The ATLAS Trigger and Data Acquisition System

1
The ATLAS Trigger and Data Acquisition System

• John Strong
• Royal Holloway, University of London

2
Talk outline

• Trigger and DAQ basics
• The LHC and ATLAS
• 14 TeV Physics
• The ATLAS Detector
• ATLAS Trigger and DAQ design
•  Level 1 trigger
•  High level trigger and data acquisition
• Current status

3
TDAQ basics

• The DAQ challenge is to
• get information from the detectors and put it on
  permanent storage media quickly and accurately
• supply the trigger with information in a timely
  fashion
• buffer (temporarily store) data while the
  trigger does its job
• Zero or very low dead-time

• The trigger (filter or event selection) challenge
  is to
• reduce the event rate to one the DAQ can
  transfer to permanent storage by
• selecting interesting interactions
• throwing away background
• Take care, once rejected they are non-recoverable

• TDAQ also has to deal with
• Calibration runs, run control, data monitoring,
  bookkeeping etc.

4
TDAQ starting point

• from Physics
• what is the experimental programme
• TDAQ should be flexible enough to accommodate
  changes to programme
• from the Detector
• what data are available and when
• size, granularity and occupancy of detectors
• from the Accelerator
• what rates and structures
• start-up and design luminosity

5
TDAQ design process

• develop algorithms to match the physics programme
  and off-line selections
• off-line algorithms not fast enough
• need high, unbiased and known efficiency
• need large rate reduction from non-relevant
  processes
• develop systems to collect data required and run
  algorithms at rates needed to match accelerator
  and detector performance
• use trigger to remove backgrounds as soon as
  possible
• Get as much interesting physics data as
  possible to tape for off-line analysis

6
Trigger Design

• Inclusive and exclusive triggers
• inclusive - select events 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
• As selection criteria are tightened
• Background rejection improves
• BUT event selection efficiency tends to decrease

7
The matching problem

• Ideally
• off-line algorithms select phase space which just
  encloses the physics channel
• trigger algorithms just enclose the off-line
  selection
• In practice, this doesnt happen
• Would need to match the off-line algorithm
  selection
• BUT off-line the algorithm can be changed, data
  re-processed and recalibrated
• On-line algorithms have tight time constraints
• SO, make sure on-line algorithm selection is well
  known, controlled and monitored

8
Matching problem (cont.)

9
TDAQ basics

• Trigger and DAQ not an exact science
• NO truth - NO 'right choice'
• Main question asked is
• Does it do the job can we afford it?
• One major problem is interconnection and data
  flow.

ALEPH barrel end-view
Partially cabled TPC
10
The LHC experiment layout

• 7 TeV on 7 TeV pp collider
• 27 km of 8.3T superconducting dipoles at 1.8K
  
• Luminosity of 2.1033 cm-2s-1 initially, design
  1.1034 cm-2s-1

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LHC Physics (1)

• Still many unknowns in the Standard Model
• Origin of mass symmetry breaking generation
  hierarchy
• A possible solution the Higgs boson
• Next step hunt the Higgs
• Unknown mass cover wide range
• Small x-section need high luminosity

• ALSO - Explore new energy domain
• Supersymmetry compositeness the unexpected

• AND - Something has to happen by 1 TeV
• Higgs mechanism regulates divergences in the
  Standard Model
• If no Higgs, then should see effects e.g. in
  the W-W x-section
• Other theories supersymmetry, technicolor
  predict particle production at, or before, the
  TeV scale

12
LHC Physics (2)
Rate at design luminosity Rate at design luminosity Rate at design luminosity
Channel X-section Rate/s
Ineleastic 0.1 b 109
B-physics 200 µb 2.106
Jet (gt250GeV) 100 nb 103
W?l? 20 nb 2.102
t-t production 1 nb 10
Higgs (100 GeV) 20 pb 2.10-1
Z (1 TeV) 10 pb 10-1
Higgs (500 GeV) 1 pb 10-2

Lepton decay branching ratio 10-2
selection power for Higgs 1013
A special piece of hay in a haystack 109
13
LHC Physics (3)

• Higgs signal extraction very difficult
• Searches for H?ZZ ?leptons (e or µ), H ??? also
  H ?tt, H ?bb
• but a lot of other interesting physics
• SUSY and other new physics
• High-pT particles particularly leptons - are
  likely to be signature of such physics (and
  Higgs)
• Of interest in their own right and must be
  understood as backgrounds to new physics
• B physics and CP violation quarks, gluons and
  QCD top quarks
• W and Z bosons

