Precision Synchronization of the ATLAS Level-1 Calorimeter Trigger with Collision Data in 2010 and 2011

The ATLAS Level-1 Calorimeter trigger (L1Calo) selects LHC collision events based on the identification of high pT-objects like electrons, jets and taus as well as the determination of total and missing ET in the Tile and Liquid Argon Calorimeters. Operating at 40MHz LHC bunch-crossing frequency, the hardware based L1Calo system processes 7168 so-called Trigger Tower (TT) signals from the calorimeters. Synchronizing these TT signals as well as maintaining and refining the L1Calo synchronization are important measures to ensure a stable and reliable functioning of the ATLAS trigger system, including high Level-1 trigger efficiencies. The fit method for L1Calo precision synchronization emulates the analogue calorimeter signal shape on digitized TT pulses to derive the required synchronization settings. Systematic tests have shown the validity of the method within a statistical and systematical accuracy of +-3 ns, well within the required precision for bunch-crossing identification and Level-1 energy measurement. Over the course of 2010 and 2011, 6 updates of the L1Calo precision synchronization plus 3 additional cross-checks based on the fit method were conducted for data from pp collisions at center-of-mass energies of 7 TeV. Since the first update in July 2010, the mean required timing adjustments per calorimeter layer have been ~< +-1.5 ns with the average spread between TTs of one calorimeter layer decreasing to around 1 ns towards the end of 2011. Thus, the fit method for L1Calo timing has enabled a very stable precision synchronization of the L1Calo trigger system well within the required accuracy limits throughout 2010 and 2011.