Masanobu Yonenaga ; Hidekazu Kakuno

2020
Tokyo Metropolitan University Hachioji

Abstract: The Standard Model (SM) has been completed by observing the Higgs boson which is a last particle expected by the SM in 2012. However, the SM cannot explain non-zero neutrino mass, existence of dark matter and lack of gravity and so on. Therefore, we need the new physics (NP) beyond the SM to explain those things. The energy frontier experiment which directly produce the NP particles using high energy collision doesn’t find the sign of the NP so far, and the NP is expected to exist in the more high energy region. The luminosity frontier experiment which find the signature of NP in the intermediate state can reach more high energy region by precise measurement, and expected to discover the NP. The Belle II experiment is a luminosity frontier experiment and is searching NP by studying CP asymmetry and rare decays of B and D mesons and τ leptons using high statistic data. Discrimination for CP eigenstate and rare decay process in wide momentum region is required to study precisely. Therefore, particle identification has an important role to measure rare decays with high precision to search NP. We have developed a new particle identification device in the forward end-cap region at the Belle II spectrometer called the aerogel Ring Imaging Cherenkov (ARICH) counter to realize particle identification in high accuracy for charged particles with momenta up to 4 GeV/c. The ARICH counter consists of a silica aerogel layer as the radiator and Hybrid Avalanche Photo Detector (HAPD) as the photon detector. The construction was completed in October 2018 and the ARICH counter was installed into the Belle II spectrometer in December 2018. We evaluated the particle identification performance of ARICH using D∗ → D0(→ Kπ)π decays with early beam collision data which correspond to an integrated luminosity of 5.15 ab−1. The result of overall performance of ARICH shows that K(π) efficiency is 93.5 ± 0.6% (87.5 ± 0.9%) with π(K) misidentification probability of 10.9±0.9%(5.6±0.3%). We also evaluated the dependance of the performance on the particle momentum or polar angle, and the results demonstrate that ARICH has capability to identify particles for all regions. A b → s(d)γ process induced by the Flavor Changing Neutral Current (FCNC), and is forbidden at the tree level diagram and occur through a loop level diagram in the SM. The particles indicated in new physics beyond the SM such as charged Higgs boson and supersymmetric particles can contribute in the loop, therefore, the process is a good probe to search new physics. b → s(d)γ process can be observed in Bmeson decays such as B → ρ(→ ππ)γ and B → K∗(→ Kπ)γ. We search B → K∗ γ decays using the early Belle II data collected between March and May in 2019 which corresponds to an integrated luminosity of 2.62 fb−1, aiming to demonstrate that the Belle II operation in total. The result shows that the number of signals are 19.1 ± 5.2, 9.8 ± 3.4 and 6.6 ± 3.1 for B0 → K∗0(→ K+π −)γ , B+ → K∗+(→ K+π0)γ and B+ → K∗+(→ K0S π+)γ , respectively. This results are consistent with a Monte Carlo simulation of Belle II and the world average. The combined significance of the B → K∗ γ decays is 6.2 σ, and we rediscover the B → K∗ γ decays with the significance more than 5 σ. We demonstrate that the Belle II has capability to search rare B decays as expected through rediscovering B → K∗ γ decays.

Note: Presented on 31 08 2020
Note: PhD