Search for $CP$ violation in the decay $D^{\pm }\to K_S^0{\pi }^{\pm }$

We report on a search for $CP$ violation in the decay $D^{\pm }\to K_S^0{\pi }^{\pm }$ using a data set corresponding to an integrated luminosity of $469{\mathrm{fb}}^{-1}$ collected with the BABAR detector at the PEP-II asymmetric energy $e^{+}{e}^{-}$ storage rings. The $CP$-violating decay rate asymmetry $A_{CP}$ is determined to be $(-0.44\pm 0.13(\mathrm{stat})\pm 0.10(\mathrm{syst}))\%$, consistent with zero at $2.7\sigma$ and with the standard model prediction of $(-0.332\pm 0.006)\%$. This is currently the most precise measurement of this parameter.

Figure 1

Invariant mass distribution for $K_S^0{\pi }^{\pm }$ candidates in the data (black points). The solid curve shows the fit to the data. The dashed line is the sum of all backgrounds, while the dotted line is combinatorial background only. The vertical scale of the plot is logarithmic.

Figure 2

Map of the ratio between detection efficiency for ${\pi }^{+}$ and ${\pi }^{-}$ (top) plus the corresponding statistical errors (bottom). The map is produced using the numbers of ${\pi }^{-}$ and ${\pi }^{+}$ tracks in the selected control sample.

Figure 3

$A_{CP}$ (top) and $A_{FB}$ (bottom) asymmetries for $D^{\pm }\to K_S^0{\pi }^{\pm }$ candidates as a function of $|\cos {\theta }_D^{*}|$ in the data sample. The solid line represents the central value of $A_{CP}$ and the hatched region is the $\pm 1\sigma$ interval, both obtained from a ${\chi }^2$ minimization assuming no dependence on $|\cos {\theta }_D^{*}|$.