CMS High Level Trigger Selection

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CMS High Level Trigger Selection
EPS-HEP 2003 Aachen, Germany

• Giuseppe Bagliesi
• INFN-Pisa
• On behalf of the CMS collaboration

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Outline

• LHC Environment
• High Level Trigger strategy
• Object selection
• e/g, m, Jet/n, t, b
• HLT rates and efficiencies
• Conclusions

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p-p collisions at LHC
Event rate
Crossing rate 40 MHz Event Rates
109 Hz Max LV1 Trigger 100 kHz Event
size 1 Mbyte Readout network
1 Terabit/s Filter Farm 106
Si95 Trigger levels 2 Online rejection
99.9997 (100 Hz from 50 MHz) System dead
time Event Selection 1/1013
Luminosity Low 2x1033 cm-2 s-1 High 1034
cm-2 s-1
Discovery rate
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Trigger environment
40 MHz Clock driven Custom processors 100
kHz Event driven PC network Totally software 100
Hz To mass storage
two trigger levels
Level-1 (µs) 40 MHz High-Level ( ms-sec) 100
kHz Event Size 106 Bytes
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High Level Trigger requirements and operation

• HLT Reconstruction and selection of electrons,
  photons, muons, jets, missing ET, and b and t
  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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Regional Reconstruction
Global process (e.g. DIGI to RHITs) each
detector fully then link detectors then make
physics objects
Regional process (e.g. DIGI to RHITs) each
detector on a "need" basis link detectors as
one goes along physics objects same
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e/g selection
Level-1
Level-2
ECAL reconstruction Threshold cut
Level-2.5
Pixel matching

• In addition
• Isolation cuts (ECAL, pixel, track)
• Had/EM isolation
• p0 rejection

Level-3
Photons Threshold cut
Electrons Track reconstruction E/p, matching (Dh)
cut
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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 lt0.05
• Select highest ET cluster

• Brem recovery
• Seed cluster with ETgtETmin
• Road in f around seed
• Collect all clusters in road
• ? superclusterand
• add all energy in road

Supercluster
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HLT Electron selection (II)

• Level-2.5 selection add pixel information
• Very fast, high rejection
• high efficiency (e95), 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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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 Dh(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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HLT muon track reconstruction

• 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

• 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

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L2 L3 muon pT resolution and efficiency
PT resolution barrel
Efficiency vs PT threshold
L2
s0.11
10 GeV threshold
L3
L3
L1
s0.013
30 GeV
L2
10.
30.
50.
(1/pTrec-1/pTgen) /(1/pTgen)
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Isolation and physics content after muon
Level-3
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
muon Pixel 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)
Muons from b,c,K,p decays are greatly suppressed
by isolation
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HLT efficiencies on H? WW ? 2m2n
L3 muon thresholds at low luminosity Single
m 19 GeV Double m 7 GeV
L3 threshold

Efficiency _at_ low lumi MH120 GeV single mu 74
, di-mu exclusive 14 , combined 87 MH160
GeV single mu 87 , di-mu exclusive 5 ,
combined 92
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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 condition
• to have acceptably low threshold
• MET, leptons, isolation, vertices

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MET Rates

• Calorimeter coverage hlt5
• Generator level
• real neutrinos -gt ETmissgt60 GeV
• ETmisslt60 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
Reconstruct only a ROI (Region Of Interest) from
LVL1 candidate objects (regional tracking) Use a
reduced number of hits (conditional tracking)

• At HLT ultimate resolution is not needed
• Good track parameter resolution is obtained
  already with 4 or more hits
• The time for track reconstruction increases
  linearly with the number of hits

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Tracker _at_ HLT tau tagging

• TEST CHANNELS
• A0/H0 (200, 500 GeV) -gt t-jet t-jet, t-jet lepton
• H(200, 400 GeV) -gt t-jet n
• Efficiencies 40-50,
• Background rej. after LVL1 103

• Regional Tracking
• Look only in Jet-track matching cone
• Loose Primary Vertex association

Conditional Tracking Stop track as soon as Pixel
seed found (PXL) / 6 hits found (Trk) If Ptlt1 GeV
with high C.L.
Reject event if no leading track found
Regional seeding look for seeds in a specific
region

• Regional Tracking
• Look only inside isolation cone
• Loose Primary Vertex association

Conditional Tracking Stop track as soon as Pixel
seed found (PXL) / 6 hits found (Trk) If Ptlt1 GeV
with high C.L.
Essential at High Luminosity activity well
advanced
Reject event as soon as additional track found
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Inclusive b tagging at HLT
Regional Tracking Look only in Jet-track
matching cone Loose Primary Vertex association
Conditional Tracking Stop track as soon as Pixel
seed found (PXL) / 6 hits found (Trk) If Ptlt1 GeV
with high C.L.
300 ms low lumi 1 s high lumi
Performance of simple signed IP track counting
tags same as after full track reconstruction
Use tracks to define Jet axis (if rely on L1 Calo
Jet randomize signed IP)
Inclusive b tag at HLT possible, provided
alignment under control
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HLT table LHC start
Trigger Threshold (e90-95) (GeV) Indiv. Rate (Hz) Cumul rate(Hz)
1e, 2e 29, 17 34 34
1g, 2g 80, (4025) 9 43
1m, 2m 19, 7 29 72
1t, 2t 86, 59 4 76
Jet Miss-ET 180 123 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

• Level-1 rate DAQ staging
• 50 KHz
• Total Rate 105 Hz
• Average HLT CPU
• 300ms1GHz
• Improvements are possible

Channel Efficiency (for fiducial objects)
H(115 GeV)?gg 77
H(160 GeV)?WW ?2m 92
H?ZZ?4m 92
A/H(200 GeV)?2t 45
SUSY (0.5 TeV sparticles) 60
With RP-violation 20
W?en 67 (fid 60)
W?mn 69 (fid 50)
Top?m X 72

• HLT performances
• Priority to discovery channels

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HLT CPU usage

• All numbers for a 1 GHz, Intel Pentium-III CPU

Trigger CPU (ms) Rate (kHz) Total (s)
1e/g, 2e/g 160 4.3 688
1m, 2m 710 3.6 2556
1t, 2t 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/event Time
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 Moores 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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Summary

• The 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
• Conclusions
• Start-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/algor
  ithms