ATLAS Level-1 trigger timing-in strategies

The ATLAS detector at CERN’s LHC will be exposed to proton-proton collisions at a bunch-crossing rate of 40 MHz. In order to reduce the data rate, a three-level trigger system selects potentially interesting events. Its first level is implemented in electronics and firmware, and aims at reducing the output rate to under 100 kHz. The Central Trigger Processor (CTP) combines information from the calorimeter and muon trigger processors, and makes the final Level-1 Accept (L1A) decision, which is transferred to all sub-detector front-ends. The functioning of the Level-1 Trigger is based on the correct timing of the signals. In this paper we present various strategies for sub-detector timing-in, in particular how to arrive at a decent initial timing setup using test-pulses in standalone mode, and in global mode with the CTP. In addition we describe how the beam pick-up detectors are a powerful tool to further refine the timing with bunches in the LHC machine. In this context we describe new developments on a proposal for precision read-out of the ATLAS beam pick-up detectors with commercial oscilloscopes in order to monitor the phase of the clock with respect to the LHC bunches. I. I NTRODUCTION