# Search for the decay $D^0\to\pi^+\pi^-\mu^+\mu^-$

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## Abstract

A search for the D0 to \pi+ \pi- \mu+ \mu- decay, where the muon pair does not originate from a resonance, is performed using proton-proton collision data corresponding to an integrated luminosity of 1.0 fb-1 recorded by the LHCb experiment at a centre-of-mass energy of 7 TeV. No signal is observed and an upper limit on the relative branching fraction with respect to the resonant decay mode D0 to \pi+ \pi- \phi(to \mu+ \mu-), under the assumption of a phase-space model, is found to be B(D0 to \pi+ \pi- mu+ mu-)/B(D0 to \pi+ \pi- \phi(to \mu+ \mu-)) < 0.96 at the 90% confidence level. The upper limit on the absolute branching fraction is evaluated to be B(D0 to \pi+ \pi- \mu+ \mu-) < 5.5 x 10-7 at 90% confidence level. This is the most stringent to date.

## Figures and captions

 Leading Feynman diagrams for the FCNC decay $D ^0 \rightarrow \pi ^+ \pi ^- \mu ^+\mu ^-$ in the SM. Fig_1.pdf [36 KiB] HiDef png [59 KiB] Thumbnail [40 KiB] *.C file tex code Distributions of $m(\pi^+\pi^-\mu^+\mu^-)$ for $D ^0 \rightarrow \pi ^+ \pi ^- \mu ^+\mu ^-$ candidates in the (a) low- $m(\mu ^+\mu ^- )$ , (b) $\rho$ / $\omega$ , (c) $\phi$ , and (d) high- $m(\mu ^+\mu ^- )$ regions, with $\Delta m$ in the range $144.4-146.6$ ${\mathrm{ Me V /}c^2}$ . The data are shown as points (black) and the fit result (dark blue line) is overlaid. The components of the fit are also shown: the signal (filled area), the $D ^0 \rightarrow \pi ^+ \pi ^- \pi ^+ \pi ^-$ background (green dashed line) and the non-peaking background (red dashed-dotted line). Fig2a.pdf [18 KiB] HiDef png [194 KiB] Thumbnail [159 KiB] *.C file Fig2b.pdf [18 KiB] HiDef png [268 KiB] Thumbnail [229 KiB] *.C file Fig2c.pdf [18 KiB] HiDef png [260 KiB] Thumbnail [225 KiB] *.C file Fig2d.pdf [17 KiB] HiDef png [172 KiB] Thumbnail [145 KiB] *.C file Distributions of $\Delta m$ for $D ^0 \rightarrow \pi ^+ \pi ^- \mu ^+\mu ^-$ candidates in the (a) low- $m(\mu ^+\mu ^- )$ , (b) $\rho$ / $\omega$ , (c) $\phi$ , and (d) high- $m(\mu ^+\mu ^- )$ regions, with the $D ^0$ invariant mass in the range $1840-1888$ ${\mathrm{ Me V /}c^2}$ . The data are shown as points (black) and the fit result (dark blue line) is overlaid. The components of the fit are also shown: the signal (filled area), the $D ^0 \rightarrow \pi ^+ \pi ^- \pi ^+ \pi ^-$ background (green dashed line) and the non-peaking background (red dashed-dotted line). Fig3a.pdf [19 KiB] HiDef png [197 KiB] Thumbnail [153 KiB] *.C file Fig3b.pdf [20 KiB] HiDef png [212 KiB] Thumbnail [182 KiB] *.C file Fig3c.pdf [21 KiB] HiDef png [210 KiB] Thumbnail [182 KiB] *.C file Fig3d.pdf [19 KiB] HiDef png [160 KiB] Thumbnail [139 KiB] *.C file Observed (solid curve) and expected (dashed curve) $\mathrm{CL}_{s}$ values as a function of $\cal B ( D ^0 \rightarrow \pi ^+ \pi ^- \mu ^+\mu ^- )/\cal B ( D ^0 \rightarrow \pi ^+ \pi ^- \phi(\rightarrow \mu ^+\mu ^- ) )$. The green (yellow) shaded area contains 68.3% and 95.5% of the results of the analysis on experiments simulated with no signal. The upper limits at the 90(95)% $\mathrm{CL}$ are indicated by the dashed (solid) line. Fig4.pdf [14 KiB] HiDef png [180 KiB] Thumbnail [148 KiB] *.C file Observed (solid curve) and expected (dashed curve) $\mathrm{CL}_{s}$ values as a function of $\cal B$ ( $D ^0 \rightarrow \pi ^+ \pi ^- \mu ^+\mu ^-$ ). The green (yellow) shaded area contains 68.3% and 95.5% of the results of the analysis on experiments simulated with no signal. The upper limits at the 90(95)% $\mathrm{CL}$ are indicated by the dashed (solid) line. Fig5.pdf [14 KiB] HiDef png [167 KiB] Thumbnail [135 KiB] *.C file Animated gif made out of all figures. PAPER-2013-050.gif Thumbnail

## Tables and captions

 $D^0\rightarrow \pi^+\pi^-\mu^+\mu^-$ fitted yields in the four $m(\mu^+\mu^-)$ regions. The corresponding signal fractions under the assumption of a phase-space model, as described in Section 7, are listed in the last column. Table_1.pdf [47 KiB] HiDef png [39 KiB] Thumbnail [18 KiB] tex code Relative systematic uncertainties averaged over all the $m(\mu ^+\mu ^- )$ regions for the efficiency ratio. Table_2.pdf [18 KiB] HiDef png [65 KiB] Thumbnail [31 KiB] tex code Upper limits on $\mathcal{\cal B }( D ^0 \rightarrow \pi ^+ \pi ^- \mu ^+\mu ^- )/\mathcal{\cal B }( D ^0 \rightarrow \pi ^+ \pi ^- \phi (\rightarrow \mu ^+ \mu^-))$ at 90 and 95% $\mathrm{CL}$ , and p-values for the background-only hypothesis in each $m(\mu ^+\mu ^- )$ region and in the full $m(\mu ^+\mu ^- )$ range (assuming a phase-space model). Table_3.pdf [37 KiB] HiDef png [68 KiB] Thumbnail [31 KiB] tex code Upper limits on $\mathcal{\cal B }( D ^0 \rightarrow \pi ^+ \pi ^- \mu ^+\mu ^- )$ at 90 and 95% $\mathrm{CL}$ in each $m(\mu ^+\mu ^- )$ region and in the full $m(\mu ^+\mu ^- )$ range (assuming a phase-space model). Table_4.pdf [43 KiB] HiDef png [47 KiB] Thumbnail [23 KiB] tex code

Created on 19 October 2019.