Study of charmonium decay and $CP$ violation in $B$ meson charmless decay

Sumitted to PubDB: 2025-10-10

Category: Phd Thesis

Tags: Run 1

Principal Authors Xiantao Hou
Date 2025-10-10
Belle II Number BELLE2-PTHESIS-2025-022
Abstract This thesis focuses on studies of charmonium decays at the BESIII experiment and investigations of $CP$ violation in charmless $B$ meson decays at the Belle II experiment. Using a data sample of 2.93 fb$^{-1}$ collected at $\sqrt{s}=3.773$ GeV with BESIII, we performed the first search for the scalar partner of the $X(3872)$, namely the $X(3700)$, through the radiative decay $\psi(3770)\to\gamma\eta\eta'$. No significant signal was observed. Upper limits at the 90% confidence level on $\mathcal{B}(\psi(3770)\to\gamma X(3700))\cdot\mathcal{B}(X(3700)\to\eta\eta')$ were determined to be $(0.9$--$1.9)\times10^{-5}$, depending on the assumed $X(3700)$ mass. We also searched for the decay $X(3700)\to\pi^+\pi^- J/\psi$, motivated by possible isospin-violating effects analogous to those in $X(3872)\to\pi^+\pi^- J/\psi$, but no significant signal was found. The corresponding upper limits on $\mathcal{B}(\psi(3770)\to\gamma X(3700))\cdot\mathcal{B}(X(3700)\to\pi^+\pi^- J/\psi)$ were determined to be $(0.9$--$3.4)\times10^{-5}$ at the 90% confidence level. Using BESIII data collected at center-of-mass energies between 3.508 and 4.951 GeV, we measured the cross section of $e^+e^- \to \phi\eta'$, obtaining values ranging from $0.71\pm0.20$ pb to $7.46\pm1.20$ pb, significantly lower than the predictions of the Nambu--Jona-Lasinio (NJL) model. The fitted energy dependence yields a power-law parameter of $n=4.35\pm0.14$, consistent with the NJL prediction within uncertainties. We also searched for the non-charmed decay $\psi(3770)\to\phi\eta'$, and obtained an upper limit of $\mathcal{B}(\psi(3770)\to\phi\eta')<2.3\times10^{-5}$ at the 90% confidence level. Measurements of $CP$ violation in charmless neutral $B$ meson decays provide an important test of the CKM mechanism and sensitivity to possible new physics. Using Belle II generic Monte Carlo samples and all available data collected at the $\Upsilon(4S)$ resonance, we studied the sensitivity to the $CP$ violation parameters in the decay $B^0\to\rho^+\rho^-$. A multidimensional fit to six observables was performed to extract the branching fraction, longitudinal polarization fraction, and $CP$ violation parameters. Using a 1 ab$^{-1}$ generic MC sample, we obtained $\mathcal{B}(B^0 \to \rho^+ \rho^-) = (\times\times \pm 0.14 \pm 0.15)\times10^{-5}$, the longitudinal polarization fraction $f_{L} = \times\times \pm 0.016 \pm 0.003$, and the $CP$ violation parameters $\mathcal{A}_{CP} = \times\times \pm 0.08 \pm ^{+0.04}_{-0.01}, \mathcal{S}_{CP} = \times\times \pm 0.11 \pm 0.02$, where the first uncertainties are statistical and the second are systematic. In this study, machine learning techniques were employed to further suppress the continuum background. Compared with the luminosity-scaled results from the Belle and BABAR experiments, the precisions of $\mathcal{S}_{CP}$ and $\mathcal{A}_{CP}$ were improved by 14% and 7%, and by 10% and 13%, respectively.
Institute IHEP-China

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