ATLAS Level-1 Calorimeter Trigger:\nStatus and Development

The ATLAS Level-1 Calorimeter Trigger seeds all the\ncalorimeter\nbased triggers in the ATLAS experiment at LHC.\nThe inputs to the system are analogue signals of\nreduced\ngranularity, formed by summing cells from both the ATLAS Liquid\nArgon and Tile calorimeters.\nSeveral stages of analogue then\ndigital processing, largely performed in FPGAs, refine these\nsignals via configurable and flexible algorithms into identified\nphysics objects, for example electron, tau or jet candidates.\nThe complete processing chain is performed in a pipelined\nsystem at the LHC bunch-crossing frequency, and with a fixed\nlatency of about 1$\mu$s.\n\nThe first LHC run from 2009-2013 provided a varied and\nchallenging environment for first level triggers.\nWhile\nthe energy and luminosity were below the LHC design, the pile-up\nconditions were similar to the nominal conditions.\nThe physics ambitions of the experiment also tested the performance\nof the Level-1 system while keeping within the rate limits\nset by detector readout.\nThis presentation will show how the\nLevel-1 Calorimeter rose to this challenge, including the\nrate and efficiency for important physics channels.\nIt will illustrate how the flexibility of FPGAs was utilised to\nenhance the performance and that there are still more\nideas to improve the trigger performance during the current\nshutdown using the spare capacity and flexibility built into\nthe processing modules along with small hardware additions.\nFinally, the longer term upgrade plans will also be reviewed.