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LVL1 Workshop, CERN, 16/07/99Alan Watson (by proxy) ATLAS Jet Trigger Algorithm Performance Requirements: Good jet E T resolution Sharp threshold Minimise rate for inclusive trigger Good multijet performance Ability to resolve nearby (non-overlapping) jets Ability to classify events by multiplicity Good jet coordinate precision Required for RoI-driven Level 2 Robustness and flexibility Robust against noise & pileup Flexible as do not know what we will want to trigger on in 2005/6/7/8/... Technical Requirements: Manageable complexity of implementation! Not necessarily minimal complexity Must integrate well with rest of system
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LVL1 Workshop, CERN, 16/07/99Alan Watson (by proxy) Elements of Jet Algorithm Overlapping, sliding windows Architecture forces sliding window algorithm Overlapping to optimize jet containment Cluster Size Main determinant of single jet resolution Jet E T containment vs noise/pileup summation Same size may not be optimal for all conditions and tasks (e.g. low-E T multijets) Step Size Constrains possible cluster sizes Major factor in multijet performance Limits how close jets can be resolved Limits RoI coordinate precision “Declustering” Algorithm Resolves overlaps & allows jet count Main factor in multijet performance Determines how close jets can be resolved Major factor in RoI coordinate precision
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LVL1 Workshop, CERN, 16/07/99Alan Watson (by proxy) Overlapping Windows: Problem of Boundaries: Unless windows >> jets, overlap required to optimise E T containment Drawback of Overlap: Several windows overlapping jet may all pass threshold cluster of hits from 1 object Additional “declustering” logic required to resolve overlaps and allow jet multiplicity to be determined Non-OverlappingOverlapping 2 mid-E T objects1 high-E T object
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LVL1 Workshop, CERN, 16/07/99Alan Watson (by proxy) Declustering and RoIs Two Tasks: Count jets, resolving overlapping objects Identify RoI coordinates for LVL2 One Solution: Define an “RoI Cluster” jet cluster or part of jet cluster (“jet core”) Require it is more energetic than neighbouring objects of same type Centre of such a “local ET maximum” = RoI coordinate Since neighbouring RoI clusters cannot both pass, 1 jet cannot produce > 1 RoI so same requirement resolves multiple- counting due to overlaps R Allow for 2 adjacent RoI clusters having equal E T. Final algorithm uses 0.4 0.4 RoI cluster, sliding by 0.2
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LVL1 Workshop, CERN, 16/07/99Alan Watson (by proxy) The ATLAS Algorithm Main Features Self-contained algorithm Implement in independent 4 4 windows Require RoI cluster = E T maximum Avoids multiple counting of single object Provides RoI coordinate for Level 2 Different cluster sizes available Can choose 0.4 0.4, 0.6 0.6 or 0.8 0.8 Can mix cluster sizes for different tasks Elements = 0.2 0.2 (= step by which window slides) RoI = 2 2 cluster (0.4 0.4) Window = 4 4 elements (0.8 0.8)
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LVL1 Workshop, CERN, 16/07/99Alan Watson (by proxy) Main Properties Fine Step Size: Precise RoI coordinate ( ) Ability to resolve nearby jets ( ) Allows = 0.2 for E T miss trigger Variable Cluster Size: Maximum size = 0.8 0.8 Good jet E T containment/inclusive trigger Minimum size = 0.4 0.4 Useful where noise/pileup significant cf jet E T (e.g. very low E T multijet triggers or secondary RoIs)
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LVL1 Workshop, CERN, 16/07/99Alan Watson (by proxy) Jet Definitions The Problem: A “jet” is defined by a “jet finder” No single, unambiguous definition Many different algorithms used So, what should trigger performance be judged against? What we used: Fixed-width Cone Algorithm (width = 0.8) Most widely used in ATLAS physics studies Most similar to trigger algorithms (risk of biassing trigger algorithm choice?) KT algorithm (M. Seymour) Variant on widely-used Durham scheme, modified for hadron colliders In all studies, performance of different trigger algorithms was compared with both types of jet finder.
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LVL1 Workshop, CERN, 16/07/99Alan Watson (by proxy) Rate vs Cluster Size Threshold vs Cluster Size Trade-off between containing jet E T and summing noise/pileup Favours larger clusters except for lowest E T Inclusive Rate vs Size For thresholds giving 95% jet efficiency
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LVL1 Workshop, CERN, 16/07/99Alan Watson (by proxy) Cluster & RoI Sizes & Multijet Triggers Jet Separation & Multijet Efficiency Too large an RoI (poor jet separation) degrades efficiency for multijet trigger Top efficency of 4jet trigger vs Rate Multijet Efficency vs Cluster/Step Size Choose thresholds for 1.5 kHz 4jet rate Compare efficiencies for different physics processes.
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LVL1 Workshop, CERN, 16/07/99Alan Watson (by proxy) RoI Multiplicities RoI multiplicity vs Cluster Size Choose thresholds to give 95% efficiency for jet of required p T. Compare mean RoI multiplicities for different cluster sizes. Fast simulation used RoI Efficency vs p T High L, full simulation
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