s \rightarrow \mu^+\mu^$ branching fraction for the 2011 data and for the combination of 2010 and 2011 data. The expected limits are computed allowing the presence of $ B ^0_s \rightarrow \mu^+\mu^$ events according to the SM branching fraction. 
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Expected and observed limits on the $ B ^0 \rightarrow \mu^+\mu^$ branching fraction for 2011 data and for the combination of 2010 and 2011 data. The expected limits are computed in the background only hypothesis. 
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Created on 06 March 2021.