# Search for the rare decays $B^{0}_{s} \to \mu^{+} \mu^{-}$ and $B^{0} \to \mu^{+} \mu^{-}$

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## Abstract

A search for the decays Bs-->mumu and Bd-->mumu is performed with about 37 pb^{-1} of pp collisions at sqrt{s} = 7 TeV collected by the LHCb experiment at the Large Hadron Collider at CERN. The observed numbers of events are consistent with the background expectations. The resulting upper limits on the branching ratios are BR(Bs-->mumu) < 5.6 x 10^{-8} and BR(Bd-->mumu) <1.5 x 10^{-8} at 95 confidence level.

## Figures and captions

 Probability of signal events in bins of GL obtained from the inclusive sample of TIS $B^0_{q}\rightarrow h^+h^{'-}$ events (solid squares). The background probability (open circles) is obtained from the events in the sidebands of the $\mu\mu$ invariant mass distribution in the $B ^0_s$ mass window. GL_pdf[..].pdf [13 KiB] HiDef png [96 KiB] Thumbnail [113 KiB] Fit of the invariant mass distribution for $B^0_{q}\rightarrow h^+h^{'-}$ candidates in the ${\rm GL}$ range [0.25, 1.0]. The pink dashed curve is the combinatorial background component, while the physical background is shown with a light-blue dotted curve. The remaining contributions are from the two-body decays of the $B ^0$ , $B ^0_s$ and $\Lambda^0_b$ . Mass_B2hh.pdf [31 KiB] HiDef png [296 KiB] Thumbnail [200 KiB] Distribution of the $\mu\mu$ invariant mass for different ${\rm GL}$ bins: (a) $[0,0.25]$, (b) $[0.25,0.5]$, (c) $[0.5,0.75]$, (d) $[0.75,1.0]$. The blue solid lines show the interpolation model used and the dashed line shows the result of the interpolation in the search windows. sideba[..].pdf [31 KiB] HiDef png [155 KiB] Thumbnail [151 KiB] sideba[..].pdf [27 KiB] HiDef png [136 KiB] Thumbnail [126 KiB] sideba[..].pdf [27 KiB] HiDef png [141 KiB] Thumbnail [132 KiB] sideba[..].pdf [27 KiB] HiDef png [133 KiB] Thumbnail [126 KiB] Observed distribution of selected dimuon events in the GL {\it vs} invariant mass plane. The orange short-dashed (green long-dashed) lines indicate the $\pm 60 {\mathrm{ Me V /}c^2}$ search window around the $B ^0_s$ ( $B ^0$ ). GLvsmass.pdf [317 KiB] HiDef png [387 KiB] Thumbnail [250 KiB] (a) Observed (solid curve) and expected (dashed curve) $\textrm{CL}_{\textrm{s}}$ values as a function of ${\cal B}( B ^0_s \rightarrow \mu ^+ \mu ^- )$ . The green shaded area contains the $\pm 1 \sigma$ interval of possible results compatible with the expected value when only background is observed. The 90 % (95 %) C.L. observed value is identified by the solid (dashed) line. (b) the same for ${\cal B}( B ^0 \rightarrow \mu ^+ \mu ^- )$ . cls_Bs.pdf [68 KiB] HiDef png [165 KiB] Thumbnail [148 KiB] cls_Bd.pdf [59 KiB] HiDef png [183 KiB] Thumbnail [163 KiB] Animated gif made out of all figures. PAPER-2011-004.gif Thumbnail

## Tables and captions

 Summary of the factors and their uncertainties needed to calculate the normalization factors ($\alpha_{ B^0_{q}\rightarrow \mu^+\mu^- }$) for the three normalization channels considered. The branching ratios are taken from Refs. [14,16]. The trigger efficiency and number of $B ^0 \rightarrow K^+\pi^-$ candidates correspond to only TIS events, as described in the text. Table_1.pdf [75 KiB] HiDef png [46 KiB] Thumbnail [20 KiB] tex code Probability of signal events in bins of GL obtained from the inclusive sample of TIS $B^0_{q}\rightarrow h^+h^{'-}$ events. The background probability in the $B ^0_s$ mass window is obtained from the events in the sidebands of the dimuon invariant mass distribution. Table_2.pdf [51 KiB] HiDef png [86 KiB] Thumbnail [40 KiB] tex code Expected background, expected SM signal and observed number of events in bins of ${\rm GL}$ and invariant mass, in the $\pm 60 {\mathrm{ Me V /}c^2}$ mass window around the $B ^0_s$ mass central value of 5363.1 ${\rm MeV}/c^2$ . Table_3.pdf [56 KiB] HiDef png [188 KiB] Thumbnail [82 KiB] tex code Expected background, expected SM signal and observed number of events in bins of ${\rm GL}$ and invariant mass, in the $\pm 60 {\mathrm{ Me V /}c^2}$ mass window around the $B ^0$ central value of 5275.0 ${\rm MeV}/c^2$ . Table_4.pdf [56 KiB] HiDef png [177 KiB] Thumbnail [64 KiB] tex code

Created on 06 March 2021.