UCL Seminar, 16/2/07 Costas Foudas, Imperial College 1 Overview of the CMS Trigger Overview of the CMS Trigger and and Plans for LHC Start-up Plans for LHC Start-up Overview of this talk: • Trigger Challenges at LHC and Goals • The CMS Trigger System • Pilot run Triggers (2007) • Physics Triggers (2008)
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UCL Seminar, 16/2/07Costas Foudas, Imperial College London 1 Overview of the CMS Trigger and Plans for LHC Start-up Overview of this talk: Trigger Challenges.
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UCL Seminar, 16/2/07 Costas Foudas, Imperial College London
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Overview of the CMS Trigger andOverview of the CMS Trigger andPlans for LHC Start-upPlans for LHC Start-up
Overview of this talk:
• Trigger Challenges at LHC and Goals• The CMS Trigger System• Pilot run Triggers (2007)• Physics Triggers (2008)
UCL Seminar, 16/2/07 Costas Foudas, Imperial College London
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Minimum Bias EventsMinimum Bias Events
• At full LHC Luminosity we have 22 events superimposed on any discovery signal.
• First Level Event Selection requires considerable sophistication to limit the enormous data rate.
• Typical event size: 1-2 Mbytes.
22
70 mb deep inelastic component
UCL Seminar, 16/2/07 Costas Foudas, Imperial College London
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Higgs -> 4
Trigger Challenge at LHCTrigger Challenge at LHC
• We want to select this type of event (for example Higgs to 4 muons) which are superimposed by this……
+30 MinBias
UCL Seminar, 16/2/07 Costas Foudas, Imperial College London
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Challenge 2: PileupChallenge 2: Pileup
• In-time pile up: Same crossing different interactions• Out-of-time pile up: Tails from previous event• New events come every 25 nsec 7.5 m radial separation.• Out-of-time pile up: Due to events from different crossings.• Need a to identify the bunch crossing that a given event comes
from.
P.S
ph
icas
UCL Seminar, 16/2/07 Costas Foudas, Imperial College London
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Trigger Goals at LHCTrigger Goals at LHC• At LHC we want to select events that have: (1) Isolated leptons and photons, (2) -, central- and forward-jets (3) Events with high ET
(4) Events with missing ET.• The QCD- are orders of magnitude larger than any exotic channel .
• QCD events must be rejected early in the DAQ chain and selecting them using high ET cuts in the trigger will simply not work. Need to select events at the 1:1011 level with almost no dead-time.• HLT must then be able to run full blown reconstruction software and selection filters
UCL Seminar, 16/2/07 Costas Foudas, Imperial College London
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The CMS Trigger SystemThe CMS Trigger System
• 40 MHz input • 100 KHz FLT rate• 3.2 sec Latency• 100 Hz written at the output• Event Size 1-2 Mbytes• The requirements on the
Level-1 Trigger are demanding.
• Level-1 Trigger: Custom made hardware processor.
• High Level Trigger: PC Farm using reconstruction software and event filters similar to the offline analysis.
UCL Seminar, 16/2/07 Costas Foudas, Imperial College London
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CMS Trigger and DAQCMS Trigger and DAQ
• The First Level decision is distributed to the Front-end as well as the readout units.
• Front-end and readout buffers take care of Poisson fluctuations in the trigger rate.
• Hand-shaking using back-pressure guarantees synchronization
Detector Frontend
Computing Services
ReadoutSystems
FilterSystems
Event Manager Builder Networks
Level-1Trigger
RunControl
UCL Seminar, 16/2/07 Costas Foudas, Imperial College London
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The CMS L1 Calorimeter TriggerThe CMS L1 Calorimeter Trigger
• FE: Front End • P: Pipeline• RCT: Regional Calorimeter Trigger• GCT: Global Calorimeter Trigger• GT: Global Trigger
Y/N
TPG
RCT
GT
GCT
P
FE
Detector data stored in Front End Pipelines. Trigger decision derived from Trigger Primitives generated on the detector. Regional Triggers search
for Isolated e/ and and compute the transverse, missing energy of the event. Event Selection Algorithms run on the Global Triggers 128x25ns=3.2 µsec
later i.e. 128 bunch-crossings latency
UCL Seminar, 16/2/07 Costas Foudas, Imperial College London
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Level-1 StrategyLevel-1 Strategy
• Selecting events using physics filters at the High Level Trigger level (HLT CPU farms) will not do. The rate must be cut earlier before the HLT is overwhelmed by MHz of background QCD jet events.
• It follows that the first level of selection, the First Level Trigger, should include algorithms of considerable sophistication which can find Isolated Electrons, Jets and detect specific event topologies.
• This is a challenging task because we only have 15x25 ns = 375 ns to accomplish it for all sub-triggers. Jets take longer: 24x25 nsec = 600 nsec; which is many orders of magnitude faster than offline.
• An example of this is CMS Global Calorimeter Trigger (GCT)
UCL Seminar, 16/2/07 Costas Foudas, Imperial College London
UCL Seminar, 16/2/07 Costas Foudas, Imperial College London
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CMS GCT CardsCMS GCT Cards
UCL Seminar, 16/2/07 Costas Foudas, Imperial College London
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The Leaf Card (eThe Leaf Card (e±±, Jets, E, Jets, ETT))
• Main processing devices: Xilinx Virtex II Pro P70• 32 x 5 Gbit/sec Links with Serializers/Deserializers • Each serves 1/6 of the detector in Jet finding mode.
