ATLAS trigger and DAQ

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ATLAS trigger and DAQ

• a short introduction

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Starting points

• physics programme for the experiment
• what are you trying to measure
• detector performance
• what data is available
• accelerator parameters
• what rates and structures

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Physics programme

• see lhc_basics for more detail
• luminosity dependent
• low luminosity (first 2 years)
• high PT programme (Higgs etc.)
• b-physics programme (CP measurements)
• high luminosity
• high PT programme (Higgs etc.)
• searches for new physics

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Physics and triggering

• trigger must select the required physics
• with good (high) efficiency
• well known and monitored efficiency (well
  matched to off-line selection)
• with high reliability
• in shortest possible time (and lowest cost)
• trigger must reject background
• repeat all above!

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Matching problem
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Matching problem (cont.)

• ideally
• off-line algorithms select phase space which
  shrink-wraps the physics channel
• trigger algorithms shrink-wrap the off-line
  selection
• in practice, this doesnt happen
• need to match the off-line algorithm selection
• BUT off-line can change algorithm, re-process and
  recalibrate at a later stage
• SO, make sure on-line algorithm selection is well
  known, controlled and monitored

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Trigger design

• Inclusive and exclusive triggers
• inclusive - select all physics with certain
  characteristics
• single (or few) particle triggers e.g. high pT
  leptons
• unbiased sample (or relatively so)
• does not exclude new physics
• exclusive - select physics channel under study
• use to recognise well known processes
• accept, scale (sample) or reject
• need to monitor efficiency

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Trigger design (cont.)

• Level 1
• inclusive triggers
• Level 2
• confirm Level 1, some inclusive, some
  semi-inclusive,some simple topology triggers,
  vertex reconstruction(e.g. two particle mass
  cuts to select Zs)
• Level 3
• confirm Level 2, more refined topology
  selection,near off-line code

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Selection and rejection

• as selection criteria are tightened
• background rejection improves
• BUT event selection efficiency decreases

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Detector and accelerator parameters

• sell lhc_basics for main details
• detector channel numbers

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Trigger and DAQ design process

• develop algorithms to match the physics programme
  and off-line selections
• run time constraints mean you cant use off-line
  algorithms
• develop systems to collect data required and run
  algorithms at rates needed to match accelerator
  and detector performance
• use multi-level trigger to remove backgrounds as
  soon as possible
• get interesting physics to tape for off-line
  analysis

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Selected (inclusive) signatures (high L)
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ATLAS trigger and DAQ
Level 1 latency 2 µs
Level 2 latency 10 ms
Level 3 latency few s
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Level-1 trigger system
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Level 1 (cont.)

• calorimeter and muon triggers
• trigger on inclusive signatures
• data held in pipelines (2.5µs) for Level-1
  decision
• bunch crossing identified
• information (Regions of Interest) passed to
  level-2
• on calorimeter clusters (ETgt10GeV)
• and muon tracks (µ pT gt 6 GeV)
• 4000 e.m. and 4000 hadronic channels

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em cluster trigger algorithm
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Trigger efficiency vs cluster threshold

• 1-cell, 2-cell and 4-cell groupings (50 GeV
  electrons)

2 x 1 cell sharper threshold than 1 x 1 2 x 1
cell and 2 x 2 cell nearly identical. lower rate
than 2 x 2 half the background rate.
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Level-1 estimated accept rates
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Front-end to buffer data flow
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Level 2 system philosophy

• fundamental granularity of detectors
• no special readout from front-ends
• no inherent loss of data quality
• guidance from LVL1 - Region of Interest (RoI)
• reduces data to be moved to T2 processors
• RoI - 'cone' from interaction point
• Processing scheme
• extract features from sub-detector data
• combine features from one RoI into object
• combine objects to test event topology

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Regions of Interest (RoIs)
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RoI data rate reduction

• only a few percent of the total data for each
  event are transferred from the buffers to the
  level-2 system for processing

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Level-2 system hardware overview
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Level-2 trigger rates (estimated)
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System Overview
Technologies Level-1 custom built, ASICs and
FPGA based system Higher level triggers and event
builder mainly commercial - µPs and networks
(Ethernet/ATM)
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Summary

• To achieve TDAQ aims, the following are studied
• trigger performance
• physics studies, algorithm development and
  testing
• hardware development
• buffers, networks, processor systems
• software development
• data flow, fault handling, control and monitoring
• modelling
• mont-carlo physics studies
• simulation of hardware and software systems
• A broad knowledge of the expeiment needed