norbert neumeister october 25, 2001 iii international ... · r inner,outer[mm] 1238,1750 316,1711...

III International Symposium on III International Symposium on LHC Physics and DetectorsLHC Physics and Detectors

ChiaChia, Sardinia, SardiniaOctober 25, 2001October 25, 2001

Norbert NeumeisterNorbert NeumeisterCERN EP / HEPHY Vienna

2III International Symposium on LHC Physics and DetectorsIII International Symposium on LHC Physics and DetectorsChia Sardinia, 25th October 2001

Norbert NeumeisterCERN EP / HEPHY Vienna

OutlineOutline

• Introduction• Electrons, Photons

– ECAL– Preshower– Electron reconstruction– High Level Trigger strategy– H → γγ

• Muons– Detectors– Reconstruction– High Level Trigger strategy

• Summary



IntroductionIntroduction

• Simulation + Reconstruction– Detector simulation done with GEANT 3 (FORTRAN)– Reconstruction with ORCA (Object-oriented Reconstruction for

CMS Analysis )• Have been working with OO software (C++ reconstruction code)

– Regional reconstruction• High Level Trigger

– Concentrate on startup luminosity L = 2×1033cm–2s–1

• pileup is included



Provide a precise measurement of the energy of electrons and photons, particularly important for the Higgs decay signature H →→→→ γγγγγγγγ

Electromagnetic CalorimeterElectromagnetic Calorimeter



ECALECAL



Barrel Endcaps

coverage |η| < 1.48 1.48<|η|<3.0r inner,outer[mm] 1238,1750 316,1711z inner,outer[mm] 0,±3045 ±3170,±3900 ∆η∆η∆η∆η

x ∆Φ∆Φ∆Φ∆Φ 0.0175 x 0.0175 0.021x0.021 to

0.05 x 0.05depth in X0 25.8 23off pointing 3° 3°Nb of crystals 61200 16000Volume 8.14 m3 2.2 m3

Radiation Length 0.89 cm

Decay time 10 ns

Lead Tungstate Crystals

PbWO4 CrystalsPbWO4 Crystals



PreshowerPreshower

• Coverage: – 1.65 < |η| < 2.6– r inner,outer [mm] 420,1250– z inner,outer [mm] ±2970,±3170– ∆η x ∆Φ 2x63

• 2 lead radiators – 2 and 1 radiation length thick

• 2 planes of silicon microstrip detectors– First layer: vertical strips– Second layer: horizontal strips

• Depth: 3 X0

• 4512 silicon detectors• Modularity: 500 ladders• Area: 16.4 m2

π0 decay

Single photon



ECAL ReconstructionECAL Reconstruction• Main challenge is reconstruction in ECAL of electrons after a thick

(~1X0 ) tracker in a 4T magnetic field– Electrons radiate in the material between the interaction point and the

ECAL. The bending of the electron in the 4T magnetic field results in a spray of energy reaching the ECAL. The spreading of this spray is, to good approximation, only in the φ-direction.

• First step: clustering of the energy deposits in the ECAL and the estimation of the electron’s energy and position– Barrel: energy deposited in the lead tungstate crystals– Endcaps: energy is also deposited in the ~3X0 thick preshower detector– Electron energy can be collected by making a cluster of clusters along a

road• Concentrate first on electron reconstruction because the target

transverse momentum cuts and thresholds for triggering on electrons are much lower than those for photons– We are interested here in electrons in the range 10 < pT < 40 GeV– Photons, at the significantly higher transverse momenta, are adequately

reconstructed with electron algorithms



Electron HLTElectron HLT• Signal = electrons (photons wanted at significantly larger pT)• Background = jets (largely dominated by jets where a single π0

takes a large fraction of the jet ET)

Level-2 algorithm:• Clustering: define a basic cluster as a collection of cells with energy deposition• Bremsstrahlung recovery• Electron energy calibration• Position correction

Level-2.5 : match ECAL super-clusters with hits in the pixel detectorLevel-3 : add full tracker information, electron tracking

