The trigger system of the COMPASS experiment

In the following article the trigger system of the CERN COMPASS experiment is presented. Special emphasis is put on the so called Quasi Real Photon Trigger which was designed to select muon scattering events with a relative energy loss in excess of a certain threshold y_min over the full range of momentum transfers (Q²) down to the quasi real limit (Q² ≈ 0). This trigger system consists of two stages to achieve high selectivity. The first stage is composed of pairs of plastic scintillator hodoscopes which select the scattered muons by measuring their energy. The second stage comprises a hadron trigger based on the two hadron calorimeters of COMPASS. This system detects hadrons in coincidence with the scattered muon to reduce background stemming from radiative andμ-e-scattering events. The Quasi Real Photon Trigger which can be considered as a tagger for quasi real photon events is complemented by larger standard trigger hodoscope systems covering the region of (Q² > 1 GeV²).