Virtex-II Pro-P70
3x12 Channel 5 Gbit/s
Optical Links (eventually)
UCL Seminar, 16/2/07 Costas Foudas, Imperial College London
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Data Sharing SchemeData Sharing Scheme
• Each Jet Leaf Card Serves 3 Regional calorimeter crates or 1/3 of half Barrel calorimeter (forward calorimeters have been included as edges of the barrel).
• Each Leaf Searches for Jets using a 3x3 region sliding window.• Each Leaf has access to boundary data from neighbours via data
duplication at the input of each Leaf
η- η+
UCL Seminar, 16/2/07 Costas Foudas, Imperial College London
UCL Seminar, 16/2/07 Costas Foudas, Imperial College London
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Trigger Commissioning and Testing Trigger Commissioning and Testing without Beamwithout Beam : : Patterns TestsPatterns Tests
• Install, integrate trigger chain and connect TTC system. • Propagate patters from the Trigger front end all the way to HLT and DAQ.• GCT is given here as an example but other systems will perform similar tests,• GCT: Electron Patterns Tests (March 07)
(1) The Source Cards will be loaded with events containing 4 electrons in various parts of the detector. (2) Empty crossings will be loaded in between the electron events. Each Source Card can store half and orbit worth of data (~1500 thousand crossings) which can be either empty or test events. (3) The data will be propagated from the Source Cards via the optical links, to the two electron Leaf Cards and from there to the concentrator all the way to the Global Trigger and also to the DAQ. (4) The data will also be processed by the GCT emulator and the results of the emulator and the hardware will be compared.• Goals: (a) Exercise and validate a given trigger path. (b) Establish synchronization: 4 electrons should arrive at GT at the correct crossings with the correct energy, rapidity and phi. (c) Establish agreement between software and firmware
UCL Seminar, 16/2/07 Costas Foudas, Imperial College London
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Trigger Commissioning and Testing Trigger Commissioning and Testing without Beamwithout Beam : : Cosmic Ray TestsCosmic Ray Tests
• Take Cosmic Ray (CR) runs. Trigger using the muon detectors (RPC,DT,CSC) However be aware that CR do not come synchronously with the clock and do not necessarily go through the interaction point where the muon systems are optimized to trigger.
• Raw rate estimated ~ 1.8 KHz for muon momentum above 10 GeV. This should decrease a lot after cuts on
timing and muon direction are folded in.
• Goals: (a) Exercise and validate the data taking system. (b) Establish coarse synchronization. (c) Start aligning the detectors.
• Almost no Level-1 cuts; HLT runs Level-1 simulation to validate the Level-1 trigger; Muon reconstruction at HLT but no momentum cuts.
UCL Seminar, 16/2/07 Costas Foudas, Imperial College London
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Pilot Run in 2007 (900 GeV)Pilot Run in 2007 (900 GeV)
UCL Seminar, 16/2/07 Costas Foudas, Imperial College London
UCL Seminar, 16/2/07 Costas Foudas, Imperial College London
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Particle Distributions:Particle Distributions:
• Particles go forward and have energy below 1 GeV.
• Need to be able to Trigger forward at low energy.
• Obviously you do not want
a transverse energy trigger.
Will not be better at 900 GeV
UCL Seminar, 16/2/07 Costas Foudas, Imperial College London
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Trigger for Pilot Run 2007 Trigger for Pilot Run 2007 CMS Trigger Mode of Operation:
• Ideas on how to do this: (1) Random Level-1 triggers at 1% level. (2) CMS will be using the OPAL scintillators mounted at the front face of the Hadron Forward Calorimeter (HF). (3) Energy/Et over threshold from the first 2 rings around the beams pipe (both sides) in coincidence. (4) Feature Bits from the forward regions will be used to count trigger towers over threshold.
• L1T identifies collisions and accepts all events
• HLT verifies L1T bits, and stream events to calibration, e, , jets..
• In other words we need a Beam Bias Trigger just to ‘see’ beams.
UCL Seminar, 16/2/07 Costas Foudas, Imperial College London
• GCT can compute the energy or transverse energy in rings around the beam pipe form both sides of the calorimeter; energy is better.• Global Trigger can set threshold on energy or transverse energy.• Forward and Rear in Coincidence
UCL Seminar, 16/2/07 Costas Foudas, Imperial College London
• For luminosities above 1031 cm–2s–1 we need to set thresholds at L1 and refine object ID at HLT
• 75ns operation possible till we see a luminosity of 1033 cm–2s–1
• Average of 5 interactions per crossing at the peak. Only in-time pileup relevant
• Going to 25ns - 1 operation, i.e. at 33% bunch intensity, keeps the luminosity about the same but pileup goes down:
• Now about 1-2 interactions per crossing • Pileup plays a less significant role
UCL Seminar, 16/2/07 Costas Foudas, Imperial College London
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CMS Trigger OverviewCMS Trigger Overview
• CMS is gearing up for the first data. Preparations of almost two decades come to a conclusion and I am sure it will be a very exciting time.
• However, the trigger and DAQ systems at LHC are orders of magnitude more complicated than before but also more sophisticated producing samples of purity not seen before .
• Understanding the first samples will not be easy, it will take time and requires a methodical and systematic approach.
• But you can be sure that he who understands his detector first will be closer to discovery using the data after 2008.