Sum Rules of Charm CP Asymmetries beyond the SU(3)$_F$ Limit

Abstract

We find new sum rules between direct CP asymmetries in $D$ meson decays with coefficients that can be determined from a global fit to branching ratio data. Our sum rules eliminate the penguin topologies $P$ and $PA$, which cannot be determined from branching ratios. In this way we can make predictions about direct CP asymmetries in the standard model without ad hoc assumptions on the sizes of penguin diagrams. We consistently include first-order SU(3)$_F$ breaking in the topological amplitudes extracted from the branching ratios. By confronting our sum rules with future precise data from LHCb and Belle~II one will identify or constrain new-physics contributions to $P$ or $PA$. The first sum rule correlates the CP asymmetries $a_{CP}^{\mathrm{dir}}$ in $D^0\to K^+K^-$, $D^0\to \pi^+\pi^-$, and $D^0\to \pi^0\pi^0$. We study the region of the $a_{CP}^{\mathrm{dir}}(D^0\to \pi^+\pi^-)$--$a_{CP}^{\mathrm{dir}} (D^0\to \pi^0\pi^0)$ plane allowed by current data and find that our sum rule excludes more than half of the allowed region at 95\% C.L. Our second sum rule correlates the direct CP asymmetries in $D^+\rightarrow \bar{K}^0 K^+$, $D_s^+\rightarrow K^0 \pi^+$, and $D_s^+\rightarrow K^+\pi^0$.