Level-2 : reconstruct an ECAL super-cluster in a region specified by the Level-1 triggerreject background after Level-1 trigger using ECAL/preshower only



ClusteringClustering

• Collection of energy resulting from an electromagnetic shower in a fine grained crystal calorimeter – can be approached as a pattern recognition procedure

• The shower appears as a local maximum (bump) in a spatial array of crystal energy deposits

• Looking for single crystal local maxima (“seeds”), which are then extended to collect as large a fraction of the original shower energy deposition as possible, while avoiding the collection of energy depositions from nearby particles and noise



Bremsstrahlung Bremsstrahlung RecoveryRecoveryAverage bremsstrahlung loss (~44%) corresponding to an average thickness of 0.57 X0 (in barrel)

• define a road in φ• use narrow η window• collect all clusters in road• make a “cluster of clusters”• recalculate the energy and position• in barrel: take advantage of the η - φgeometry of the crystals

�Produces a super-cluster(collection of ECAL clusters)�Removes large tails



Hybrid AlgorithmHybrid Algorithm

• Use ηηηη−−−−φφφφgeometry of barrel crystals to exploit the knowledge of the lateral shower shape in the ηηηη direction– Start from a seed crystal– Take a fixed domino of 3 or 5 crystals in η– Search dynamically in φ

• In more detail:– Start if ET

seed>EThyb

– Make 1x3 domino– If center of domino>Ewing

• Extend to 1x5– Proceed Nstep crystals from the

origin – Remove dominoes below Ethresh

– Disconnected domino preclusters with E>Eseed are then reclustered in φ (producing a super-cluster)

η

ϕ



Position MeasurementPosition Measurement• Determine the position where an unradiated electron would go• Lateral position of the crystal axis depends on depth• Dependence of shower max on energy ~log(E)• Tmax = A[T0+log(E)]• Specific for electrons and photons• Log weighting technique:

crystal axis

nominal pos @ front face

correctedposition

tmax

0

1000

2000

3000

-25 0 2 5φmeas - φtrue (mrad)

Even

ts σgauss = 1.6mradσeff = 2.1mrad

electronspT = 35GeV

0

2000

4000

-25 0 2 5ηmeas - ηtrue (x 10 3)

σgauss = 1.0x10 -3

σeff = 1.0x10 -3

-4

-2

0

2

4

0.18 0.2 0.22 0.24 0.26

-4

-2

0

2

4

0.18 0.2 0.22 0.24 0.26ηtrue

η mea

s -

η true

(x10

00)

+=

∑i i

ii E

EWW log0

W0 ~ smallest fractional energy to contribute to position calculation

S-shape before log-weighting

Position resolution after correction



Energy ScaleEnergy Scale• Energy is estimated by the sum of energy deposits• Emeas/Etrue : Gaussian + tail, peaking at <1

– incomplete containment, unrecovered bremsstrahlung energy

• Set the energy scale such that the Gaussian peaks at 1• Parametrize correction as a function of the

number of crystals included in the cluster• Endcaps: energy deposited in the preshower added

0.93

0.94

0.95

0.96

0.97

0.98

0 1 0 2 0 3 0 4 0 5 0Ncrystal

Ere

c/E

true

0

1000

2000

0.8 1 1.2Emeas /Etrue (hyb cor)

Even

ts µ = 1.002σgauss /µ = 0.80%

σeff/µ = 1.61%

electronspT = 35GeV



Photon PerformancePhoton Performance

• Performance on unconverted photons: σeff/E ~0.9%

0

1000

2000

3000

4000

0.8 1 1.2Emeas /Etrue (Σ25 cor)

Even

ts µ = 0.998σgauss /µ = 0.66%

σeff/µ = 0.76%

Barrel photons25 <pT<50GeV

0

500

1000

1500

0.94 0.96 0.98 1 1.02Emeas /Etrue (Σ9 cor)

Even

ts µ = 1.001σgauss /µ = 0.72%

σeff/µ = 0.88%

Endcap photons25 <pT<50GeV

Effective width is defined as half-width containing 68.3% of the distribution



Pixel MatchingPixel Matching

• Propagate ECAL cluster to pixel search area • Look for pixel hits in the 1st pixel layer• Vertex estimate from 1st pixel hit + ECAL cluster• Accept ECAL cluster as electron if compatible hits are found

