Yuma Uematsu ; Yutaka Ushiroda

2020
The University of Tokyo Tokyo

Abstract: The Belle II experiment at KEK, Tsukuba, Japan, is conducted to search for new physics beyond the Standard Model (SM) through the precise measurement of the SM parameter in the flavor physics. For this purpose, we measure e+e− collision events using the high luminosity accelerator SuperKEKB. To precisely measure the decay vertices, we locate the Vertex Detector (VXD) at the innermost part of the Belle II detector. VXD is a silicon tracker composed of two types of detectors; (1) a pixel detector and (2) a double-sided microstrip detector (Silicon Vertex Detector, SVD). This thesis presents studies on SVD to improve data readout and track reconstruction. The stable operation of SVD with a large amount of beam background is challenging: studies in the first operation of VXD reveal the higher background hit-rate than our expectation, which affects both online data-taking and offline analysis at the design luminosity. Firstly, the online data rate exceeds the data bandwidth. Secondly, the offline track finding becomes difficult at such a high occupancy of background. To expand these limitations, we study modifications for each. For the former, we reduce the number of sampling points of the silicon-strip hit signals, thereby reducing the data rate. For the latter, we newly develop a hit selection method using the hit-time information, which is currently not used. This hit-time cut selectively removes the background hits because the hit-time of backgrounds is uncorrelated to that of physics signals. We review the possible effects of these two modifications and confirm their validity, analyzing the first physics data taken in the spring of 2019, with VXD fully installed. Then, we evaluate the performance of the modifications in the safety factor of the above limitations for the simulated background, using the Monte Carlo simulation with various amounts of background hits. As a result, we increase the safety factor from 1.5 to almost 5 with these modifications, which ensures the stable operation of SVD at the design luminosity.

Note: Presented on 21 01 2020
Note: MSc