A New Scheme for the Injection Veto System of the Belle II Experiment

• Main Authors: Bae, H., Koga, T., Kusudo, A.
• Format: Conference Proceeding
• Language: English
• Subjects: Colliding beam accelerators; Data acquisition; Microwave integrated circuits; Positrons; Semiconductor detectors; Storage rings; Timing
• ISSN: 2577-0829
• DOI: 10.1109/NSS/MIC/RTSD57108.2024.10658627

The Belle II experiment is a high-energy particle physics experiment using the SuperKEKB accelerator at Tsukuba-shi, Japan, an asymmetric electron-positron accelerator with the world's highest luminosity. In the accelerator, a linear accelerator makes two beams of energetic electrons (7 \mathrm{GeV}) and positrons (4 \mathrm{GeV}), which are injected into a storage ring with a 3km circumference to store and collide the beams. Especially, SuperKEKB injects the beams even during the physics data taking is ongoing in Belle II, which maintains the beam current always maximally and realizes the highly efficient data taking of Belle II. However, we have inefficiency and loss of a part of the beams in the injection, which turns out background events for the Belle II detector. We adopt a veto system for the detector called "injection veto" to prevent the effects of the injection backgrounds. This system pauses the data acquisition system of Belle II at the time of arrival of the injection backgrounds at the detector along revolutions of the injected beams in the storage ring. The current injection veto scheme uses the beam injection signal and the revolution timing information only, which makes the redundant deadtime of about 15% at maximum for the data acquisition system of Belle II. To solve this problem, we developed a new scheme for the injection veto on the L1 trigger system implemented with FPGA. The scheme uses the information from a wire chamber and an electromagnetic calorimeter of Belle II, which are sensitive to charged particles and gammas. The injection backgrounds generate those particles many times more than the typical collision events. Thus, the new scheme issues the pausing signals to the detector only if the hit number from the subdetectors is above the threshold, in addition to the pausing signals from the current scheme. We began using the new scheme in 2024 and confirmed it halved the deadtime. We will discuss the details of this improvement in the presentation.