Measurement of the $ D^{0}\rightarrow K^{-}π^{+}e^{+}e^{-} $ branching fraction and search for $ D^{0}\rightarrow π^{+}π^{-}e^{+}e^{-} $ and $D^{0}\rightarrow K^{+}K^{-}e^{+}e^{-} $ decays at Belle

Abstract

We present a study of the rare charm meson decays $ D^{0}\rightarrow K^{+}K^{-}e^{+}e^{-} $, $ π^{+}π^{-}e^{+}e^{-} $, and $ K^{-}π^{+}e^{+}e^{-} $ using a 942 fb$^{-1}$ data set collected by the Belle detector at the KEKB asymmetric-energy $ e^{+}e^{-} $ collider. We use $ D^{0} $ candidates identified by the charge of the pion in $ D^{*} \rightarrow D^{0} π$ decays and normalize the branching fractions to $ D^{0} \rightarrow K^{-}π^{+}π^{-}π^{+} $ decays. The branching fraction for decay $ D^{0} \rightarrow K^{-}π^{+}e^{+}e^{-} $ is measured to be (39.6 $\pm$ 4.5 (stat) $\pm$ 2.9 (syst)) $\times$ $10^{-7}$, with the dielectron mass in the $ ρ/ω$ mass region $ 675 < m_{ee} < 875 $ MeV$/c^{2}$. We also search for $ D^{0}\rightarrow h^{-} h^{(\prime)+}e^{+}e^{-} $ ($ h^{(\prime)}=K,\,π$) decays with the dielectron mass near the $η$ and $φ$ resonances, and away from these resonances for the $ K^{+}K^{-}e^{+}e^{-} $ and $ π^{+}π^{-}e^{+}e^{-} $ modes. For these modes, we find no significant signals and set 90$\%$ confidence level upper limits on their branching fractions at the $\mathcal{O}$(10$^{-7}$) level.

Figures (3)

• Figure 1: SD contributions to FCNC $D^0$ decays through an electroweak penguin diagram (left). LD contributions to $D^0\xspace\rightarrow\xspace X^0V^0\rightarrow\xspace X^0 l^+l^-\xspace$ decays through the VMD diagram (right).
• Figure 1: $dN/dm_{ee}\xspace^{2}$ vs $m_{ee}\xspace^{2}$ distributions for $D^0$$\rightarrow$$h^- h^{(\prime)+}e^+e^-$ candidates. The background has been subtracted using the $s\mathcal{P}lot$ technique. A significant $D^0$$\rightarrow$$K^-\pi^+e^+e^-$ signal with $m_{ee}$ in $\rho/\omega$ mass region is visible in the (0.4, 0.8${{\mathrm{\,Ge V}^{2}\!/}c^4}$) $m_{ee}\xspace^{2}$ bins. The negative bins are due to low statistics after final selections. In the analysis, the selections are optimized separately in each resonance region and in the combined non-resonant regions. For these plots, the $p^*(D^{*+})$, $\Delta m$, and PID selections for the non-resonant region are applied to all $m_{ee}$ regions.
• Figure 2: $D^0$$\rightarrow$$h h^{(\prime)}ee$ decays $m_{h h^{(\prime)}ee}$ distributions for $m_{ee}$ in the $\eta$, $\rho/\omega$ and $\phi$ mass regions. The $\eta\rightarrow\xspace e^+e^-\xspace\gamma$ decay background is shown for $D^0$$\rightarrow$$K^-\pi^+e^+e^-$ with $m_{ee}$ in the $\eta$ mass region (cyan dashed curve). These background PDF parameters are obtained from $D^0\xspace\rightarrow\xspace K^-\xspace\pi^+\xspace \eta\,\,[\rightarrow\xspace e^+e^-\xspace\gamma]$ MC simulation in which the $\gamma$ is not reconstructed. The $K^+K^-e^+e^-$ mode with $m_{ee}$ in the $\eta$ mass region is not fitted since only one event is observed.