Trigger system of the NA61/SHINE experiment at the CERN SPS

This paper describes technical aspects and details of the trigger system of NA61/SHINE experiment. The SPS Heavy Ion and Neutrino Experiment (SHINE) is a large acceptance hadron spectrometer designed for comprehensive studies of hadron production in hadron-proton, hadron-nucleus and nucleus-nucleus collisions at the CERN Super Proton Synchrotron. The NA61/SHINE physics programme requires measurements of a large number of reactions recorded using different trigger conditions. This motivated a flexible solution for the NA61/SHINE trigger system. The trigger uses signals from scintillation, Cherenkov and calorimeter detectors. In total up to 16 signals have to be processed for a trigger decision. The core of the system is based on a single field-programmable gate array (FPGA), running with a 120 MHz clock which offers a resolution of 8.3 ns. Moreover, it is capable of parallel selecting events which satisfy up to four trigger definitions. Events corresponding to these trigger definitions are recorded with relative frequencies which can be selected using 12 bit prescalers. The working parameters of the trigger, such as coincidences and delays, are set up remotely via a Java application, designed to be handled by users. The performance parameters are monitored by Python based monitoring software, which is capable of displaying values of beam counters and beam time structure. Furthermore, the monitoring software calculates summary information, such as trigger probabilities, which are crucial from a user´s point of view. It also detects pre-failure states of the trigger system, such as false scaler counts, and gives the opportunity to prepare the necessary repair procedure before reaching an irrecoverable state.