Installation of a Cosmic-Ray Trigger System to Commission the Belle II Experiment VerteX Detector with Cosmic Rays

In this paper, we describe the design, construction, testing, and installation of the cosmic ray trigger system used to commission the VerteX Detector (VXD) of the Belle II experiment at the High-Energy Accelerator Research Organization (KEK) in Japan. The system consists of two rows of scintillators; with six scintillators being on top of the VXD, and six at the bottom of it. The scintillators were characterized (plateaus, threshold values, coincidences rate), and when compared with the simulation values a concordant match was found for all cosmic coincidence measurements. In Phase 3 of SuperKEKB accelerator, the VXD was the last sub-detector to be integrated into the heart of Belle II detector, after it was absent from being integrated within Belle II in Phase 1 and Phase 2 of SuperKEKB machine studies. A system consisting of VXD prototype parts had been installed, into Belle II detectors, at the VXD location during Phase 2 to study background and it was found that the VXD can cope with the level of measured background: allowing the VXD to be present during Phase 3 Belle II data taking. The VXD was tested with cosmic rays outside of Belle II before it was integrated within Belle II just before Phase 3 data taking, which commenced in March 2019.