# 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). Fig1.pdf [28 KiB] HiDef png [630 KiB] Thumbnail [115 KiB] *.C file 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). Fig0.pdf [21 KiB] HiDef png [153 KiB] Thumbnail [32 KiB] *.C file 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. Fig2a.pdf [18 KiB] HiDef png [151 KiB] Thumbnail [31 KiB] *.C file Fig2b.pdf [18 KiB] HiDef png [156 KiB] Thumbnail [33 KiB] *.C file Animated gif made out of all figures. PAPER-2015-043.gif Thumbnail

## 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. Table_1.pdf [72 KiB] HiDef png [74 KiB] Thumbnail [6 KiB] tex code 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. Table_2.pdf [71 KiB] HiDef png [28 KiB] Thumbnail [6 KiB] tex code Systematic uncertainties on $\cal B$ ( $D ^0 \!\rightarrow K ^- \pi ^+ \mu ^+ \mu ^-$ ). Table_3.pdf [81 KiB] HiDef png [133 KiB] Thumbnail [21 KiB] tex code

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