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

TL;DR

This paper reports a precise measurement of the $CP$ asymmetry in $D^0 o ext{pi}^+ ext{pi}^- ext{pi}^0$ decays using Belle II data, finding results consistent with no $CP$ violation, thus testing the Standard Model predictions.

Contribution

First measurement of $CP$ asymmetry in $D^0 o ext{pi}^+ ext{pi}^- ext{pi}^0$ decays at Belle II with unprecedented precision.

Findings

Measured $A_{CP} = (0.29 extpm 0.27 extpm 0.13)\%$

Result is consistent with $CP$ conservation

Most precise $A_{CP}$ measurement for this decay mode to date

Abstract

We measure the time- and phase-space-integrated $CP$ asymmetry $A_{CP}$ in $D^0\to\pi^+\pi^-\pi^0$ decays reconstructed in $e^+e^-\to c\bar c$ events collected by the Belle II experiment from 2019 to 2022. This sample corresponds to an integrated luminosity of 428 fb$^{-1}$. We require $D^0$ mesons to be produced in $D^{*+}\to D^0\pi^+$ decays to determine their flavor at production. Control samples of $D^0\to K^-\pi^+$ decays are used to correct for reconstruction-induced asymmetries. The result, $A_{CP}(D^0\to\pi^+\pi^-\pi^0)=(0.29\pm0.27\pm0.13)\%$, where the first uncertainty is statistical and the second systematic, is the most precise result to date and is consistent with $CP$ conservation.