Measurement of$ D^0-\bar{D}^0$ Mixing in $D^0 \to 𝐾_𝑆^0 \pi^+ \pi^−$ Decays and Measurement of $\Sigma$ Effective Electromagnetic Form Factors

Sumitted to PubDB: 2026-05-15

Category: Phd Thesis

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Principal Authors Guanda Gong
Date 2026-05-15
Belle II Number BELLE2-PTHESIS-2026-018
Abstract The phenomenon of a neutral meson oscillating into its anti-particle is called the neutral meson mixing. This process plays an important role in the study of flavor physics. Among the mesons that mix, the $D^0$ meson is the only open flavor meson of the up type quark. A precise measurement of the $D^0-\bar{D}^0$ mixing can serve as a probe to new physics beyond the standard model. $D^0\to K_S^0\pi^+\pi^-$ is the golden channel to measure the mixing parameter $x$. A model independent measurement for the $D^0-\bar{D}^0$ mixing parameters $x$ and $y$ is performed using the data collected by Belle and Belle II experiment, with integrated luminosities of $943~\mathrm{fb}^{-1}$ and $407~\mathrm{fb}^{-1}$ , respectively. Approximately $2\times 10^6~D^0$ mesons are reconstructed with the $e^+e^-\to\gamma^\ast \to c\bar{c}\to D^{\ast +} + X, D^{\ast +} \to D^0 \pi^+,D^0\to K_S^0 \pi^+\pi^-$ process. By a simultaneously fit to the decay-time and decay-time uncertainty distributions in 16 Dalitz-plot bins, the mixing parameters are measured to be $x=(3.6\pm 1.7 \pm 0.4)\times 10^{-3}$ and $y=(3.5 \pm 1.4\pm 0.3) \times 10^{-3}$ . The first error is statistical and second is systematic. This is the most precise result in an electron-positron collider. This is also the first model independent measurement result in an electron-positron collider. The study of strange baryons plays an important role in the understanding of strong interaction, strange quark and the neutron star. The electromagnetic form factors reveal the structure and dynamics of a strange baryon. Using a data sample of $980~\mathrm{fb}^{-1}$ collected by Belle experiment, the $e^+e^-\to \Sigma^0 \bar{\Sigma}^0$ and $e^+e^-\to \Sigma^+\Sigma^-$ cross sections from the $\Sigma\bar{\Sigma}$ mass threshold to $3~\mathrm{GeV}$ are measured. The $e^+e^-\to \Sigma^+\Sigma^-$ cross section in the $\sqrt{s}\in (2.4, 2.5)$ and $\sqrt{s} = (2.5, 2.6) ~\mathrm{GeV}$ regions are measured for the first time. The effective electromagnetic form factors are extracted. All the results are consistent with that of BESIII and BaBar. To validate the cross section measurements, the branching fractoins of $J/\psi \to \Sigma^0 \bar{\Sigma}^0$ and $J/\psi \to \Sigma^+ \Sigma^-$ are also measured. The results are consistent with world average values.
Institute USTC

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