Precision Timing of the ATLAS Level-1 Calorimeter Trigger

The ATLAS Level-1 Calorimeter Trigger is one of the main elements of the first-stage online selection of LHC collision events measured at the ATLAS experiment. Using 7168 pre-summed trigger tower signals from the Liquid Argon and Tile calorimeters as input, the hardware-based system identifies high-pT objects and determines the total and missing transverse energy sums within a fixed latency of 2.5 us. The Preprocessor system digitizes the analogue calorimeter signals at the LHC bunch-crossing frequency of 40MHz and provides bunch-crossing identification and energy measurement. Prerequisite for high stability and accuracy of this procedure is a timing synchronization at the nanosecond level of the signals which belong to the same collision event. The synchronization of the trigger tower signals was first established in the analysis of beam splash events in November 2009 and then refined and sustained with data from proton-proton collisions at a centre-of-mass energy of 7TeV, recorded at the LHC in 2010 and 2011. In order to extract nanosecond precision from signals sampled in 25 ns-steps, a specifically developed fit method was applied to the signals from the energy depositions in the calorimeters. This procedure requires a good understanding of the signal shapes and, especially, its variations between the different calorimeter regions. Overall, a precision synchronization with collision data better than, on average, approximately ± 1.5 ns has been achieved throughout 2010 and 2011. In this talk, details on the method as well as selected results from the different synchronizations with beam splashes and proton-proton collisions from 2009 to 2011 are presented.