– look at 2nd layer; demand two out of three pixel hits– if at least one candidate matches -> electron stream

• Matching hits are given by most electrons and by few photons

η=1.5

η=2.5

zvtx = ± 15 cm



Pixel Matching Pixel Matching

Nominal vertex (0,0,0)

B→

Predict a track

Cluster ECluster position

Propagate tothe pixel layersand look forcompatible hits

If a hit is found,estimate z vertex

Predicta new trackand propagate

Estimated vertex (0,0,z)

Pixel hit



Electron/Photon HLTElectron/Photon HLT

Efficiencies:H(170GeV) → ZZ* → 4efid:99%; Level-1: 99%; Level-2: 97.5%

H(120GeV) → 2γ

fid/phys:52%; Level-1: 99%; Level-2: 96%

85

90

95

100

10 15 20 25 3085

90

95

100

10 15 20 25 30Jet rejection

ε(e± )

(%)

|η| < 2.1

|η| < 2.5

2.10 33/cm 2/s

After pixel matching:Jet rejection = 16, for electron efficiency of 97.7% (|η| <2.1)



Higgs event into two PhotonsHiggs event into two Photons



H H →→→→→→→→ γγγγγγγγγγγγγγγγUnconverted photons

Higgs mass resolution (mH = 110 GeV)

pTγ1 > 40 GeV, pT

γ2 > 25 GeV

barrel+

endcaps

σfit = 0.68 GeV



• Muon Barrel• Four layers of Drift Tube chambers with Bunch crossing identification capability (DTBX)

• Resistive Plate Chambers to detect muon hits for triggering purpose (RPC)

• Muon Endcap• Cathode Strip Chambers (CSC) up to |ηηηη| < 2.4

• Resistive Plate Chambers (RPC) up to |ηηηη| < 2.1

Muon System Muon System



Muon SystemMuon System

0

100

200

300

400

600

700

800

0 200 400 600 800 1000 1200Z (cm)

R (c

m)

RPC

CSC

Drift Tubes η=0.8 η=1.04 η=1.2

MB1

MB2

MB3

MB4

ME1/3

ME2/2

ME1/1

ME1/2

ME3/2

ME4/2

ME2/1

ME3/1

ME4/1

500



Muon DetectorsMuon Detectors

Three types of gaseous particle detectors for muon identification:• Drift Tubes (DT) in the central barrel region• Cathode Strip Chambers (CSC) in the endcap region• Resistive Plate Chambers (RPC) in both the barrel and endcaps

The DT and CSC detectors are used to obtain a precise measurement of the position and thus the momentum of the muons, whereas the RPCchambers are dedicated to providing fast information for the Level-1 trigger



Each DT Chamber comprises4 layers of DT cells in r-ϕ, 4 in r-z and

further 4 in r-ϕ

(No r-z layer in the fourth station)

Nominal Operating Parameters• Nominal Mixture Ar - CO2 (85% -15%)• Nominal voltages strips at 1800V,

wires at 3600,I-Beams at -1800V

• Gain (nominal) 9x104

• Typical charge 1pC

Anode wire ElectrodesCathode

Muon Drift Tube ChambersMuon Drift Tube Chambers

wire pitch = 4.2 cmmax. drift time = 380 ns

250 chambers192 000 channels

42 mm

11 mm



CSC stations are arranged in 4 disks of chambers

Inner rings have 18 CSCs each covering 20°. Outer rings have 36 CSCs covering 10° each

540 chambers : trapezoidal shape

4 Endcap Stations each comprising6 layers of CSCs with (radial) strip and anode wire readout

• Coordinate in bending plane is precisely measured by interpolation of induced charge on strips (σ=63.4 µm)

• Nominal mixture: Ar -CO2 -CF4 (30% 50%20%)

