Measurement of the time-integrated $CP$ asymmetry in $D^0\to\pi^0\pi^0$ decays at Belle II

TL;DR

This study measures the $CP$ asymmetry in $D^0 o ext{pi}^0 ext{pi}^0$ decays at Belle II, finding results consistent with no $CP$ violation, using 428 fb$^{-1}$ of data collected from 2019 to 2022.

Contribution

First measurement of the time-integrated $CP$ asymmetry in $D^0 o ext{pi}^0 ext{pi}^0$ decays at Belle II with a novel correction method for detection asymmetries.

Findings

$A_{CP}(D^0 o ext{pi}^0 ext{pi}^0) = (0.30\pm 0.72 ext{(stat)}\pm 0.20 ext{(syst)}) ext{ extpercent}$

Result is consistent with $CP$ symmetry, indicating no observed $CP$ violation.

Analysis used control samples of $D^0 o K^- ext{pi}^+$ decays for asymmetry correction.

Abstract

We measure the time-integrated $CP$ asymmetry, $A_{CP}$, in $D^0\to\pi^0\pi^0$ decays reconstructed in $e^+e^-\to c\bar{c}$ events collected by Belle II during 2019--2022. The data corresponds to an integrated luminosity of 428$\mathrm{fb}^{-1}$. The $D^0$ decays are required to originate from the flavor-conserving $D^{*+} \to D^0 \pi^+$ decay to determine the charm flavor at production time. Control samples of $D^0\to K^- \pi^+$ decays, with or without an associated pion from a $D^{*+}$ decay, are used to correct for detection asymmetries. The result, $A_{CP}(D^0\to\pi^0\pi^0) = (0.30\pm 0.72\pm 0.20)\%$, where the first uncertainty is statistical and the second systematic, is consistent with $CP$ symmetry.