Trigger%20Systems%20at%20LHC%20Experiments

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Trigger Systems at LHC Experiments
Institute of High Energy Physics of the Austrian
Academy of Sciences

• Manfred Jeitler
• Instr-2008, Novosibirsk, March 2008

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• first particle physics experiments needed no
  trigger
• were looking for most frequent events
• people observed all events and then saw which of
  them occurred at which frequency

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• later physicists started to look for rare events
• frequent events were known already
• searching good events among thousands of
  background events was partly done by auxiliary
  staff
• scanning girls for bubble chamber photographs

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• due to the extremely small cross sections of
  processes now under investigation it is
  impossible to check all events by hand
• 1013 background events to one signal event
• it would not even be possible to record all data
  in computer memories
• we need a fast, automated decision (trigger) if
  an event is to be recorded or not

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• detectors yielding electrical output signals
  allow to select events to be recorded by
  electronic devices
• thresholds (discriminators)
• logical combinations (AND, OR, NOT)
• delays
• available in commercial modules
• connections by cables (LEMO cables)

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• because of the enormous amounts of data at major
  modern experiments electronic processing by such
  individual modules is impractical
• too big
• too expensive
• too error-prone
• too long signal propagation times
• ? use custom-made highly integrated electronic
  components (chips)

10 logical operations / module ?
40000 logical operations in one
chip
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example trigger logic of the L1-trigger of the
CMS experiment
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When do we trigger ?

• bunch structure of the LHC collider
• bunches of particles
• 40 MHz
• a bunch arrives every 25 ns
• bunches are spaced at 7.5 meters from each other
• bunch spacing of 125 ns for heavy-ion operation
• at nominal luminosity of the LHC collider (1034
  cm-2 s-1) one expects about 20 proton-proton
  interactions for each collision of two bunches
• only a small fraction of these bunch crossings
  contains at least one collision event which is
  potentially interesting for searching for new
  physics
• in this case all information for this bunch
  crossing is recorded for subsequent data analysis
  and background suppression
• luminosity quoted for ATLAS and CMS
• reduced luminosity for LHCb (b-physics
  experiment)
• heavy-ion luminosity much smaller

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the LHC experiments

• 4 major experiments
• 3 different main physics goals
• general purpose Higgs, Susy, .....
  ATLASCMS
• b-physics LHCb
• heavy ion physics ALICE
• different emphasis on trigger
• ATLASCMS high rates, many different trigger
  channels
• LHCb lower luminosity, need very good vertex
  resolution (b-decays)
• ALICE much lower luminosity for heavy ions,
  lower event rates, very high event multiplicities

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trigger first level high
level ATLAS, CMS 40 MHz ? 100 kHz ? 100
Hz LHCb 40 MHz ? 1 MHz ? 2
kHz ALICE 10 kHz ? 1 kHz ? 100 Hz
Event size (bytes)
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How do we trigger ?

• use as much information about the event as
  possible
• allows for the best separation of signal and
  background
• ideal case complete analysis using all the
  data supplied by the detector
• problem at a rate of 40 MHz it is impossible to
  read out all detector data
• (at sensible cost)
• have to take preliminary decision based on part
  of the event data only
• be quick
• in case of positive trigger decision all detector
  data must still be available
• the data are stored temporarily in a pipeline
  in the detector electronics
• short term memory of the detector
• ring buffer
• in hardware, can only afford a few µs
• how to reconcile these contradictory requirements
  ?

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? multi-level trigger

• first stage takes preliminary decision based on
  part of the data
• rate is already strongly reduced at this stage
• 1 GHz of events ( 40 MHz bunch crossings) ?
  100 kHz
• only for these bunch crossings are all the
  detector data read out of the pipelines
• still it would not be possible (with reasonable
  effort and cost) to write all these data to tape
  for subsequent analysis and permanent storage
• the second stage can use all detector data and
  perform a complete analysis of events
• further reduction of rate 100 kHz ? 100 Hz
• only the events thus selected (twice filtered)
  are permanently recorded

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How does the trigger actually select events ?

