Study of the trigger mode of LHAASO-KM2A

The Large High Altitude Air Shower Observatory (LHAASO), built 4410 m a.s.l. near Daocheng, in the Sichuan province of China, is a new-generation complex EAS array. The Square Kilometre Array (KM2A), one of LHAASO's three detector arrays, is composed of 5195 electromagnetic particle detectors and 1171 muon detectors, which are distributed in an area of 1.3 km2. A lower trigger threshold would increase the number of low-energy showers detected, although this would also import many noise events because of the numerous KM2A detectors. In this work, the KM2A trigger logic adopted a software trigger system. A toy simulation was developed to estimate the chance coincidence rate of noise hits. Taking into account the characteristics of real cosmic ray shower events, the trigger logic was investigated by adjusting the space and time windows simultaneously using the detector simulation sample. In order to improve trigger efficiency and reduce the effect of noise, an optimized trigger criterion was achieved. The KM2A threshold energy can be reduced to 5 TeV. The performance of KM2A with this trigger criterion is presented. The expectation on observation of sun shadow and cosmic ray large-scale anisotropy are also shown.