Measurement of the Direct $CP$-violating Charge Asymmetry in $D_s^{\pm }\to \varphi {\pi }^{\pm }$ Decays

We present a measurement of the direct $CP$-violating charge asymmetry in $D_s^{+}\to \varphi {\pi }^{\pm }$ decays where the $\varphi$ meson decays to $K^{+}{K}^{-}$, using the full Run II data set with an integrated luminosity of $10.4{\mathrm{fb}}^{-1}$ of proton-antiproton collisions collected using the D0 detector at the Fermilab Tevatron Collider. The normalized difference $A_{CP}$ in the yield of $D_s^{+}$ and $D_s^{-}$ mesons in these decays is measured by fitting the difference between their reconstructed invariant mass distributions. This results in an asymmetry of $A_{CP}=[-0.38\pm 0.27]\%$, which is the most precise measurement of this quantity to date. The result is consistent with the standard model prediction of zero $CP$ asymmetry in this decay.

Figure 1

The polarity-weighted $\varphi {\pi }^{\pm }$ invariant mass distribution. The lower mass peak is due to the decay $D^{\pm }\to \varphi {\pi }^{\pm }$ while the second peak is due to the $D_s^{\pm }$ meson decay. Note the zero suppression on the vertical axis. The bottom panel shows the fit residuals. The error bars represent the statistical uncertainties.

Figure 2

The fit to the differences between the numbers of $D_s^{+}$ and $D_s^{-}$ mesons as a function of the $\varphi {\pi }^{\pm }$ mass (for clarity the data has been rebinned).