The ATLAS Trigger System: Past, Present and Future

F Pastore

doi:10.1016/j.nuclphysbps.2015.09.167

Abstract

The ATLAS trigger has been used very successfully for the online event selection during the first run of the LHC between 2009-2013 at a centre-of-mass energy between 900 GeV and 8 TeV. The trigger system consists of a hardware Level-1 (L1) and a software based high-level trigger (HLT) that reduces the event rate from the design bunch-crossing rate of 40 MHz to an average recording rate of a few hundred Hz.We will briefly review the performance of the ATLAS trigger system during the past data-taking period and point out the challenges for the trigger system during the next LHC run in early 2015 with a smaller bunch spacing, almost twice the centre-of-mass energy and higher peak luminosity. We will show the ongoing improvements and upgrades to the existing system that will ensure an even better performing trigger system despite the harsher machine conditions. This includes changes to the L1 calorimeter trigger, the introduction of a new L1 topological trigger module, improvements in the L1 muon system and the merging of the previously two-level HLT system into a single event filter farm. In addition, we will give an overview of the algorithmic improvements in the various HLT algorithms used to identify leptons, hadrons and global event quantities like missing transverse energy.

Highlights

TeV 13 TeVLHC energy increase from 7/8 to 13 TeV: x2-4 production cross-sections for EW processes (W, Z, H) Higher peak luminosity by increasing the number of interactions per p-p bunch collision higher density of interactions in space and time, and detectors occupancy Smaller bunch spacing (25 ns): smaller in-time pile-up (same bunch), increased out-of-time pile-up (close bunches)
LHC design parameters will be reached after long shutdown one (LS1)
Increase ~3% trigger coverage in the barrel (70%) completing installation in the holes of the toroid structure Reduce the dominant rates in the forward regions with additional coincidence chambers upstream of the toroids and/or additional coincidence with the outer-most layer of the Tile Calorimeter and/or excluding the low field regions (< 5% acceptance loss)

Summary

TeV 13 TeV

LHC energy increase from 7/8 to 13 TeV: x2-4 production cross-sections for EW processes (W, Z, H) Higher peak luminosity by increasing the number of interactions per p-p bunch collision higher density of interactions in space and time, and detectors occupancy Smaller bunch spacing (25 ns): smaller in-time pile-up (same bunch), increased out-of-time pile-up (close bunches). ATLAS priorities are the precision measurements of the Higgs boson parameters and the extended search of New Physics (NP). Higgs mass at 125 GeV opens access to many Higgs decay channels, and to the measurements of its coupling constants. Higgs studies: with leptons at relatively low momentum and increased importance of final states with forward jets and taus. Selects a few hundred events/s for permanent storage, ATLAS Run[1]: discarding online low-momentum-transfer interactions 20 MHz -> 700 Hz: rejection factor 30,000. Event display of a 2-tau event in the ATLAS detector. The taus decay into an electron (blue line) and a muon (red line). Efficiency wrt offline ~86 (70)% in endcap Large variety of triggers e.g. for (barrel); robust against pileup various sizes of jets down to ~15 GeV

40 MHz 20 MHz

Findings

Conclusions