Search for the radiative decay of $\Upsilon$(10753) and integrated luminosity measurement at Belle II experiment

Sumitted to PubDB: 2026-06-17

Category: Master Thesis

Tags: Run 2 (partial)

Principal Authors Weitao Xiong
Date 2026-06-17
Belle II Number BELLE2-MTHESIS-2026-050
Abstract $\Upsilon(10753)$ is a new structure discovered at the Belle experiment in 2019. Extensive research has been conducted on its internal structure and properties, with various theoretical interpretations emerging, including conventional bottomonium, hybrid, or tetraquark states. The study of $\Upsilon(10753)$ not only reveals its internal structure but also tests theories such as Quantum Chromodynamics. The Belle II experiment, as a high-precision frontier experiment in high-energy physics, has collected a large data sample around 10.75 GeV (near the mass of $\Upsilon(10753)$) to conduct more in-depth studies of this new structure. Using this data sample around 10.75 GeV, this paper investigates the radiative decay of $\Upsilon(10753)$, focusing on the processes $e^+ e^- \to \gamma \chi_{bJ}$ ($J$ = 0, 1, 2) at center-of-mass energies $\sqrt{s}$ = 10.653, 10.701, 10.746, and 10.804 GeV. By optimizing event selection criteria, including charged track selection, photon reconstruction, $\Upsilon(1S)$ reconstruction, and kinematic fitting, we fit the $\gamma\Upsilon(1S)$ mass spectrum and do not observe significant signal. We set upper limits on the Born cross sections for the processes $e^+ e^- \to \gamma\chi_{bJ}$ ($J$ = 0, 1, 2) at $\sqrt{s}$ = 10.653, 10.701, 10.746, and 10.804 GeV at the 90\% confidence level. These results provide important insights into the internal structure of $\Upsilon(10753)$. Additionally, we performed an offline measurement of the integrated luminosity for the Belle II experiment data set. Integrated luminosity, which quantifies the size of a data sample, is fundamental to all analyses. Precise measurement of the integrated luminosity enhances the accuracy of other Belle II analyses. In this paper, we measure the integrated luminosity of the data collected by the Belle II detector from 2024 to 2025 (early Run 2 data) using three highly precise Quantum Electrodynamics processes: Bhabha scattering ($e^+ e^- \to e^+ e^- (n\gamma)$), digamma process ($e^+ e^- \to \gamma \gamma (n\gamma)$), and dimuon process ($e^+ e^- \to \mu^+ \mu^- (n\gamma)$). By taking the weighted average of the three independent measurements, the integrated luminosity of the early Run 2 data set is determined to be 150.82 $\pm$ 0.73 fb$^{-1}$, successfully achieving a total relative uncertainty below 0.5\%. This work provides a crucial data foundation for subsequent physics analyses at Belle II.
Institute Fudan

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