14
Effect of pT cut on minimum-bias events
Simulated H?4µ event 17 minimum-bias events
Can try to use this in trigger to select
interesting events
15
ATLAS Detector
Diameter 25 m Barrel toroid length 26 m Total
length 44 m, height 22 m Overall weight 7000 Tons
16
ATLAS Collaboration

• Albany, Alberta, NIKHEF Amsterdam, Ankara, LAPP
  Annecy, Argonne NL, Arizona, UT Arlington,
• Athens, NTU Athens, Baku, IFAE Barcelona,
  Belgrade, Bergen, Berkeley LBL and UC, Bern,
• Birmingham, Bonn, Boston, Brandeis,
  Bratislava/SAS Kosice, Brookhaven NL, Bucharest,
• Cambridge, Carleton/CRPP, Casablanca/Rabat, CERN,
  Chinese Cluster, Chicago, Clermont-Ferrand,
• Columbia, NBI Copenhagen, Cosenza, INP Cracow,
  FPNT Cracow, Dortmund, JINR Dubna, Duke,
• Frascati, Freiburg, Geneva, Genoa, Glasgow, LPSC
  Grenoble, Technion Haifa, Hampton, Harvard,
• Heidelberg, Hiroshima, Hiroshima IT, Indiana,
  Innsbruck, Iowa SU, Irvine UC, Istanbul Bogazici,
• KEK, Kobe, Kyoto, Kyoto UE, Lancaster, Lecce,
  Lisbon LIP, Liverpool, Ljubljana, QMW London,
• RHBNC London, UC London, Lund, UA Madrid, Mainz,
  Manchester, Mannheim, CPPM Marseille,
• MIT, Melbourne, Michigan, Michigan SU, Milano,
  Minsk NAS, Minsk NCPHEP, Montreal,
• FIAN Moscow, ITEP Moscow, MEPhI Moscow, MSU
  Moscow, Munich LMU, MPI Munich,
• Nagasaki IAS, Naples, Naruto UE, New Mexico,
  Nijmegen, Northern Illinois, BINP Novosibirsk,
• Ohio SU, Okayama, Oklahoma, LAL Orsay, Oslo,
  Oxford, Paris VI and VII, Pavia, Pennsylvania,
  Pisa,
• Pittsburgh, CAS Prague, CU Prague, TU Prague,
  IHEP Protvino, Ritsumeikan, UFRJ Rio de Janeiro,
• Rochester, Rome I, Rome II, Rome III, Rutherford
  Appleton Laboratory, DAPNIA Saclay,
• Santa Cruz UC, Sheffield, Shinshu, Siegen, Simon
  Fraser Burnaby, Southern Methodist Dallas,
• NPI Petersburg, Stockholm, KTH Stockholm, Stony
  Brook, Sydney, AS Taipei, Tbilisi, Tel Aviv,
• Thessaloniki, Tokyo ICEPP, Tokyo MU, Tokyo UAT,
  Toronto, TRIUMF, Tsukuba, Tufts, Udine,
• Uppsala, Urbana UI, Valencia, UBC Vancouver,
  Victoria, Washington, Weizmann Rehovot,

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The LHC and ATLAS

• LHC has
• a high luminosity 1034 cm-2s-1
• short bunch separation 25 ns (bunch length 1 ns)

• This results in
• 23 interactions / bunch crossing at design
  luminosity
• beam lifetime of day (beam-beam interactions
  major effect)

• 70 charged particles (mainly soft pions) /
  interaction
• 1000 charged particles / bunch crossing

• 7.5 m bunch separation
• debris from 3 bunch crossings in ATLAS
• one entering inner tracker
• one exiting calorimeter
• one in muon system
• bunch crossing identification needed

18
The ATLAS Sub-Detectors

• Inner tracker
• pixels (silicon)
• (3 layers) precision 3-D points
• 1.4 108 channels occupancy 10-4
• silicon strips
• (4 layers) precision 2-D points
• 5.2 106 channels occupancy 10-2
• transition radiation tracker (straw tubes)
• (40 layers) continuous tracker electron
  identification
• 4.2 105 channels 12-33 occupancy

19
Inner Detector Layout

20
ATLAS event in the tracker

21
Tracker end-view of event

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Sub-Detectors (cont.)

