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CMS High Level Trigger Selection
Giuseppe BagliesiINFN-Pisa
On behalf of the CMS collaboration
EPS-HEP 2003Aachen, Germany
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G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003 2
Outline
LHC Environment High Level Trigger strategy Object selection
e/Jet , b HLT rates and efficiencies Conclusions
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G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003 3
p-p collisions at LHC
Crossing rate 40 MHzEvent Rates: ~109 Hz
Max LV1 Trigger 100 kHzEvent size ~1 MbyteReadout network 1 Terabit/sFilter Farm ~106 Si95Trigger levels 2Online rejection 99.9997% (100 Hz from 50 MHz)System dead time ~ %Event Selection: ~1/1013
Crossing rate 40 MHzEvent Rates: ~109 Hz
Max LV1 Trigger 100 kHzEvent size ~1 MbyteReadout network 1 Terabit/sFilter Farm ~106 Si95Trigger levels 2Online rejection 99.9997% (100 Hz from 50 MHz)System dead time ~ %Event Selection: ~1/1013
Event rate
“Discovery” rate
LuminosityLow 2x1033 cm-2 s-1
High 1034 cm-2 s-1
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G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003 4
Level-1 (~µs) 40 MHz High-Level ( ms-sec) 100 kHzEvent Size ~ 106 Bytes
Level-1 (~µs) 40 MHz High-Level ( ms-sec) 100 kHzEvent Size ~ 106 Bytes
Trigger environment
40 MHzClock drivenCustom processors
100 kHzEvent drivenPC networkTotally software
100 HzTo mass storage
two trigger levelstwo trigger levels
All charged tracks with pt > 2 GeV
Reconstructed tracks with pt > 25 GeV
Operating conditions:one “good” event (e.g Higgs in 4 muons )
+ ~20 minimum bias events)
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G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003 5
High Level Trigger requirements and operation
HLT: Reconstruction and selection of electrons, photons, muons, jets, missing ET, and b and tagging.
HLT has access to full event data (full granularity and resolution) maximum flexibility
Main requirements: Satisfy CMS physics program with high efficiency Inclusive selection (we like to see also unexpected physics!) Must not require precise knowledge of calibration/run conditions Efficiency must be measurable from data alone The HLT code/algorithms must be as close as possible to the offline
reconstruction Limitations:
CPU time Output selection rate (~102 Hz) Precision of calibration constants
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G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003 6
Regional Reconstruction
Regional• process (e.g. DIGI to RHITs) each detector on a "need" basis• link detectors as one goes along• physics objects: same
14
Detector
ECAL
Pixel L_1
Si L_1
Pixel L_2
HCAL
Detector
ECAL
Pixel L_1
Si L_1
Pixel L_2
HCAL
Global • process (e.g. DIGI to RHITs) each detector fully• then link detectors• then make physics objects
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G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003 7
e/selection
Level-2
Level-3
Level-1
Level-2.5
PhotonsThreshold cut
ElectronsTrack reconstruction
E/p, matching () cut
ECAL reconstructionThreshold cut
Pixel matching
In addition:
• Isolation cuts (ECAL, pixel, track)
•Had/EM isolation
• 0 rejection
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G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003 8
HLT Electron selection (I)
Level-2 electron:• 1-tower margin around 4x4 area
found by Lvl-1 trigger• Apply “clustering”• Accept clusters if EHCAL /EECAL <0.05• Select highest ET cluster
•Brem recovery:• Seed cluster with ET>ETmin
• Road in around seed• Collect all clusters in road• “supercluster”and add all energy in road
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G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003 9
HLT Electron selection (II)
Level-2.5 selection: add pixel information Very fast, high rejection
high efficiency (=95%), high background rejection (14)• Pre-bremsstrahlung:
Matching hits given by most electrons and by few photons • Require at least 2 hits (3 pixel hits available almost always)
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G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003 10
HLT Electron selection (III)
Level-3 electron: Build tracks from pixel seeds found in
pixel-matching step
Very loose track requirements for high efficiency for radiating tracks:
3-hit layers Allow 2 consecutive missing layers
Track selection: Barrel: E/p and (track-cluster) Endcap: E/p
Also (non-track): H/E
With tight cuts is always possible to select almost no-radiating electron with very high purity
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G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003 11
HLT muon track reconstruction
Inclusion of Tracker Hits: “Level-3”•Define a region of interest through tracker based on L2 track with parameters at vertex• Find pixel seeds, and propagate from innermost layers out, including muon
Standalone Muon Reconstruction: “Level-2”
• Seeded by Level-1 muons• Kalman filtering technique applied to DT/CSC/RPC track segments•GEANE used for propagation through iron• Trajectory building works from inside out• Track fitting works from outside in• Fit track with beam constraint
Single muons10<Pt<100 GeV/c
Level-3Algorithmic efficiency
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L2 & L3 muon pT resolution and efficiency
=0.11
=0.013
L2
L3
PT resolution barrel
10 GeV threshold
30 GeV
10. 30. 50.
