The ATLAS Trigger

Students and staff from RHUL are involved in a wide range of work on the ATLAS Trigger, from developing software to select electrons in a few milliseconds, to day-to day online operation of the trigger.

Cartoon_of_the_ATLAS_trigger_system Trigger_computers_in_racks The ATLAS trigger has to sift through the debris of LHC proton bunch collisions that occur at up to 40 million times per second, and select only a few hundred per second of these 'events' to save for further analysis. The rest are lost for ever. So, no pressure then!

ATLAS has designed a three-level trigger system to perform this challenging task. It was designed, built and commissioning by a community of over 500 people from universities and labs all over the world, including RHUL. The first level is realised in custom-built electronics (FPGA and ASIC in case you're an interested electronics expert). The higher level trigger (HLT) is implemented as software running on a network of around 2300 computers. We've been developing several parts of this software, including the framework and some of the algorithms.

Inside_the_ATLAS_control_room_-_on_shift RHUL staff and students regularly play their part in operating the trigger, with expert roles and shifts in the Atlas Control Room. Special training is given so you can sit at the trigger desk all night and know what to do when the trigger rate goes up - or down - unexpectedly.

RHUL staff have a long standing involvement in the management of the Atlas Trigger, with leadership in the areas of software coordination and trigger algorithms. We have also represented the interests of several key physics groups in the trigger menu coordination group, where the high-level strategies for the trigger are decided.

The trigger software project has about a million lines of C++/Python code, ~50 active developers and we build about 20 releases per year. In a typical week we have ~100 open bugs and 50 opened/closed. RHUL staff coordinate this work and develop many of the tools and procedures for managing and validating all this the software.


Trigger algorithms are developed though collaboration between trigger experts, reconstruction software developers and physics analysis groups to give the optimum balance between technical performance (memory, cpu and network usage), the best rejection and least biased selection. This performance has to be validated before use and monitored once it is on-line. Students and staff from RHUL have been involved in all parts of this process, including recent work in electron/photon triggers.

  • Academic staff: Dr. Tracey Berry, Dr. Veronique Boisvert, Dr. Pedro Teixeira-Dias
  • Engineer: Barry Green
  • Research staff: Dr. Simon George, Dr. Ricardo Goncalo, Dr. Andrzej Misiejuk
  • Ph.D students: Muhammad Aftab Alam, Neil Cooper-Smith, Clive Edwards, Matthew Tamsett, Daniel Hayden, Matthew Rose, Robert Cantrill, Timothy Brooks

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Topic revision: r3 - 31 Aug 2010 - SimonGeorge

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