# First observation of the decay $B^+ \to \pi^+ \mu^+\mu^-$

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## Abstract

A discovery of the rare decay $B^{+} \rightarrow \pi^{+} \mu^{+} \mu^{-}$ is presented. This decay is observed for the first time, with 5.2 $\sigma$ significance. The observation is made using $pp$ collision data, corresponding to an integrated luminosity of 1.0 fb$^{-1}$, collected with the LHCb detector. The measured branching fraction is (2.3 $\pm$ 0.6 (stat.) $\pm$ 0.1 (syst.))$\times 10^{-8}$, and the ratio of the $B^{+} \rightarrow \pi^{+} \mu^{+} \mu^{-}$ and $B^{+} \rightarrow \K^{+} \mu^{+} \mu^{-}$ branching fractions is measured to be 0.053 $\pm$ 0.014 (stat.) $\pm$ 0.001 (syst.).

## Figures and captions

 BDT output distribution for simulated $B ^+ \!\rightarrow \pi ^+ \mu ^+ \mu ^-$ events (black solid line) and candidates taken from the mass sidebands in the data (red dotted line). Both distributions are normalised to unit area. The vertical line indicates the chosen cut value of 0.325. BDTout.pdf [14 KiB] HiDef png [150 KiB] Thumbnail [49 KiB] *.C file Invariant mass distribution for $B ^+ \!\rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K ^+$ candidates under the (a) $K ^+ \mu ^+ \mu ^-$ and (b) $\pi ^+ \mu ^+ \mu ^-$ mass hypotheses with the fit projections overlaid. In the legend, "part. reco" refers to partially reconstructed background. The fit models are described in the text. pionfit.pdf [19 KiB] HiDef png [196 KiB] Thumbnail [67 KiB] *.C file kaonfit.pdf [18 KiB] HiDef png [185 KiB] Thumbnail [63 KiB] *.C file Invariant mass distribution of $B ^+ \!\rightarrow \pi ^+ \mu ^+ \mu ^-$ candidates with the fit projection overlaid (a) in the full mass range and (b) in the region around the $B$ mass. In the legend, "part. reco." and "combinatorial" refer to partially reconstructed and combinatorial backgrounds respectively. The discontinuity at 5500 ${\mathrm{\,Me V\!/}c^2}$ is due to the removal of data used for training the BDT. pimumufit.pdf [22 KiB] HiDef png [291 KiB] Thumbnail [89 KiB] *.C file pimumu[..].pdf [15 KiB] HiDef png [194 KiB] Thumbnail [61 KiB] *.C file Invariant mass distribution of $B ^+ \!\rightarrow K ^+ \mu ^+ \mu ^-$ candidates with the fit projection overlaid (a) in the full mass range and (b) in the region around the $B$ mass. In the legend, "combinatorial" refers to the combinatorial background. kmumufit.pdf [33 KiB] HiDef png [202 KiB] Thumbnail [59 KiB] *.C file kmumuf[..].pdf [18 KiB] HiDef png [213 KiB] Thumbnail [64 KiB] *.C file Invariant mass distribution of ${B^+\rightarrow J\!/\!\psi \pi ^+ }$ candidates with the fit projection overlaid. In the legend, "part. reco." and "combinatorial" refer to partially reconstructed and combinatorial backgrounds respectively. The fit model is described in the text. jpsipifit.pdf [32 KiB] HiDef png [233 KiB] Thumbnail [76 KiB] *.C file Animated gif made out of all figures. PAPER-2012-020.gif Thumbnail

## Tables and captions

 Summary of systematic uncertainties. Table_1.pdf [70 KiB] HiDef png [56 KiB] Thumbnail [9 KiB] tex code

Created on 20 April 2019.Citation count from INSPIRE on 25 April 2019.