• the first trigger stage has to process a limited
  amount of data within a very short time
• relatively simple algorithms
• special electronic components
• ASICs (Application Specific Integrated Circuits)
• FPGAs (Field Programmable Gate Arrays)
• something in between hardware and software
  firmware
• written in programming language (VHDL) and
  compiled
• fast (uses always same number of clock cycles)
• can be modified at any time when using FPGAs
• the second stage (High-Level Trigger) has to
  use complex algorithms
• not time-critical any more (all detector data
  have already been retrieved)
• uses a computer farm (large number of PCs)
• programmed in high-level language (C)

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How does the trigger actually select events ?

• the first trigger stage has to process a limited
  amount of data within a very short time
• relatively simple algorithms
• special electronic components
• ASICs (Application Specific Integrated Circuits)
• FPGAs (Field Programmable Gate Arrays)
• something in between hardware and software
  firmware
• written in programming language (VHDL) and
  compiled
• fast (uses always same number of clock cycles)
• can be modified at any time when using FPGAs
• the second stage (High-Level Trigger) has to
  use complex algorithms
• not time-critical any more (all detector data
  have already been retrieved)
• uses a computer farm (large number of PCs)
• programmed in high-level language (C)
• see Marta Felcinis talk tomorrow

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ATLASCMSwhats the difference ?

• similar task
• similar conditions
• similar technology

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ATLASCMSwhats the difference ?

• similar task
• similar conditions
• similar technology

• lets hope both ATLAS and CMS will fly ....
• .... and none will crash

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ATLASCMSwhat is common ?

• same physics objectives
• same input rate
• 40 MHz bunch crossing frequency
• similar rate after Level-1 trigger
• 50 .. 100 kHz
• similar final event rate
• 100 .. 200 Hz to tape
• similar allowed latency
• pipeline length
• within this time, Level-1 trigger decision must
  be taken and detectors must be read out
• 3 µs
• 2.5 µs for ATLAS, 3.2 µs for CMS

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ATLASCMSwhat is different ?

• different magnetic field
• toroidal field in ATLAS (plus central solenoid)
• get track momentum from ? (pseudorapidity)
• solenoidal field only in CMS
• get track momentum from f (azimuth)
• number of trigger stages
• two stages (Level-1 and High-Level Trigger)
  in CMS
• ATLAS has intermediate stage between the two
  Level-2
• Level-2 receives Regions of Interest (RoI)
  information from Level-1
• takes a closer look at these regions
• reduces rate to 3.5 kHz
• allows to reduce data traffic

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ATLASCMSwhich signals are used by the
first-level trigger ?

• muons
• tracks
• several types of detectors (different
  requirements for barrel and endcaps)
• in ATLAS
• RPC (Resistive Plate Chambers) barrel
• TGC (Thin Gap Chambers) endcaps
• not in trigger MDT (Monitored Drift Tubes)
• in CMS
• DT (Drift Tubes) barrel
• CSC (Cathode Strip Chambers) endcaps
• RPC (Resistive Plate Chambers) barrel
  endcaps
• calorimeters
• clusters
• electrons, jets, transverse energy, missing
  transverse energy
• electromagnetic calorimeter
• hadron calorimeter
• only in high-level trigger tracker detectors
• silicon strip and pixel detectors, in ATLAS also
  straw tubes
• cannot be read out quickly enough

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how to find muon tracks ?(CMS solenoidal field)
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calorimeter trigger
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ATLASCMSprinciple of the first-level trigger

• data are stored in pipelines for a few
  microseconds
• e.g. in CMS 3.2 ?s 128 clock cycles at 40
  MHz
• question of cost
• decision must never take longer! (must be
  synchronous!)
• ? no iterative algorithms
• decision based on look-up tables
• all possible cases are provided for
• OR, AND, NOT can also be represented in this way

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principle of event selection ATLAS vs. CMS

• CMS
• no cuts at individual low-level trigger systems
• only look for muons, electrons, jets, choose the
  best of these candidate objects and forward to
  Level-1 Global trigger
• so, all possible kinds of events may be combined
• selection is only made by Level-1 Global
  Trigger
• trigger information from all muon detectors and
  calorimeter systems available completely
  flexible
• High-Level trigger analyzes complete detector
  data for the whole detector

