ELECTROMAGNETIC CALORIMETER

at CMS

EVANGELOS XAXIRISJune 2005

Experimental Physics Techniques

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ues • 4 detectors

Tracker, Electromagnetic Calorimeter, Hardronic Calorimeter, Muon Chambers

• Rapidity Coverage|η|= 5 equivalent to θ = 0.8º

• RadiusR = 7.5 m

• Weight12.500 tons

• MagnetSuperconductive solenoid, B = 4 Tesla

COMPACT MUON SOLENOID

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ECAL: General Purposes

• Need for a high-resolution electromagnetic calorimeter comes from the Higgs decay channel H 2γ, for Higgs mass 100 < mH < 140 GeV

ECAL just outside the tracker, in the magnetic fieldECAL will operate in a challenging environment of B = 4 T, 25nsec bunch crossings and radiation flux of a few kGy/year

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Homogeneous Crystals50% Lead Oxide PbO - 50% Tungsten Oxide WO3

Approximately 80,000 Crystals (22X22mm2)

• PropertiesSmall Radiation Length (0.89cm)Small Moliere Radius (22mm)Quick Scintillation decay timeEasy production from raw materialsLarge radiation hardness

ECAL: Lead Tungsten Crystals, PbWO4

Compact Calorimeter

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ECAL: Lead Tungsten Crystals, PbWO4

Nal Nal (Tl)(Tl)

BGOBGO CSICSI BAFBAF22 CeFeCeFe33 PbWOPbWO44

Density [g/cmDensity [g/cm33]] 3.673.67 7.137.13 4.514.51 4.884.88 6.166.16 8.288.28

Radiation [cm]Radiation [cm] 2.592.59 1.121.12 1.861.86 2.062.06 1.681.68 0.890.89

Interaction Length Interaction Length [cm][cm]

41.441.4 21.821.8 37.037.0 29.929.9 26.226.2 22.422.4

Moliere Radius [cm]Moliere Radius [cm] 4.804.80 2.332.33 3.503.50 3.393.39 2.632.63 2.192.19

Light decay time Light decay time [ns][ns]

230230 6060

3003001616 0.90.9

63063088

25255 (39%)5 (39%)

14 (60%)14 (60%)

100 (1%)100 (1%)

Retractive Index Retractive Index 1.851.85 2.152.15 1.801.80 1.491.49 1.621.62 2.302.30

Maximum of Maximum of Emission [nm]Emission [nm]

410410 480480 315315 210210

310310300300

340340440440

Temperature Temperature Coefficient [%/ºC]Coefficient [%/ºC]

App 0App 0 -1.6-1.6 -0.6-0.6 -2/0-2/0 0.140.14 -2-2

Relative Light Relative Light OutputOutput

100100 1818 2020 20/420/4 88 1.31.3

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ECAL: Lead Tungsten Crystals, PbWO4

Optical Properties

Light Emission SpectrumGaussian at 440nm (360-570nm)

5ns 39%15ns 60%100ns 1%

All light collected in 100ns

Decay timeLarge slow reducing molybdenum component impurities

80% quantum efficiency in APDs at that region

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ECAL: Lead Tungsten Crystals, PbWO4

Light YieldThermal quenching of scintillation mechanism gives the photon yield coefficient a strong dependence on temperature temperature stability to a tenth of a degree at crystals and APDs is needed

10 photoelectrons/MeV

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ECAL: Lead Tungsten Crystals, PbWO4

Radiation Hardness

Not Affected• Scintillation mechanism• Longitudinal uniformity

Affected• Transparency of crystal (self-

absorption from colour centres)• Loss in the amount of collected

light

Correction by the monitoring system, results in no effect on the energy resolution

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ECAL: Photodetectors

Strong axial magnetic field in the barrel High levels of radiation in the endcap

No photomultiplier to deal with both aspects

Avalanche Photodiodes

in barrel

Vacuum Phototriodes in endcap

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ECAL: Photodetectors, APDs

• 2 in each crystal• Cover 50mm2 crystal surface• Compactness (2mm thickness)• Fast rise time (2ns)• 70-80% quantum efficiency• Insensitive to magnetic fields• Gain at approximately 50• Receiving a small flux of 2X1013 neutrons/cm2

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ECAL: Photodetectors, VPTs

• Cover 180mm2 crystal surface• Quantum efficiency 15%• Gain approximately 12 (B=0)• Insensitive to bias voltage• faceplates of C96-1 radiation hard

glass

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ECAL: Barrel

• Rapidity Coverage |η| < 1.48• Τ = 16ºC±0.1ºC• No. crystals: 61.200• Crystal Volume: 8.14• Crystal dimensions: 21.8X21.8X230mm3

