Claws - An Injection Background Monitoring System for the Second and Third Phase of the SuperKEKB Commissioning

The Belle 2 experiment is a collider-based high energy physics experiment at the intensity and precision frontier and is based at the only collision point of the SuperKEKB collider located at the KEK research institute in Tsukuba, Japan. The B-factory circulates electrons and positrons at asymmetric beam energies of 7 GeV and 4 GeV, respectively, and aims at reaching the world highest instantaneous luminosity of 6 · 1035 cm−2 s−1 within a decade of commissioning in 2025. Substantially decreasing beam currents due to collisions are compensated by top-up injections at 50 Hz. These injections cause a considerable injection induced beam background at the interaction point (IP) eventually decreasing the overall performance especially of the Belle 2 vertex detectors. A detailed knowledge of the background conditions at the IP is the basis for a successful mitigation of the injection induced beam background. The development and performance of a dedicated injection background monitoring system for SuperKEKB is presented in this thesis. The Claws system is developed in two distinct detector designs for the Phases 2 and 3 of the SuperKEKB commissioning campaign which started at the end of March 2018. The detector design consists of a scintillating tile with silicon photomultiplier read-out detecting mostly charged particles. In Phase 2, two detectors ladders with 8 independent channels each have been placed in 22 mm distance around the interaction point inside the Belle 2 detector. A C++-based DAQ software applies a fast reconstruction and other analyses to the recorded waveforms featuring up to 10 000 continuous particle revolutions at a sampling rate of 1.25 GHz. The analysis results are displayed with an update rate of around 1 Hz in the SuperKEKB and Belle 2 control rooms considerably improving the understanding of the background conditions at the IP. For Phase 3, a total of 32 single-channel newly developed Claws modules are placed at multiple locations on the final focussing magnets in the interaction area and read out by an extended DAQ system. Furthermore, this thesis investigates the capabilities of the developed detector systems and studies the injection induced background behavior using the data collected in Phase 2. It is shown that the Claws system achieves a time resolution of (347 ± 2) ps for 10 MIP signals and a MIP response stability of −3.0 % to +5.5 % over the course of 3.5 month. This precision allows studying bunch-by-bunch beam behavior and enables performing advanced time dependent analyses. The thesis demonstrates in detail the potential of Claws of detecting minimal irregularities of the beam which builds the basis for the present use of the system as a beam abort system substantially outperforming all previous systems