HEP 2005 WorkShop, Thessaloniki

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Studies on the High Level Trigger System of the
ATLAS Detector Stefanidis E. University College
London
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Outline

• ? ATLAS Trigger System
• The challenge.
• Overview.
• ? Level-2 Trigger
• The Region of Interest mechanism.
• ? Reduced Region of Interest
• Motivation.
• Methodology.
• Implementation.
• ? Results.
• ? Conclusions.

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The ATLAS Trigger System The challenge

• Proton bunch crossing rate of 40 MHz
• Design (high) luminosity of 1034 cm-2s-1
• spp-inel 80 mb _at_ ?s 14 TeV

• Signal events are very rare
• H?gg rate is 10-13 of the LHC interaction rate.
  (mH120 GeV)
• Limited storage capability High Granularity
  of the sub-systems
• event size 1.6 MB just before storage.
• storage rate of O (200) Hz.

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The ATLAS Trigger System Overview

• LVL1
• 40 MHz ? 75 kHz
• lt 2.5 µs
• Hardware trigger
• Coarse granularity
• Calorimeter and Muon Trigger stations only

• LVL2
• 75 kHz ? 2 kHz
• 10 ms
• Full granularity information from both ID and
  Calorimeter
• RoI mechanism

• Event Filter (EF)
• 2 kHz ? 100 Hz
• ? 2s
• Sophisticated algorithms
• Alignment data available

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Level-2 Trigger The RoI mechanism
How the RoI is currently constructed

• With the RoI mechanism we manage to
  minimise the
• Processing time.
• Network traffic.

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Reduced Region of Interest motivation

• The size of the RoI in (eta, phi) space has been
  determined almost arbitrarily.
• The RoI currently takes into account the full
  range of the beam spread.
• Better algorithm design can reduce the size.
• Faster decisions (and more efficient?),
  especially in a demanding environment.
• Never before has been a detailed study for
  optimising this size.
• Main tool the 1st and 2nd samplings of the EM
  Calorimeter.
• High granularity information at Level-2.

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Reduced Region of Interest methodology
1. Determine the position of the middle of the
1st and 2nd sampling.
2. Extrapolate back to the beam.
3. Open 3sigma around the Z found by the
Calorimeter
The RoI is reduced by 30!
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Reduced Region of Interest implementation
1. Parameterize the middle of the 1st and the 2nd
EM samplings
2. Parameterize the width to open.
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Results

• Using our Tracking Algorithm we can test the
  method

1. in terms of efficiency

1. in terms of timing

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Conclusions

• A new method has been applied in order to reduce
  the size of the Region of Interest.
• Important improvement in terms of efficiency and
  timing even at this preliminary stage.
• Further/high order corrections and studies are
  being carried out in order to optimize the size.
• Contacts have started to integrate the new method
  into the official ATLAS Trigger Software.