CMS High Level Trigger Selection

CMS High Level Trigger Selection

Giuseppe BagliesiINFN-Pisa

On behalf of the CMS collaboration

EPS-HEP 2003Aachen, Germany

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


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


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)


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


Regional Reconstruction

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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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


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


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


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)


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


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

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)


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)


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


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…


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


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


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


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

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

CMS High Level Trigger Selection

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