Upgrading the ATLAS trigger system

The ATLAS Level 1 Topo chip (Image: ATLAS)

Although not all proton–proton collisions have interesting characteristics that lead to discoveries, the more data the better the chances of spotting something new. ATLAS is improving its trigger system to be able to select even more collision events with potentially interesting physics.

"The Higgs boson discovery has changed the landscape and the focus is now on measuring its properties," says David Francis, Trigger and Data Acquisition System project leader. "ATLAS has defined what it wants to analyse as the highest priority and our improved trigger system will be optimised to select these events as much as possible."

Until now, the ATLAS trigger system consisted of three levels: Level 1, where decisions were made by specialised electronics within 2.5 microseconds after a collision occurred; Level 2, where specific regions of the events identified by Level 1 were analysed; and Event Filter, where entire events were analysed in full detail. Only a few thousand events per second made it from Level 2 to Event Filter, which in turn reduced the selection to about 400 events per second.

Detector electronics are being upgraded to increase the Level-1 event acceptance rate from 70 kHz to 100 kHz. To handle this new acceptance rate, the size of computer farms will also have to be increased. Plans to make the system more efficient include merging Level 2 and Event Filter, together with the introduction of a hardware-based topological trigger in Level 1, which raises the selectivity of events at the earliest stage.

This hardware is an electronics board developed especially for ATLAS and is an example of how trigger hardware is evolving to meet new challenges. The electronics board combines existing information using new criteria for selecting events on the basis of their angular and other correlations. The board’s selection capability will be very important in identifying potentially interesting physics from the increased amount of data. For instance, the newly announced ATLAS result of Higgs decaying into two tau leptons could profit from this.

"The trigger team coordinates with different physics groups in the collaboration to optimize the selections made," says Brian Petersen, former trigger coordinator and now a supersymmetry sub-group convener. “With more data for analysis, we have a better chance of finding the unknown."

Next year, the new trigger structure will be tested. If all goes well, when protons begin colliding again in the heart of the ATLAS detector, the trigger system will continue to select interesting data with high efficiency in an increasingly difficult environment.