# Measurement of the $C\!P$ violation parameter $A_\Gamma$ in $D^0 \to K^+K^-$ and $D^0 \to \pi^+\pi^-$ decays

## TL;DR

This paper reports precise measurements of $CP$ violation parameters in $D^0$ meson decays, finding no evidence of $CP$ violation and significantly improving the measurement precision over previous results.

## Contribution

The study provides the most precise measurements to date of $A_$ in $D^0$ decays, using LHCb data from Run 1, and constrains $CP$ violation in charm meson systems.

## Key findings

- Measured $A_(K^+K^-) = (-0.30 \, ext{±} \, 0.32 \, ext{±} \, 0.10) imes 10^{-3}$
- Measured $A_(\u03c0^+) = (0.46 \, ext{±} \, 0.58 \, ext{±} \, 0.12) imes 10^{-3}$
- No evidence for $CP$ violation was observed.

## Abstract

Asymmetries in the time-dependent rates of $D^0 \to K^+K^-$ and $D^0 \to \pi^+\pi^-$ decays are measured in a $pp$ collision data sample collected with the LHCb detector during LHC Run 1, corresponding to an integrated luminosity of $3\,\mathrm{fb}^{-1}$. The asymmetries in effective decay widths between $D^0$ and $\overline{D}^0$ decays, sensitive to indirect $CP$ violation, are measured to be $A_\Gamma(K^+ K^-) = (-0.30 \pm 0.32 \pm 0.10)\times 10^{-3}$ and $A_\Gamma(\pi^+\pi^-) = (0.46 \pm 0.58 \pm 0.12)\times 10^{-3}$, where the first uncertainty is statistical and the second systematic. These measurements show no evidence for $C\!P$ violation and improve on the precision of the previous best measurements by nearly a factor of two.