Abstract
At the high luminosity HL-LHC, upwards of 140 individual proton-proton interactions (pileup) are expected per bunch-crossing at luminosities of around 5×1034 cm-2s-1. A proposal by the ATLAS collaboration to split the ATLAS first level trigger in to two stages is briefly detailed. The use of fast track finding in the new first level trigger is explored as a method to provide the discrimination required to reduce the event rate to acceptable levels for the read out system while maintaining high efficiency on the selection of the decay products of electroweak bosons along with other high pT physics signatures at HL-LHC luminosities. It is shown that the available bandwidth in the proposed new strip tracker is sufficient for a region of interest based track trigger given certain optimisations. Further methods for improving upon the proposal are discussed.
Highlights
The current event discrimination at L1 is based on low-granularity calorimetric information and fast muon systems which both use custom electronics to issue a L1 trigger decision within 2.5 μs
L0 will operate with an increased latency window of 6 μs and rate of ∼ 500 kHz while the L1 decision will need to be broadcast within 20–30 μs at around 200 kHz to stay within the constraints of legacy muon electronics whose replacement is not certain due to their inaccessible location inside the toroidal magnet systems
Using technologies similar to ATLAS Fast Tracker project [6], L1 track trigger (L1TT) will utilise track pattern banks to quickly identify particles with pT ∼> 2 GeV based upon supplied hits from the new Inner Tracker (ITK)
Summary
In order to obtain increased rejection power before the full detector readout is requested, it is envisaged to add a new L1 hardware layer to the trigger, with the current L1 being re-designated to L0. A key proposal for this new design is the inclusion of a L1 track trigger (L1TT). Studies have shown that even for modest track reconstruction resolution, the matching of e and τ candidates to the calorimeter and μ candidates to the muon spectrometer will reduce rates by a factor 3–10 with only small (∼ 10%) losses in efficiency [5]. Such additional rejection could not be obtained by continuing to use only muon trigger chambers and low granularity calorimeter information at L0/L1
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