Search for $CP$ violation in the $D^0\to\pi^+\pi^-\pi^0$ decay at the Belle II experiment

Sumitted to PubDB: 2026-04-25

Category: Phd Thesis

Tags: Run 1

Principal Authors Ludovico Massaccesi, Francesco Forti, Giulia Casarosa
Date 2025-06-29
Belle II Number BELLE2-PTHESIS-2026-004
Abstract The Standard Model of particle physics is the most successfully tested theory describing nature at a fundamental level. Still, it is unable to provide answers to several fundamental questions, for instance the origin of the matter-antimatter asymmetry of the universe: the amount of the charge-parity ($CP$) symmetry violation necessary to explain it is, within the assumptions of the Standard Model, more than what has been observed so far. There must exist, therefore, undiscovered sources of $CP$ violation. $CP$ violation was first observed in the $K$ system in 1964, then in the $B$ system in 2001 and in the $B_s$ system in 2007, and finally in the $D$ system only in 2019. The measurement in the $D$ system is particularly interesting because the charm quark is the only up-type quark that exhibits mixing and $CP$ violation. In this thesis, I measure the time-integrated $CP$ asymmetry of the $D^0$ meson decay to three pions $\pi^+\pi^-\pi^0$. The Standard Model predicts, with large theoretical uncertainties, that the $CP$ asymmetry in this decay is at or below the level of current experimental sensitivity; however, extensions of the Standard Model may enhance it. To perform the measurement, I use the sample of electron-positron collisions collected by the Belle II experiment between 2019 and 2022 with a collision energy at or around the $\Upsilon(4S)$ resonance, corresponding to an integrated luminosity of 428 fb$^{-1}$. My result, $A_{CP}=(0.29\pm0.27(stat)\pm0.13(sys))\%$, is compatible with conservation of the $CP$ symmetry, and improves the precision of the current world's best determination by 34%. With an increase in integrated luminosity of only 10%, most of the improvement is due to the superior Belle II detector performance compared to previous B-factory experiments, and the improved selection criteria and analysis strategy.
Institute Pisa

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