The trigger system for the MEG II experiment

Intending to improve the current sensitivity on μ→eγ decay by one order of magnitude, the MEG II experiment at Paul Scherrer Institute completed the integration phase in 2021 with all detectors successfully operated throughout the subsequent beamtime. Earlier in 2021, the WaveDAQ integrated Trigger and Data Acquisition (TDAQ) system, developed for the readout of the experiment, was completely commissioned. Receiving almost 9000 channels from the detectors, the MEG II TDAQ system is the largest WaveDAQ deployment so far, proving the scalability of the overall design, from bench-top setup through various smaller-size experiments. We will describe how MEG II trigger system reduces the ∼107 muon decays at the experiment target down to a 10 Hz event rate by exploiting the signal event characteristics at the online level. The trigger system performs the calorimetric reconstruction of the photon shower and then compares the timing and direction with positron candidates within a 600 ns hard latency time. The first release of the online reconstruction, deployed in 2021, achieved a 2.4% photon energy resolution at the signal energy of 52.8MeV and a ∼2ns coincidence time resolution among the child particles.