# Search for the lepton-flavour violating decay $D^0 \to e^\pm\mu^\mp$

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## Abstract

A search for the lepton-flavour violating decay $D^0 \to e^\pm \mu^\mp$ is made with a dataset corresponding to an integrated luminosity of 3.0 fb$^{-1}$ of proton-proton collisions at centre-of-mass energies of $7$ TeV and $8$ TeV, collected by the LHCb experiment. Candidate $D^0$ mesons are selected using the decay $D^{*+} \to D^0 \pi^+$ and the $D^0 \to e^\pm \mu^\mp$ branching fraction is measured using the decay mode $D^0 \to K^-\pi^+$ as a normalisation channel. No significant excess of $D^0 \to e^\pm \mu^\mp$ candidates over the expected background is seen, and a limit is set on the branching fraction, $\mathcal{B}(D^0 \to e^\pm \mu^\mp) < 1.3 \times 10^{-8}$, at 90 % confidence level. This is an order of magnitude lower than the previous limit and it further constrains the parameter space in some leptoquark models and in supersymmetric models with R-parity violation.

## Figures and captions

 Mass spectra from simulation for $D^0 \rightarrow e^\pm \mu^\mp$ decays (solid line) and $D^0 \rightarrow \pi ^+ \pi ^-$ decays reconstructed as $D^0 \rightarrow e^\pm \mu^\mp$ (dashed line). Each spectrum is normalized to unit area. The vertical line indicates the mass of the $D ^0$ meson. Fig1.pdf [25 KiB] HiDef png [200 KiB] Thumbnail [180 KiB] *.C file Distributions of (left) $m( D ^0 )$ and (right) $\Delta m$ for $D^0 \rightarrow e^\pm \mu^\mp$ candidates reconstructed in the combined $7$ $\mathrm{ Te V}$ and $8$ $\mathrm{ Te V}$ data, with fit functions overlaid. The rows correspond to the three bins of BDT output, with the top row corresponding to the most background-like and the bottom row to the most signal-like. The solid (blue) lines show the total fit results, while the thick (grey) lines show the total $D^0 \rightarrow e^\pm \mu^\mp$ component, the thin (purple) lines show the total misidentified $D^0 \rightarrow \pi ^+ \pi ^-$ and the dashed (grey) lines indicate the combinatorial background. Fig2a.pdf [34 KiB] HiDef png [184 KiB] Thumbnail [153 KiB] *.C file Fig2b.pdf [37 KiB] HiDef png [160 KiB] Thumbnail [140 KiB] *.C file Fig2c.pdf [35 KiB] HiDef png [200 KiB] Thumbnail [163 KiB] *.C file Fig2d.pdf [38 KiB] HiDef png [185 KiB] Thumbnail [156 KiB] *.C file Fig2e.pdf [35 KiB] HiDef png [202 KiB] Thumbnail [159 KiB] *.C file Fig2f.pdf [38 KiB] HiDef png [189 KiB] Thumbnail [153 KiB] *.C file Distributions of (left) $m( D ^0 )$ and (right) $\Delta m$ for $K^-\pi^+$ candidates for the $8$ $\mathrm{ Te V}$ data. The dark (blue) line shows the overall fit, the lighter grey line shows the signal, and the dot-dash line shows genuine $D ^0$ events where the soft pion does not come from a $D ^{*+}$ decay. The combinatorial background is too small to be visible. Fig3a.pdf [51 KiB] HiDef png [213 KiB] Thumbnail [174 KiB] *.C file Fig3b.pdf [47 KiB] HiDef png [190 KiB] Thumbnail [149 KiB] *.C file Distribution of CL$_{\text S}$ as a function of ${\cal B}(D^0 \rightarrow e^\pm \mu^\mp)$ . The expected distribution is shown by the dashed line, with the $\pm 1\sigma$ and $\pm 2\sigma$ regions shaded. The observed distribution is shown by the solid line connecting the data points. The horizontal line indicates the $90\%$ confidence level. Fig4.pdf [112 KiB] HiDef png [211 KiB] Thumbnail [173 KiB] *.C file Animated gif made out of all figures. PAPER-2015-048.gif Thumbnail

Created on 17 August 2019.Citation count from INSPIRE on 20 August 2019.