Abstract
A description of the ATLAS Inner Detector (ID) software trigger algorithms and the performance of the ID trigger for LHC Run 1 are presented, as well as prospects for a redesign of the tracking algorithms in Run 2. The ID trigger HLT algorithms are essential for a large number of signatures within the ATLAS trigger. During the shutdown, modifications are being made to the LHC machine, to increase both the beam energy and luminosity. This in turn poses significant challenges for the trigger algorithms both in terms of execution time and physics performance. To meet these challenges the ATLAS HLT software is being restructured to run as a single stage rather than in the two distinct levels present during the Run 1 operation. This is allowing the tracking algorithms to be redesigned to make optimal use of the CPU resources available and to integrate new detector systems being added to ATLAS for post-shutdown running. Expected future improvements in the timing and efficiencies of the Inner Detector triggers are also discussed. In addition, potential improvements in the algorithm performance resulting from the additional spacepoint information from the new Insertable B-Layer are presented.
Highlights
A BSTRACT: A description of the ATLAS Inner Detector (ID) software trigger algorithms and the performance of the ID trigger for LHC Run 1 are presented, as well as prospects for a redesign of the tracking algorithms in Run 2
To meet these challenges the ATLAS high level trigger (HLT) software is being restructured to run as a single stage rather than in the two distinct levels present during the Run 1 operation
This is allowing the tracking algorithms to be redesigned to make more optimal use of the CPU resources available and integrate new detector systems being added to ATLAS for post-shutdown running
Summary
During Run 1 the ATLAS Trigger [3] consisted of three distinct, sequential trigger levels - the hardware Level 1 (L1) trigger, running on limited granularity data predominantly from the muon spectrometer and calorimeter subsystems, and the two high level trigger (HLT) systems - the Level (L2) and the Event Filter (EF), each running on its own large farm of commodity processors. An L2 decision, if the event is to be kept, is followed by the EF reconstruction which runs modified versions of the offline reconstruction algorithms in the RoIs. Between 2010-2012 the EF output rate was in range of 350 to 1000 Hz. The efficiency of the HLT for electrons was measured using the tag and probe method. A tag lepton was required to be reconstructed in the electromagnetic calorimeter and the ID, and a probe lepton was required to be reconstructed in the calorimeter only ( leptons with ID hits are not vetoed) These were matched to offline electron tracks in order to reduce the contribution from background processes. By including a possible additional hit from the IBL for each track, close to the beam line, it was expected that the reconstruction efficiency should increase and the resolution on the impact parameter measurement should improve. An improvement of around 25% is seen and a similar improvement is observed for other track parameter resolutions [6]
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