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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.

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BDTout.pdf

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]
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pionfit.pdf
kaonfit.pdf [18 KiB]
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kaonfit.pdf

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]
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pimumufit.pdf
pimumu[..].pdf [15 KiB]
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pimumufitzoom.pdf

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]
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kmumufit.pdf
kmumuf[..].pdf [18 KiB]
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kmumufitzoom.pdf

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]
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jpsipifit.pdf

Animated gif made out of all figures.

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Tables and captions

Summary of systematic uncertainties.

Table_1.pdf [70 KiB]
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tex code
Table_1.pdf

Created on 24 August 2019.Citation count from INSPIRE on 24 August 2019.