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First Evidence for the Decay $B^0_s \to \mu^+\mu^-$

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Abstract

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 proton-proton 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 maximum-likelihood 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.

Figures and captions

$\textrm{CL}_{\textrm{s}}$ 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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Tables and captions

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, cross-feed, 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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Supplementary Material [file]

This ZIP file contains supplemetary material for the publication LHCb-PAPER-2012-043. The files are: Bsmm2012_additional.pdf: synthetic description of the Bsmm analysis, containing also all the blessed plots with the needed caption. All figures availables in various formats (*.pdf, *.png, *.eps) when available; the numbering refers to the one used in the Bsmm2012_supporting.pdf file. Two more figures (*.pdf only) with cartoon for a Bsmm and a combinatorial background events

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Created on 09 December 2018.Citation count from INSPIRE on 09 December 2018.