• ATLAS
• Level-1 trigger delivers numbers of candidate
  objects
• muon tracks, electron candidates, jets over
  appropriate threshold
• no topological information used (no angular
  correlation between different objects)
• Level-2 trigger carries out detailed analysis for
  Regions of Interest
• using complete detector information for these
  regions
• High-Level trigger analyzes complete detector
  data for the whole detector

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LHCb the challenge

• study b-meson decays
• need very good vertex resolution
• lower luminosity
• 2 ? 1032
• 50 times lower than ATLAS and CMS
• aim at having only one proton-proton collision
  per bunch crossing
• to correctly attribute primary and secondary
  vertices pile-up protection
• important difference from ATLAS CMS !
• achieved by deliberately defocusing the beam
• detector looks rather like a fixed-target
  experiment
• concentrate on forward direction
• single-arm forward spectrometer
• can take useful data at initial low-luminosity
  LHC operation

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LHCb the approach

• Level-0 trigger
• implemented in hardware
• reduction from 40 MHz bunch-crossing rate down to
  1 MHz
• 10 times more than ATLAS or CMS !
• allowed latency 4 µs
• reject pile-ups (several proton collisions in one
  bunch crossing)
• uses also vertex detector (VELO, VErtex
  LOcator)
• different from other LHC experiments
• alongside muon and calorimeter information
• High-Level trigger
• computer farm, using full detector information
• reduce data rate to 2 kHz
• 20 times more then ATLAS or CMS

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ALICE the challenge

• study heavy-ion collisions
• quark-gluon plasma
• study also proton-proton interactions for
  comparison
• lower bunch-crossing frequency and luminosity for
  heavy ions
• bunches arrive every 125 ns
• luminosity 1027 cm-2 s-1
• factor of 10 millions below proton-proton
  luminosity
• rate 10 kHz for lead-lead collisions
• 200 kHz for proton collisions
• enormous complexity of events
• tens of thousands of tracks per event
• per pseudorapidity interval dN / d? 8000

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ALICE the approach

• detectors can be slower, but must cope with high
  track multiplicity
• ? choice of Time Projection Chamber (TPC) as
  principal tracking detector (drift time up to
  100 µs !)
• no spatial correlation of objects in first-level
  trigger
• is done by High Level Trigger, which has much
  bigger time budget
• past-future protection protect against pile-up
  of events from different bunch crossings
• some detectors are faster than the TPC ? for
  certain studies, read out only them
• detector clusters
• only LHC detector which aims at analyzing partial
  events

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ALICE the approach

• detectors can be slower, but must cope with high
  track multiplicity
• ? choice of Time Projection Chamber (TPC) as
  principal tracking detector (drift time up to
  100 µs !)
• no spatial correlation of objects in first-level
  trigger
• is done by High Level Trigger, which has much
  bigger time budget
• past-future protection protect against pile-up
  of events from different bunch crossings
• some detectors are faster than the TPC ? for
  certain studies, read out only them
• detector clusters
• only LHC detector which aims at analyzing partial
  events

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ALICE the trigger structure

• no pipeline (as in the other LHC experiments)
• trigger dead-time
• only one or a few events can be buffered at low
  level before readout
• several low-level triggers with different
  latencies for different subdetectors
• Level-0 1.2 µs
• Level-1 6.5 µs
• Level-2 88 µs
• computer farm for High-Level Trigger

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Where we are today ...

• trigger is one of the most challenging and
  important tasks in major experiments at modern
  hadron colliders
• at LHC, very similar setup for most experiments
• only heavy-ion experiment ALICE differs
  significantly
• when accelerator goes online, we will see which
  solutions are most appropriate
• draw lessons for the future

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... and where do we go from here?

• upgrading the LHC to Super-LHC makes sense only
  if trigger systems are upgraded at the same time
• ATLAS and CMS will have to use their trackers in
  the first-level trigger
• but this is easier said than done
• remember Hans-Jürgen Simonis comments on the CMS
  tracker
• probably, computers will get faster and more
  (all?) trigger processing will be done in
  software
• stay tuned ... and help!

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THANK YOU

• ??????? to the organizers of Instr-2008 for
  inviting me to give this presentation
• Thanks to all members of the ATLAS, CMS, LHCb and
  ALICE collaborations who helped me to prepare it

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backup
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structure of the ATLAS Level-1 trigger
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structure of the CMS Level-1 trigger
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LHCb the detector