Thesis BELLE2-PTHESIS-2021-011

Development of the Aerogel RICH counter for a super B-factory experiment

Shuichi Iwata

Tokyo Metropolitan University Hachioji

Abstract: The framework to explain various phenomena in elementary particle physics, known as the standard model (SM), had completely been established by the experimental discoveries, such as the CP violation in a B decay system and the existence of a Higgs boson expected by the SM. The next aim in elementary particle physics is a search for the new physics (NP) beyond the standard model. As one of the approach of explore NP, a super B-factory experiment, which is named as Belle II experiment, is in construction stage at High Energy Accelerator Research Organization (KEK), Tsukuba, Japan. Belle II is the successor to Belle II is an asymmetric energy e+e− collider experiment, which uses the superKEKB accelerator, and the Belle II spectrometer. That is the project upgraded from the Belle experiment that contributed to discover the CP violation in B meson system and achieved the world record of the highest peak luminosity. The aims of a super B-factory experiment is to search for NP and to investigate properties e.g. flavor structure of NP through a measurement of CP asymmetry and/or the branching fraction of rare B decays, which had been difficult to be measured in the previous B-factory experiments. For an example of the rare B decay, B ! and B ! K∗ are strongly suppressed in the SM because these processes are the Flavor Changing Neutral Current (FCNC) process, that is forbidden at the tree level diagram in the SM. They are, therefore, one of the good probe to verify the existence of NP. In order to discuss the effect of NP from those experimental result, we have to suppress the various experimental uncertainty at a super B-factory experiment. Due to this requirement, a super B-factory commonly requires a high precision particle identification (PID) device. The =K separation with high precision is required to discriminate B ! and B ! K∗ efficiently as they decays into and K respectively. As a new PID device for the forward end-cap region of the Belle II spectrometer, an proximity-focusing type Ring Imaging Cherenkov (RICH) counter with a silica aerogel as a radiator, which is named Aerogel RICH (ARICH) counter, has been developed. The ARICH counter is designed to provide the =K separation at 4 level in the wide momentum range up to about 3:5 GeV=c. In particular, we aim to perform identification with a few % inefficiency and with a few % K fake probability at 3:5 GeV=c, which is the highest momentum xof daughter particles of a rare B decay. The ARICH counter have 144-ch multi-anode Hybrid Avalanche Photo Detectors (HAPDs) as position sensitive photon detector. One of the important concern in the ARICH development had been the radiation hardness of an HAPD. We had studied and improved the HAPD to sustain the radiation damage during the 10-year Belle II operation of the expected fluence (< 1012 one MeV-equivalent neutrons=cm2) and the dose (< 100 Gy for -ray). As the final step of the development, we constructed a prototype ARICH counter using the developed HAPDs and large size aerogel tiles. We verified the performance of the prototype ARICH counter using a beam test, and succeeded to observe the Cherenkov ring images for 5 GeV=c electrons. As a result, we obtained the number of the detected photoelectrons and Cherenkov angle resolution to be 10.71 photoelectrons per a track and 14:47 mrad, respectively. In order to estimate the PID performance of the ARICH counter at Belle II, we demonstrated an event-by-event analysis based on the likelihood method for the beam test data. We defined the probability density functions (PDFs) for distribution of the accumulated Cherenkov angle and the number of detected photoelectrons per a track. In order to emulate =K identification of the ARICH counter, we prepare the PDFs for signal and background assumption, and calculated likelihoods for an event of the beam test. Finally we estimate the identification efficiency " as 97.4% with the K fake probability as 4.9% at 3:5 GeV=c. We demonstrated that these results were mostly acceptable to our target performance. In this thesis, we present the development of the ARICH counter especially the study and improvement of the HAPD, and estimation of the PID performance through a beam test using the prototype ARICH counter.

Note: Presented on 28 02 2016
Note: PhD

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 Record created 2021-06-29, last modified 2021-06-29

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