Speaker
Description
$CP$ violation in charm decays is predicted to be very small, only $\sim\!10^{-3}$ or less. Thus, observing significant $CP$ violation could indicate new physics beyond the Standard Model. The Belle experiment has searched for $CP$ violation in charm decays by measuring an asymmetry about zero for the $T$-odd triple product $\vec{p}^{}_1\cdot (\vec{p}^{}_2\times \vec{p}^{}_3)$, where $\vec{p}^{}_1,\,\vec{p}^{}_2$, and $\vec{p}^{}_3$ are the momenta of three final-state particles produced in a four-body decay. The difference between this asymmetry for $D$ decays and that for $\overline{D}$ decays is $CP$-violating and proportional to $\sin\phi\cos\delta$, where $\phi$ and $\delta$ are the weak and strong phase differences, respectively, between two contributing amplitudes. Such $CP$ violation is largest for $\delta=0$, which differs from the $CP$ asymmetry in partial decay widths: the latter is proportional to $\sin\phi\sin\delta$ and thus vanishes as $\delta\rightarrow 0$. Belle has used its full data set of over 900 fb$^{-1}$ to search for a triple-product $CP$ asymmetry in the following four-body decays: $D^0\rightarrow K^0_S\,K^0_S \pi^+\pi^-$, $D^+\rightarrow K^+ K^0_S \pi^+\pi^-$, $D^+_s\rightarrow K^+ K^0_S \pi^+\pi^-$, $D^+\rightarrow K^0_S\,K^+ K^- \pi^+$, $D^+\rightarrow K^+ K^- \pi^+ \pi^0$, $D^+\rightarrow K^+ \pi^- \pi^+ \pi^0$, $D^+\rightarrow K^- \pi^+ \pi^+ \pi^0$, $D^+_s\rightarrow K^+ \pi^- \pi^+ \pi^0$, and $D^+_s\rightarrow K^+ K^- \pi^+ \pi^0$. Here we report the results of these searches.
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