Efficiency vs PT threshold
L1
L2
L3
(1/pTrec-1/pT
gen) /(1/pTgen)
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G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003 13
Isolation and physics content after muon Level-3
Before isolation After isolation
Muons from b,c,K, decays are greatly suppressed by isolation
Isolation is based on transverse energy (ET ) or momentum (PT ) measurements in cones around the muon Calorimeter isolation - ET from calorimeter towers in a cone around the muonPixel isolation - PT of 3-hit tracks in the pixel detector in cone around the muon - Requires that contributing tracks come from same primary vertex as the Level-3 muon (to reduce pile-up contamination)Tracker isolation - PT of tracks in the Tracker (regional reconstruction around L3 muon)
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G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003 14
HLT efficiencies on H WW 22
Efficiency @ low lumi:MH=120 GeV: single mu 74% , di-mu exclusive 14% , combined: 87 %MH=160 GeV: single mu 87% , di-mu exclusive 5% , combined: 92 %
L3 threshold
L3 muon thresholdsat low luminosity:
Single 19 GeVDouble 7 GeV
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G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003 15
Jet rates and thresholds Low luminosity:
1 kHz at Level-1: 177 GeV (1 jet), 85 GeV (3 jet), 70 GeV (4 jet) 1 Hz at HLT: 657 GeV (1 jet), 247 GeV (3 jet), 149 GeV (4 jet)
High luminosity: 1 kHz at Level-1: 248 GeV (1 jet), 112 GeV (3 jet), 95 GeV (4 jet) 1 Hz at HLT: 860 GeV (1 jet), 326 GeV (3 jet), 199 GeV (4 jet)
Very high rates and thresholds!•HLT triggers need some other conditionto have acceptably low threshold•MET, leptons, isolation, vertices…
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G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003 16
MET Rates
Calorimeter coverage: ||<5
Generator level:•real neutrinos -> ET
miss>60 GeV•ET
miss<60 GeV mostly due to limited coverage
Much higher ETmiss at HLT than
at generator level
•“ETmiss” objects selection is
done in association with other requirements, like a energetic jet
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Partial track reconstruction strategy at HLT
At HLT ultimate resolution is not neededGood track parameter resolution is obtained already with 4 or more hitsThe time for track reconstruction increases linearly with the number of hits
Momentum resolution
Full tracker
Impact parameter resolution
Full tracker
Reconstruct only a ROI (Region Of Interest) from LVL1 candidate objects (regional tracking)Use a reduced number of hits (conditional tracking)
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Tracker @ HLT: tau tagging
Regional Tracking: •Look only in Jet-track matching cone•Loose Primary Vertex associationConditional Tracking: Stop track as
soon asPixel seed found (PXL) / 6 hits found (Trk)
If Pt<1 GeV with high C.L.Reject event if no “leading track” found
Regional Tracking: •Look only inside isolation cone•Loose Primary Vertex association
Conditional Tracking: Stop track as soon asPixel seed found (PXL) / 6 hits found (Trk)
If Pt<1 GeV with high C.L.
