CLAWS - a novel time resolved study of backgrounds during the first commissioning phase of SuperKEKB

Sumitted to PubDB: 2021-04-27

Category: Phd Thesis, Visibility: Public

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Authors Allen Caldwell, Miroslav Gabriel, Stephan Paul
Non-Belle II authors Nora Brambilla
Date Jan. 1, 2019
Belle II Number BELLE2-PTHESIS-2021-004
Abstract Abstract The asymmetric e+e􀀀 collider SuperKEKB is a second generation flavor factory designed to achieve an unprecedented luminosity of 8 1035 cm􀀀2s􀀀1, a factor 40 higher than its record-breaking predecessor KEKB. This ambitious design luminosity is expected to cause challenging levels of beam backgrounds for various subsystems of the corresponding Belle II experiment, in particular for its pixel vertex detector. Understanding and mitigating these beam backgrounds early on is critical for the successful operation of SuperKEKB and Belle II. In particular, backgrounds related to continuous top-up injections of new particles can not be simulated with sufficient accuracy and have to be determined by direct measurements. Phase 1 of the commissioning of SuperKEKB in 2016 focused on the basic operation of the accelerator and thus the detector and the final focusing systems were not installed and no collisions took place. A suite of dedicated beam background detectors collectively known as BEAST II used this collision-free environment to study beam-induced backgrounds. One of these detectors is the CLAWS experiment. This thesis describes the sensor technology and the overall setup of CLAWS and presents the measurements of beam-induced backgrounds it performed. The detector system consists of eight plastic scintillator tiles with directly coupled SiPMs read out continuously over up to several milliseconds. The sub-nanosecond time resolution and single particle energy resolution of the sensors allow bunch-by-bunch measurements, enable CLAWS to perform a novel time resolved analysis of beam backgrounds and make it uniquely suited for the study of injection backgrounds. We present measurements of various aspects of regular and injection backgrounds which include particle composition of regular backgrounds, time structure and decay behavior of injection backgrounds, hit energy spectra and overall background rates. These measurements show that in both rings the majority of the injection backgrounds are typically observed withing the first 500 μs after the injection. Another major finding is that the time structure of injection backgrounds is determined by different timing patterns connected to properties of the accelerator, such as betatron and synchrotron oscillations. We directly determine the frequencies of these patterns from detector data, mostly with sub-nanosecond precision. In addition, beam-gas and Touschek backgrounds are measured in a dedicated background study and are compared to simulations. We find a clear excess in data over simulation which ranges from a factor three up to two orders of magnitude. The studies presented in this thesis make several noteworthy contributions to advancing the understanding of beam-induced backgrounds and injection mechanisms in high luminosity flavor factories. The described sensor technology and analysis methodology also serve as the basis for upgraded versions of the CLAWS detector system which will monitor beam backgrounds during Phase 2 and full physics operation of SuperKEKB.
Conference Munich

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