Level-1 Calorimeter Trigger starts firing

29 November 2007

Processor

The complete processor system


L1Calo is one of the major components of ATLAS First Level trigger, along with the Muon Trigger and Central Trigger Processor. It forms all of the first-level calorimeter-based triggers, including electron, jet, tau and missing ET. The final system consists of over 250 custom designed 9U VME boards, most containing a dense array of FPGAs or ASICs.

It is subdivided into a PreProcessor, which digitises the incoming trigger signals from the Liquid Argon and Tile calorimeters, and two separate processor systems, which perform the physics algorithms. All of these are highly flexible, allowing the possibility to adapt to beam conditions and luminosity. All parts of the system are read out through Read-Out Drivers, which provide monitoring data and Region of Interest (RoI) information for the Level-2 trigger.

Plot of correlation between L1Calo and TileCal energies in M4

Production of the modules is now essentially complete, and enough modules exist to populate the full scale system in USA15. Installation is proceeding rapidly - approximately 90% of the final modules are installed and the cabling is almost complete. Installation and testing has concentrated initially on the barrel region of the calorimeters, and much useful information and data have already been derived from this region. Commissioning will soon move into the endcap regions as the final hardware appears. The complete system should be in place and cabled up by the end of 2007.

Integration into the ATLAS combined runs is progressing well. L1Calo first took part in M3 with a reduced system, but things got more serious in M4 with complete readout for the digitised barrel trigger signals, and most of the processing system for the barrel. Despite early problems, M4 ended well with stable running over the final weekend, during which the first evidence of signals from cosmic muons were seen in the data.

M5 proved even more successful. The barrel region was fully instrumented for digitisation, trigger processing, readout and Region of Interest generation. The digital part of the system was approximately timed in and almost fully functional. RoIs were sent to the Region of Interest Builder and interpreted by the Level-2 trigger. Progress was made on lining up the timing of both the input signals from the calorimeters, and output signal to the CTP. For a small region of the acceptance, the L1Calo trigger signals at the CTP were timed in to coincide with the Tile trigger currently used to trigger calorimeter events in ATLAS. By the time of the final weekend of M5, L1Calo was able to produce triggers on high energy cosmic events, and was included in the trigger menu for the weekend run.

L1Calo triggered cosmic event from a weekend combined run

As commissioning continues, we will increase our understanding of the performance of the trigger, and integrate more closely with the calorimeter systems. L1Calo has already become a stable part of the regular weekend runs performed by the calorimeters, and is now providing trigger inputs for those runs, enhancing the cosmic trigger capability, and helping to provide data from calorimeter commissioning. An event triggered by L1Calo in a weekend cosmic run is shown in the ATLANTIS picture above, showing energy deposited in both the Liquid Argon and Tile Calorimeters. We expect to learn much from combined runs with the calorimeters in preparation for LHC data next year.Colin Barras

 

Stephen Hillier

University of Birmingham