Trigger Development


The ATLAS trigger system selects potentially interesting events to be recorded from the full collision rate. In 2012, during the 8 TeV proton-proton (pp) collisions, the 20 MHz bunch crossing rate was reduced to an average output of 300-400 Hz. To accomplish this, the ATLAS trigger system currently consists of three levels of event selection: Level-1 (L1), Level-2 (L2), and Event Filter (EF). The L2 and EF together form the High-Level Trigger (HLT). The University of Geneva currenlty has responsibilities in the HLT management and has contributed significantly to the selection algorithms and software for the electron, photon, and minimum bias triggers, and to the trigger operation during the 2010-2012 data taking period. We are now involved in the preparations for the 13 TeV operation in 2015, for which the trigger architecture and selection algorithms are being redeveloped in order to cope with the much higher rates and pile-up conditions.

Level-1 trigger

The L1 trigger searches for signatures from high-pT muons, electrons/photons, jets, and tau-leptons decaying into hadrons. It also selects events with large missing transverse energy and large total transverse energy using reduced granularity. The maximum L1 accept rate during the 2012 operation was 75 kHz, which will be increased to 100 kHz for 2015.

High-level trigger

The L2 trigger is seeded by Regions-of-Interests (RoI's). These are regions of the detector where the L1 trigger has identified possible trigger objects within the event. The L2 trigger uses RoI information on coordinates, energy, and type of signatures to limit the amount of data which must be transferred from the detector readout. In 2012 the L2 trigger reduced the event rate to below 6.5 kHz, with an average event processing time of approximately 90 ms.

The EF uses offline analysis procedures on fully-built events to further select events down to a rate which can be recorded for subsequent offline analysis. In 2012 it reduced the event rate to 300-400 Hz, with an average event processing time of order 1 second.

The HLT algorithms use the full granularity and precision of calorimeter and muon chamber data, as well as the data from the inner detector, to refine the trigger selections. Better information on energy deposition improves the threshold cuts, while track reconstruction in the inner detector significantly enhances the particle identification, for example distinguishing between electrons and photons.

High-level trigger racks
High-level trigger racks
The HLT is almost entirely based on commercially available computers and networking hardware. The computers run Scientific Linux CERN and are interconnected by multi-layer gigabit-Ethernet networks, one for control functionality and another for data movement. For Run 1 the HLT had approximately 17000 CPU cores.

The electron-photon trigger

Single electron trigger efficiency as function of pile-up in 2012
Pile up robustness
Geneva has played a leading role in the electron-photon trigger signature group for many years. This group is responsible for the development, implementation, monitoring and performance studies of the triggers used to collect events containing electrons or photons in the final state. In 2012 these triggers accounted for around one third of the rate of the recorded data and are essential for a wide range of ATLAS physics analyses. The plot shows the efficiency of the single electron triggers as a function of the number of primary vertices in an event, measured using a Tag and Probe technique on Z decays to di-electrons, illustrating the pile-up robust performance achieved in 2012. The attention of the group is now on the activities of LS1, during which time the trigger system will be reoptimised for the 13 TeV running at luminosities up to 3 x 1034 cm-2s-1 from 2015.

Last modified: 2013/04/05