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First observation of the decay $D^{0}\rightarrow K^{-}\pi^{+}\mu^{+}\mu^{-}$ in the $\rho^{0}$-$\omega$ region of the dimuon mass spectrum

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Abstract

A study of the decay $D^{0}\rightarrow K^{-}\pi^{+}\mu^{+}\mu^{-}$ is performed using data collected by the LHCb detector in proton-proton collisions at a centre-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 2.0 fb$^{-1}$. Decay candidates with muon pairs that have an invariant mass in the range 675--875 MeV$/c^2$ are considered. This region is dominated by the $\rho^{0}$ and $\omega$ resonances. The branching fraction in this range is measured to be ${\cal B}$($D^{0}\rightarrow K^{-}\pi^{+}\mu^{+}\mu^{-}$) = $(4.17 \pm 0.12(stat) \pm 0.40(syst))\times10^{-6}$. This is the first observation of the decay $D^{0}\rightarrow K^{-}\pi^{+}\mu^{+}\mu^{-}$. Its branching fraction is consistent with the value expected in the Standard Model.

Figures and captions

Mass distributions of (a) $ D ^0 \!\rightarrow K ^- \pi ^+ \pi ^+ \pi ^- $ and (b) $ D ^0 \!\rightarrow K ^- \pi ^+ \mu ^+ \mu ^- $ candidates. The data are shown as points (black) and the total PDF (blue solid line) is overlaid. In (a), the two corresponding components of the fit model are the $ D ^0 \!\rightarrow K ^- \pi ^+ \pi ^+ \pi ^- $ decays (red solid line) and the non-peaking background (violet dashed line). In (b), the components are the $ D ^0 \!\rightarrow K ^- \pi ^+ \mu ^+ \mu ^- $ (long-dashed green line), the peaking background due to misidentified $ D ^0 \!\rightarrow K ^- \pi ^+ \pi ^+ \pi ^- $ decays (red solid line), and the non-peaking background (violet dashed line).

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Distributions of the BDT response of $ D ^0 \!\rightarrow K ^- \pi ^+ \mu ^+ \mu ^- $ ( circles) and $ D ^0 \!\rightarrow K ^- \pi ^+ \pi ^+ \pi ^- $ decays (triangles) in data (full markers) and simulation (open markers). In data, the background contributions are removed using the {\it sPlot} technique. The lower plot shows the ratio between the $ D ^0 \!\rightarrow K ^- \pi ^+ \mu ^+ \mu ^- $ and $ D ^0 \!\rightarrow K ^- \pi ^+ \pi ^+ \pi ^- $ distributions in data (full squares) and simulation (open squares).

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Background subtracted distribution of (a) the $ K ^-$ $\pi ^+$ invariant mass and (b) the $\mu ^+$ $\mu ^-$ invariant mass, measured in $ D ^0 \!\rightarrow K ^- \pi ^+ \mu ^+ \mu ^- $ decays using the {\it sPlot} technique.

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Animated gif made out of all figures.

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Tables and captions

Summary of the results of the fit described in Sect. 4. The yields measured in the $ D ^0 \!\rightarrow K ^- \pi ^+ \mu ^+ \mu ^- $ sample and the correlations between them, the yields measured in the normalisation sample, the common width fitted to the $ D ^0 \!\rightarrow K ^- \pi ^+ \mu ^+ \mu ^- $ and $ D ^0 \!\rightarrow K ^- \pi ^+ \pi ^+ \pi ^- $ yields, and the relative uncertainty on $\cal B$ ( $ D ^0 \!\rightarrow K ^- \pi ^+ \mu ^+ \mu ^- $ ) are presented. Uncertainties on the fitted parameters are statistical. The variation of the uncertainty on $\cal B$ ( $ D ^0 \!\rightarrow K ^- \pi ^+ \mu ^+ \mu ^- $ ) when the background yields are fixed indicates to what extent it is enhanced by the need to separate contributions in overlap and which shapes present some similarities.

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Measured efficiencies and yields for the decay $ D ^0 \!\rightarrow K ^- \pi ^+ \mu ^+ \mu ^- $ in the dimuon mass range 675--875 $ {\mathrm{\,MeV\!/}c^2}$ , and for the decay $ D ^0 \!\rightarrow K ^- \pi ^+ \pi ^+ \pi ^- $ . The uncertainties are statistical. In the case of efficiencies, it stems from the finite size of the simulated samples.

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Systematic uncertainties on $\cal B$ ( $ D ^0 \!\rightarrow K ^- \pi ^+ \mu ^+ \mu ^- $ ).

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Created on 11 December 2018.Citation count from INSPIRE on 11 December 2018.