• solenoid
• between tracker and calorimeters 4 m x 7 m x 1.8T

• calorimetry
• electromagnetic
• liquid argon (accordion) detector lead
• hadronic
• scintillator tiles liquid argon iron
• 2.3 105 channels occupancy 5-15

• muon system
• air-core toroid magnet system
• trigger - resistive plate and thin gap chambers
• precision monitored drift tubes
• 1.3 106 channels occupancy 2-7.5

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ATLAS Calorimeters and Inner Tracking Detectors
EM Accordion Calorimeters

Hadronic LAr End Cap Calorimeters
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Accordion calorimeter
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Accordion calo em shower
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A Barrel Toroid

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ATLAS Trigger

• Physics programme is luminosity dependent
• low luminosity (2.1033 cm-2 s-1) - first 2
  years
• high PT programme (Higgs etc.), b-physics
  programme (CP etc.)
• high luminosity (1034 cm-2 s-1)
• high PT programme (Higgs etc.), searches for new
  physics
• trigger must select physics and reject background
• 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)

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ARCHITECTURE
Trigger
DAQ
40 MHz
10s PB/s(equivalent)
Three logical levels
Hierarchical data-flow
LVL1 - FastestOnly Calo and MuHardwired
On-detector electronics Pipelines
3 ms
LVL2 - LocalLVL1 refinement track association
Event fragments buffered in parallel
ms
LVL3 - Full eventOffline analysis
Full event in processor farm
sec.
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Experiment TDAQ comparisons

30
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

31
Trigger rates and decision times
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T/DAQ system overview

• Latency 2.5ms (max)
• Hardware based (FPGA, ASIC)
• Calo/Muon (coarse granularity)

LVL1

• Latency 10 ms (average)
• Software (specialised algs)
• Uses LVL1 Regions of Interest
• All sub-dets, full granularity
• Emphasis on early rejection

LVL2

• Latency few sec (average)
• Offline-type algorithms
• Full calibration/alignment info
• Access to full event possible

EF
33
LVL1 Overview

• Identify basic signatures of interesting physics
• muons
• em/tau/jet calo clusters
• missing/sum ET
• Hardware trigger
• programmable and custom electronics (FPGA ASIC)
• programmable thresholds
• Decision based on multiplicities and thresholds

34
em cluster trigger algorithm
35
Em cluster trigger algorithm

• Trigger efficiency vs cluster threshold

1 x 1, 2 x 1 and 2 x 2 cell groupings (50 GeV
electrons)
2 x 1 cell sharper threshold than 1 x 1 2 x 1
cell and 2 x 2 cell threshold nearly identical. 2
x 1 half the background rate of 2 x 2 .
36
Level 1 Jet and em trigger (cont.)

EM RoI multiplicity vs. threshold
Jet RoI Multiplicity (ET gt 5 GeV)
multiplicity
ET GeV
37
Level 1 Muon trigger
RPC - Trigger Chambers - TGC

RPC Restive Plate Chambers TGC Thin Gap
Chambers MDT Monitored Drift Tubes
38
Level 1 Muon trigger (cont.)

Level-1 muon trigger from Muon Trigger
Chambers Main single-muon background comes from
hadrons (pi/K decays in flight) Steeply falling
cross section with increasing pt of muon (and
even steeper drop off of b/g) means rate can be
controlled by fine-tuning threshold.
39
Estimated Level-1 accept rates
40
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)
• Only process data from areas indicated by Level 1
• reduces data to be moved to T2 processors
• Processing scheme
• Requires updating!

41
Regions of Interest (RoIs)
42
Region of interest mechanism

• LVL1 selection is mainly based on local
  signatures identified at coarse granularity in
  muon detectors and calorimeter .
• Further rejection can be achieved by examining
  full granularity muon, calo and and inner
  detector data in the same localities
• The Region of Interest is the geometrical
  location of a LVL1 signature.
• It is passed to LVL2 where it is translated into
  a list of corresponding readout buffers
• LVL2 requests RoI data sequentially, one detector
  at a time, only transfers as much data as needed
  to reject the event.
• The RoI mechanism is a powerful and important way
  to gain additional rejection before event
  building
• Order of magnitude reduction in dataflow
  bandwidth, at small cost of more control traffic

43
HLT event selection strategy

• Processing in Steps
• Alternate steps of feature extraction /
  hypothesis testing
• Events can be rejected at any step if features do
  not fulfil certain criteria (signatures)
• Reconstruction in Regions of Interest (RoIs)
• RoI size/position derived from previous step(s)

Emphasis on early event rejection
Emphasis on minimising a. Processing time b.
Network traffic
44
Milestone schedule
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ATLAS cavern April 2002
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ATLAS cavern April 2003
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Atlas cavern April 2004
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A Toroid End Cap cryostats journey