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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.

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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{\,MeV\!/}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).

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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{\,MeV\!/}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).

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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.

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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.

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Animated gif made out of all figures.

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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.

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Relative systematic uncertainties averaged over all the $ m(\mu ^+\mu ^- )$ regions for the efficiency ratio.

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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).

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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).

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Created on 09 December 2018.Citation count from INSPIRE on 18 December 2018.