ATLAS Level-1 Calorimeter Trigger: Status and Development

The ATLAS Level-1 Calorimeter Trigger seeds all the calorimeter-based triggers in the ATLAS experiment at LHC. The inputs to the system are analogue signals of reduced granularity, formed by summing cells from both the ATLAS Liquid Argon and Tile calorimeters. Several stages of analogue then digital processing, largely performed in FPGAs, refine these signals via configurable and flexible algorithms into identified physics objects, for example electron, tau or jet candidates. The complete processing chain is performed in a pipelined system at the LHC bunch-crossing frequency, and with a fixed latency of about 1us. The first LHC run from 2009-2013 provided a varied and challenging environment for first level triggers. While the energy and luminosity were below the LHC design, the pile-up conditions were similar to the nominal conditions. The physics ambitions of the experiment also tested the performance of the Level-1 system while keeping within the rate limits set by detector readout. This presentation will show how the Level-1 Calorimeter rose to this challenge, including the rate and efficiencies for important physics channels. It will illustrate how the flexibility of FPGAs was utilised to enhance the performance and that there are still more ideas to improve the trigger performace during the current shutdown using the spare capacity and flexibility built into the processing modules along with small ha rdware additions. Finally, the longer term upgrade plans will also be reviewed.