Cathode Strip ChambersCathode Strip Chambers



Dedicated trigger detector, with fast timing responseBasic functions:• identify candidate muon track• assignment of bunch crossing

to the candidate track(s)• estimate their transverse momenta

Resistive Plate ChambersResistive Plate Chambers



Local Pattern RecognitionLocal Pattern Recognition

• Barrel:– Reconstruct φsuper-layer hits (time-space conversion)

global resolution (r-φ) position ~ 100 µm, direction ~ 1mrad)

– Cluster hits (linear fit): 2D segment– Same for z super-layer– Associate the two projections to build a 3D segment– Apply impact angle correction on time-to-distance

relation and refit– Calculate position (center of gravity) of the track-

segment and its angle in the super-layer

• Endcaps:– Reconstruct 3D hit– Associate hits with linear fit (only one hit per layer)

Reconstruct track segments in the DT and CSC detectorsup to 12 hits/station

up to 6 hits/station



Local Muon ReconstructionLocal Muon Reconstruction

• all muon detectors (DTBX, CSC and RPC) are used• Seed generation:

– external: Level-1 trigger (vector at 2nd station) – internal: track segments from local pattern recognition

• Steering: – find reconstructed hits compatible with a given seed and “grow” a trajectory

• Fit: – use all compatible hits to get best parameter estimate for the track– update trajectory with compatible hits (Kalman Filter)– use 3D(2D) segments in barrel and 3D hits in endcaps– apply χ2 cut to reject bad hits– “best” measurements at innermost muon station



MuonsMuons

Muon candidates from Level-1 can be:

• prompt muons– decays of W, Z , top, Higgs, etc.– b and c quark decays

• non prompt muons (from π±, K±, K0L decays, etc)

• fake muons (from Level-1 Trigger)• punchthrough of hadronic showers• cosmic muons• beam halo muons

at the end we only want to keep prompt muons

Output from Level-1 trigger: muons with pT, charge, η, ϕ, quality information



MuonsMuons

rate is dominated by π,K decays up to 4 GeV and by b-, c-quark decays from 4 to 25 GeV

L = 2××××1033 cm-2s-1

threshold [GeV/c]µTp

0 10 20 30 40 50 60 70 80

Rat

e [H

z]

10-2

10-1

1

10

102

103

104

105

106

107

±π/±K

L0K

c

bτ

*γ/0Z±W

all



Muon HLTMuon HLT

Level-2 : based on local muon reconstruction (using L1 seed) :

• confirm Level-1 decision• redefine pT measurement and propagate muon to interaction point

• pT refinement using full information form muon detectors• cut on ‘fit parameters’ (χ2, vtx, IP, etc) :

• reject fake Level-1 muon candidates• reduce non-prompt muon contribution

• isolation cut using calorimeter data

Level-3 : bring in tracker data• match muons with tracker

Strategy for High Level Triggers:



LevelLevel--2 Muon Reconstruction2 Muon ReconstructionLevel-2 algorithm:• L1 input extrapolated to virtual surface• fit outwards to find hits (segments)• fit inwards to remove bad hits• extrapolation to vertex and refit

σz ~ 5.5 cm

position resolution: ~0.02 - 0.04 cmL1 pT resolution:• 16% barrel, 35% endcapsL2 pT resolution:• 8% barrel, 20% endcaps

- local (chamber) pattern recognition- regional track finding

(starting from a seed, segments are associated)- track fitting (based on Kalman Filter)

point of closest approach to vtx

σx,y ~ 1.5 cm



LevelLevel--3 Muon Reconstruction3 Muon Reconstruction

Start from Level-2 reconstructed muons:• Generation of seeds

• Get muon trajectory at innermost muon station• Propagate to outer tracker surface

– Rescale errors• open window for track reconstruction

– Find start layer(s) inside tracker (outside-in)• if there are no compatible hits go to next layer

