Thesis BELLE2-PTHESIS-2024-005

Measurement of Time-Dependent CP Violation in Decays of B0 → η′ K0S at the Belle II Experiment: Belle II 実験におけるB0 → η′ K0S崩壊過程での時間依存CP非対称度測定

BAE Hanwook ; USHIRODA Yutaka

The University of Tokyo Tokyo, Japan

Abstract: B0 → η′K0S channel is a decay mode of neutral B-meson, which is mediated by the penguin process of b → sq¯q of the Standard Model, and we can observe the CP violation phenomenon in the B0 → η′K0S channel. The CP violation can be parameterized by the CP asymmetries, which consists of two real parameters of ACP and SCP. The parameters can be obtained in a systematical discussion for the difference in the temporal decay rate between B0 and B0bar by analyzing the difference in a time-dependent fashion. Also, the CP violation is deeply related to the matter-antimatter asymmetry, which is one of the unsolved problems that the Standard Model cannot describe. We expect a new theory, called New Physics, beyond the Standard Model to solve this problem, and New Physics might modify phenomena of the CP violation in the B0 → η′K0S decay from those described by the Standard Model using the CKM matrix. We can explore New Physics by comparing measurements for parameters of the CP violation between those in B0 → η′K0S decay mode and tree-dominating one such as B0 → J/ψK0S decays. In this aspect, extracting CP parameters (ACP , SCP ) in B0 → η′K0S is a key measurement of Belle II, an experiment for particle physics with asymmetrically accelerated electron and positron. We measure the CP parameters in a sub-decay mode of η′ → ηπ+π−, η → π+π−π0, and K0S → π+π− of B0 → η′K0S channel at the Belle II experiment with experimental data accumulated between 2019 March and 2022 June corresponding integrated luminosity of 362 fb−1. For the measurement, we should consider the various factor of the reconstruction, such as the inaccuracy of decay time determination for B-mesons or effects from background events. We develop a resolution function and method to calibrate and validate the function parameters using the experimental data. In detail, we developed a resolution function that considers the inaccuracy in determining the decay time and is an essential component for CP asymmetry studies for all the subchannels of the B0 → η′K0S channel. A method to calibrate the parameters of the resolution function is also devised, which has more advantages than conventional calibration methods and is an original methodology developed in this study firstly. The measured CP parameter (ACP, SCP ) from this study is ACP = −0.111+0.319−0.311(stat)+0.032−0.031(syst), and SCP = +0.248+0.470−0.526(stat) +0.053−0.053 (syst). We confirm that the measured CP parameters are consistent with the world average values of B0 → η′K0 decay or that of B0 → c¯cK0 decay considered reference values of the Standard Model prediction, within the uncertainty. We also found that when measuring CP asymmetries using all subchannels in the B0 → η′K0S channel by the method developed in this study, the statistical error of ACP and SCP at 50 ab−1, the target luminosity of Belle II, was 0.01 or less, which is lower than the current theoretical prediction of CP asymmetries in the channel.

Note: Presented on 19 07 2023
Note: PhD

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 Record created 2024-03-12, last modified 2024-03-12

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