The CMS Level1 Trigger System

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The CMS Level-1 Trigger System

• Dave Newbold, University of Bristol
• On behalf of the CMS collaboration

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Triggering at the LHC

• Tiny signals, huge background
• p-p inelastic rate GHz
• e.g. H(150) -gt gg lt mHz
• but bb MHz background!
• Complex events detector
• Typical CMS event is gt1MB
• Max storage rate 100MB/s
• Huge selectivity needed
• Trigger reduction factor 107
• L1 o/p rate lt 100kHz
• LHC collisions _at_ 40MHz
• Mean of 23 evts per BX at full luminosity
• Detector response time of flight are gt 1BX

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Level-1 trigger strategy

• Driven by LHC physics conditions
• Decays of rare and heavy particles against large
  soft QCD b/g
• Many decays involve intermediate W / Z H -gt gg
  also important
• -gt Identify high-pt leptons and photons
  (including t)
• Low pt thresholds motivated by efficiency for W /
  Z / light Higgs
• Trigger combinations
• gt20GeV limit on single-lepton thresholds due to
  quark decay p0 b/g
• Most interesting states decay to two or more
  trigger objects can use lower thresholds for
  objects in combination
• -gt Find trigger objects locally, combine and cut
  only at last stage
• Large uncertainties in background (and perhaps
  signal)
• Flexibility and control of rate are both vital
• -gt All trigger thresholds and conditions must be
  programmable
• Trigger architecture is fixed, but this is a
  function of detector geometry
• Must have high and well-understood efficiency
• -gt Need to include overlapping and minbias
  triggers to measure e

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Trigger / DAQ architecture

• Level-1 uses muon calo data only
• Tracking data too large / complex
• Local pattern recognition is possible
• Fully pipelined digital electronic system
• Physically impossible to make decision in 25ns
  (speed of light)
• All data stored on detector during fixed L1
  latency, read out upon L1A
• Memory constraints give max latency 3.2ms (of
  which 2ms is cable delay)
• Output of Level-1
• Single bit accept / reject
• Triggers may be throttled for technical reasons
  but otherwise, zero deadtime
• On L1A, data proceed via event builder switch to
  High Level Trigger (see talk of G. Bagliesi)

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Level-1 overview
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Trigger system location
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Calorimeter trigger detectors

• ECAL, HCAL cover to h 3, forward calorimeter
  to h 5
• Trig prims group crystals / scintillators into (2
  x) 32 x 72 trigger towers

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Calorimeter trigger algorithms
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Calorimeter trigger performance

• Full GEANT study
• Includes minbias background
• L1034 cm-2s-1
• Efficiencies
• For objects within fiducial acceptance
• Rates
• e/g dominated by jet (p0) background
• Steep curves allow good control of rate

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Muon trigger detectors

• Dedicated RPC detectors
• Excellent time resolution for effective BX-ID

• Main DT and CSC detectors
• Excellent position resolution for accurate pt
  reconstruction

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Muon trigger algorithms
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Muon trigger performance

• Efficiency for any muon gt3 GeV pt

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Global trigger

• Global trigger implements a wide range of
  triggers (incl. topological)
• Example low lumi (L2.1033 cm-2s-1) trigger
  selection shown above
• Total rate balanced between e/g, jets, muons for
  initial HLT input 50kHz
• Rate safety factor 3, to account for
  uncertainties in background

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Physics efficiencies

• Typical efficiencies for preceding trigger table
  _at_ L2.1033 cm-2s-1

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Practical challenges

• Technology
• Pushing (todays) digital processing and comms
  technologies to the limit
• Cannot afford huge outlay on custom components
  (-gt FPGAs)
• System must last for 10 years obsolescence.
• Synchronisation
• Time-of-flight and detector response take many BX
• Subdetector timing and bunch-crossing ID will be
  challenging
• Reliability
• Level-1 trigger performs online selection cannot
  correct mistakes
• System must be highly reliable, all data taking
  depends on it
• But some parts will fail or degrade at some time
• Some components on detector -gt radiation,
  magnetic field considerations
• Simulation / configuration control / real-time
  trigger optimisation
• Trigger integration for CMS begins 2004 - the
  real work starts now

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Summary

• Triggering at the LHC will be hard
• Leptons / photons are the key
• CMS Level-1 trigger system currently under
  construction
• Reduces 40MHz BX rate to lt 50KHz L1A
• Very large digital logic system
• Uses calorimeter and muon information only
• Simulated performance shows good efficiency for
  the interesting channels