The CMS Level-1 Trigger System
Dave Newbold, University of Bristol
On behalf of the CMS collaboration
Dave Newbold, University of Bristol HEP2003 Conference - 19/7/2003
Triggering at the LHC Tiny signals, huge background
p-p inelastic rate: ~ GHz e.g. H(150) -> : < mHz …but bb ~MHz – background!
Complex events & detector Typical CMS event is >1MB Max storage rate : 100MB/s
Huge selectivity needed Trigger reduction factor: 107
[L1 o/p rate < 100kHz]
LHC collisions @ 40MHz Mean of 23 evts per BX at full
luminosity Detector response & time of
flight are > 1BX
Dave Newbold, University of Bristol HEP2003 Conference - 19/7/2003
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 -> also important -> Identify high-pt leptons* and photons (*including ) Low pt thresholds motivated by efficiency for W / Z / light Higgs
Trigger combinations >20GeV limit on single-lepton thresholds due to quark decay + 0 b/g Most interesting states decay to two or more trigger objects – can use
lower thresholds for objects in combination -> 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 -> 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 -> Need to include overlapping and minbias triggers to measure
Dave Newbold, University of Bristol HEP2003 Conference - 19/7/2003
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 3.2s
(of which 2s 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)
Dave Newbold, University of Bristol HEP2003 Conference - 19/7/2003
ECAL, HCAL cover to = 3, forward calorimeter to = 5 Trig prims group crystals / scintillators into (2 x) 32 x 72 trigger towers
Calorimeter trigger detectors
Dave Newbold, University of Bristol HEP2003 Conference - 19/7/2003
Calorimeter trigger performance
Full GEANT study Includes
minbias background
L=1034 cm-2s-1
Efficiencies For objects
within fiducial acceptance
Rates e/ dominated
by jet (0) background
Steep curves allow good control of rate
Dave Newbold, University of Bristol HEP2003 Conference - 19/7/2003
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
Dave Newbold, University of Bristol HEP2003 Conference - 19/7/2003
Muon trigger performance
Efficiency for any muon >3 GeV pt
Dave Newbold, University of Bristol HEP2003 Conference - 19/7/2003
Global trigger
Global trigger implements a wide range of triggers (incl. topological) Example low lumi (L=2.1033 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
Trigger type Thrsh (0.95 eff) Rate (kHz) Cum. Rate (kHz)
Incl. iso e/ 29 GeV 3.3 3.3
Di- e/ 17 GeV 1.3 4.3
Incl. iso 14 GeV/c 2.7 7.0
Di- 3 GeV/c 0.9 7.9
Single 86 GeV 2.2 10.1
Di- 59 GeV 1.0 10.9
1j, 3j, 4j 177, 86, 70 GeV 3.0 12.5
j && Etmiss 88 ; 46 GeV 2.3 14.3
e/, j 21 ; 45 GeV 0.8 15.1
Minbias, calib, efficiency estimation 0.9 16.0
Dave Newbold, University of Bristol HEP2003 Conference - 19/7/2003
Physics efficiencies
Typical efficiencies for preceding trigger table @ L=2.1033 cm-2s-1
Channel Efficiency % Efficiency % from each trigger type – (each) cumulative
W - > e 70 e: (70) 70
t -> eX 91 e: (82) 82 e. (62) 86 : (55) 89 jjj: (24) 90 e.j: (54) 91
Z -> ee 94 e: (93) 93 e.e: (76) 94
H(115) -> gg 99 e: (99) 99 e.e (82) 99
H(150) -> WW 87 e: (78) 78 e.(43) 81 : (34) 83 e.j: (39) 85 j: (28) 87
H(135) -> -> ej 84 e: (70) 70 e. (62) 86 e.j: (54) 91 : (38) 84 j: (34) 84
Charged H(200) 98 : (85) 85 jj: (77) 96 j.Etm: (60) 98
H(200) -> -> jj 81 : (75) 75 : (50) 79 j: (24) 81 jj: (9) 81
H(500) -> -> jj 99 : (94) 94 : (64) 94 j: (94) 99 jj: (73) 99
t -> jets 53 Ht: (39) 39 jjjj: (26) 43 jjj: (26) 46 jj: (21) 47 j: (35) 53
mSUGRA 99 j: (99) 99
H(120) -> bb 41 jjj: (12) 12 j: (27) 30 : (26) 41 jj: (16) 41
Invisible H(120) 44 j.Etm: (39) 39 j: (22) 41 : (13) 44
Dave Newbold, University of Bristol HEP2003 Conference - 19/7/2003
Practical challenges Technology
Pushing (today’s) digital processing and comms technologies to the limit Cannot afford huge outlay on custom components (-> 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 -> radiation, magnetic field considerations
Simulation / configuration control / real-time trigger optimisation Trigger integration for CMS begins 2004 - the real work starts now
Dave Newbold, University of Bristol HEP2003 Conference - 19/7/2003
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 < 50KHz L1A Very large digital logic system Uses calorimeter and muon information only
Simulated performance shows good efficiency for the interesting channels
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