Reject event as soon as additional track found
Regional seeding: look for seeds in a specific region
Essential at High LuminosityEssential at High Luminosityactivity well advancedactivity well advanced
TEST CHANNELS•A0/H0 (200, 500 GeV) -> -jet -jet, -jet lepton•H+(200, 400 GeV) -> -jet •Efficiencies ~ 40-50%, •Background rej. after LVL1 ~103
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G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003 19
Inclusive b tagging at HLT
Inclusive b tag at HLT possible, provided alignment under control
Use tracks to define Jet axis(if rely on L1 Calo Jet ~ randomize signed IP)
Performance of simple signed IP “track counting” tags~ same as after full track reconstruction
Regional Tracking: Look only inJet-track matching cone
Loose Primary Vertex association
Conditional Tracking: Stop track as soon asPixel seed found (PXL) / 6 hits found (Trk)
If Pt<1 GeV with high C.L.
~300 ms low lumi~300 ms low lumi~1 s high lumi~1 s high lumi
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G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003 20
HLT table: LHC start…
Level-1 rate “DAQ staging”:50 KHz
Total Rate: 105 Hz
Average HLT CPU:300ms*1GHz Improvements are possible
Channel Efficiency (for fiducial objects)H(115 GeV) 77%H(160 GeV)WW* 2 92%HZZ4 92%A/H(200 GeV)2 45%SUSY (~0.5 TeV sparticles) ~60%
With RP-violation ~20%
We 67% (fid: 60%)W 69% (fid: 50%)Top X 72%
HLT performances:
Priority to discovery channels
Trigger Threshold (=90-95%) (GeV)
Indiv.Rate (Hz)
Cumul rate(Hz)
1e, 2e 29, 17 34 34
1, 2 80, (40*25) 9 43
1, 2 19, 7 29 72
1, 2 86, 59 4 76
Jet * Miss-ET180 * 123 5 81
1-jet, 3-jet, 4-jet 657, 247, 113 9 89
e * jet 19 * 52 1 90
Inclusive b-jets 237 5 95
Calibration/other 10 105
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G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003 21
HLT: CPU usageAll numbers for a 1 GHz, Intel Pentium-III CPU
Trigger CPU (ms)
Rate (kHz)
Total (s)
1e/, 2e/ 160 4.3 688
1, 2 710 3.6 2556
1, 2 130 3.0 390
Jets, Jet * Miss-ET
50 3.4 170
e * jet 165 0.8 132
B-jets 300 0.5 150
Total: 4092 s for 15.1 kHz 271 ms/eventTime completely dominated by slow GEANE extrapolation in muons – will improve!Consider ~50% uncertainty!
Today: ~300 ms/event on a 1GHz Pentium-III CPU
Physics start-up (50 kHz LVL1 output): need 15,000 CPUs
Moore’s Law: 2x2x2 faster CPUs in 2007
~ 40 ms in 2007, ~2,000 CPUs~1,000 dual-CPU boxes in Filter Farm
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G. Bagliesi – EPS 2003 – Aachen 17-24/7/2003
SummaryThe regional/conditional reconstruction is very useful to reduce CPU time and very effective in the HLT selection
Tracker at HLT: Essential for muons, electron and tau selection inclusive/esclusive b-trigger is possible
Standard Model physics: “just do it” at lower initial luminosity (“dedicated” triggers could be
implemented) Pre-scale or lower thresholds when luminosity drops through fill
ConclusionsStart-up system 50kHz (Level-1) and 105 Hz (HLT) satisfy basic “discovery menu”
The HLT design based on a purely software selection will work:
Maximum flexibility and scalability Possibility to use “off-line” reconstruction/algorithms