– Create one or more seeds for each L2 muon

• Construction of trajectories for a given seed

• Resolve ambiguities

• Final fit of trajectories

�tremendous gain in resolution



ppTT ResolutionResolution

-1 -0.8 -0.6 -0.4 -0.2 -0 0.2 0.4 0.6 0.8 10

500

1000

1500

2000

2500

3000

3500

-1 -0.8 -0.6 -0.4 -0.2 -0 0.2 0.4 0.6 0.8 10

1000

2000

3000

4000

5000 Constant = 4185

Mean = 0.008167

Sigma = 0.1398

Chi2 / ndf = 831.5 / 27

Mean = 0.008167

Sigma = 0.1398

-0.1 -0.05 0 0.05 0.10

500

1000

1500

2000

2500

3000

Constant = 2626

Mean = -0.00188

Sigma = 0.01919

Chi2 / ndf = 895.3 / 43

Mean = -0.00188

Sigma = 0.01919

L1 L3L21/pT resolution

σσσσ = 0.14σσσσ = 0.019



EfficiencyEfficiency

|µη|0 0.5 1 1.5 2 2.5

Effic

ienc

y

0

0.2

0.4

0.6

0.8

1

|µη|0 0.5 1 1.5 2 2.5

Effic

ienc

y

0

0.2

0.4

0.6

0.8

1

L1

L2

L3



L2 pL2 pTT ResolutionResolution

-1 -0.8 -0.6 -0.4 -0.2 -0 0.2 0.4 0.6 0.8 10

500

1000

1500

2000

2500

3000

3500

Constant = 3131

Mean = 0.007268

Sigma = 0.1284

Chi2 / ndf = 790.7 / 27

Mean = 0.007268

Sigma = 0.1284

-1 -0.8 -0.6 -0.4 -0.2 -0 0.2 0.4 0.6 0.8 10

50

100

150

200

250

300

Constant = 274.8

Mean = -0.008735

Sigma = 0.1636

Chi2 / ndf = 115.1 / 33

Mean = -0.008735

Sigma = 0.1636

-1 -0.8 -0.6 -0.4 -0.2 -0 0.2 0.4 0.6 0.8 10

100

200

300

400

500

600

700

800

Constant = 758.9

Mean = 0.02786

Sigma = 0.1988

Chi2 / ndf = 244.4 / 41

Mean = 0.02786

Sigma = 0.1988

barrel endcapsoverlap

Level-2 : 1/pT resolution



L3 pL3 pTT ResolutionResolution

-0.2 -0.15 -0.1 -0.05 -0 0.05 0.1 0.15 0.20

500

1000

1500

2000

2500

3000

3500

4000

4500Mean = -0.001624

Sigma = 0.01882

Chi2 / ndf = 1400 / 25

-0.2 -0.15 -0.1 -0.05 -0 0.05 0.1 0.15 0.20

50

100

150

200

250

300Constant = 263.4

Mean = -0.0009174

Sigma = 0.01998

Chi2 / ndf = 79.95 / 25

Mean = -0.0009174

Sigma = 0.01998

-0.2 -0.15 -0.1 -0.05 -0 0.05 0.1 0.15 0.20

200

400

600

800

1000

1200

Constant = 1115

Mean = -0.003264

Sigma = 0.02442

Chi2 / ndf = 205 /25

Mean = -0.003264

Sigma = 0.02442

barrel endcapsoverlap

Level-3 : 1/pT resolution



Trigger RatesTrigger Rates


0 10 20 30 40 50 60 70 80

Rat

e [H

z]

10-1

1

10

102

103

104

105

106

generator

L1

L2

L3



Muons: LevelMuons: Level--33


0 10 20 30 40 50 60 70 80

Rat

e [H

z]

10-2

10-1

1

10

102

103

104

105

all

π K/

c/b

W/Z

With Level-3 resolution: rate given by ~ prompt muons

• Rate is mainly heavy flavors (b/c) (~100 Hz)• W/Z rate: 15 Hz at pT > 20 GeV/c



SummarySummary

• Electrons:– ECAL reconstruction and pixel matching– Work on Level-3 started: tracker matching, electron tracking

• Photons: – Results for unconverted photons– Need special algorithms for converted photons

• Muons:– Local Muon reconstruction with Level-1 seed– Tracker matching: improves pT resolution

norbert neumeister october 25, 2001 iii international ... · r inner,outer[mm] 1238,1750 316,1711...

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