(25.8 X0)

Submodule10 Crystals2 in φ, 5 in η17 types

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ECAL: Barrel

Module5 submodules in φ36 modules in barrel4 types

Supermodule4 modules36 supermodules in barrel20 in φ, 85 in η1 type

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ECAL: Endcaps

• Rapidity Coverage 1.48 <|η| < 3.00 (precise measurements till n=2.6)• Τ = 18ºC±0.1ºC• No. crystals: 21.628• Crystal Volume: 3.04• Crystal dimensions: 24.7X24.7X220mm3

(24.7 X0)

Each endcap consists of 600 supercrystalsEach supercrystal is made up of an array of 6 X 6 crystals

Barrel-Endcap transition: Loss of coverage in the range 1.46 <|η|< 1.59 (5.2% of η,φ space) 4.8% loss of photons

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ECAL: Endcap Preshower

• Rapidity Coverage 1.653 <|η| < 2.60• Τ = -5ºC±0.1ºC

At channel H γγ, 1 photon falls to endcaps and must be separated by high energy π0, which also give closely spaced decay photons (π0 γγ)

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ECAL: Endcap Preshower

• 40mm neutron moderator • Thin hitting film• 10mm insulating foam• Cooling unit• 1.75Χ0 Al-Pb-Al absorber

(2 X 9.3 X 2mm)• Si detectors

(shower profile in y)• Electronics/Cooling

•Cooling unit 0.77Χ0 Al-Pb-Al absorber • Si detectors

(shower profile in x)• 10mm insulating foam • Heating film • 40mm moderator

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ECAL: Barrel Preshower

Low luminosities vertex knownHigh luminosities spread in interaction vertices in z

(5.3cm rms)

Knowledge of vertex required for good energy resolutionAngular determination (photon angle in η direction)

• Preshower section at |η| < 0.9• Τ = 12ºC η

Combining position measurements of ECAL and Preshower gives a 500 MeV/c2 contribution to the energy reconstruction

Without preshower: contribution of 1.5 GeV to energy resolution

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ECAL: Barrel Preshower

• 5mm insulating foam• 4mm Al cover• Electronics• Al-Pb-Al absorber

4mm AlVarying thickness of Pb

13.2mm at η = 09.0mm at η = 0.9

Cooling pipes in second Al• Si detectors • Front-end Electronics• 5mm insulating foam

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ECAL: Cooling systems

1st SystemCooling crystals and APDsWater flow of 50l/sec

2nd SystemPrevents heat from very-front-end electronicsWater flow of 3l/sec

temperature spread of 0.05ºC

temperature spread of 2.5ºC

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ECAL: Cooling systems

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ECAL: Calibration

1) Pre-CalibrationIn high energy electron beams (2 energies) resolution 2%

2) In Situ CalibrationIn physics events (mainly the channel Z e +e-, where e have correlated energies)

resolution reaches 0.3% (400 crystals, 250pb-1 lum.)

Combined information from ECAL and Tracker for electrons which haven’t radiated gives a typical resolution (in barrel) in E/P = 1.5%

Goal: 0.5% constant term

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ECAL: Monitoring System

• Injects light pulses into each individual PbWO4

• Measure optical transmission near the scintillation spectrum peak (~ 500nm)

Relation between

Transmission losses of an electromagnetic shower scintillation light

Correlated losses in laser transmission in the crystal

helps in the recovery (self-annealing processes) of the PbWO4

crystals from radiation damage

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ECAL: Energy reconstruction

Finding ‘clusters’ of energy

Correcting the amount of energy deposit there

Correction for the impact position

Different energy deposits for impact in the centre and in the corner of the crystal (mainly for the endcaps)

Cluster: 5X5 array of crystals centered on the crystal with the max signal

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ECAL: Energy reconstruction

Correction for intermodule gaps in the cluster

Algorithms take into account the loss in energy depositionDifferent functions for gaps on η and on φ

Only in regions and

Loss of 3.8% of photons which hit the barrel

1 24 log( / ) 1E E 1 22 log( / ) 4E E

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ECAL: Energy reconstruction

Correction of converted photons

ECAL ECAL RegionRegion

UnconvertedUnconverted ConvertedConverted

(Invisible)(Invisible)ConvertedConverted

(Visible)(Visible)

BarrelBarrel 76.2 %76.2 % 5.0 %5.0 % 18.8 %18.8 %

EndcapEndcap 65.1 %65.1 % 8.7 %8.7 % 26.2 %26.2 %

Photons convert into e+e- in materials

2 types of conversion, visible/invisible electrons

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ECAL: Energy reconstruction

Loss 4.8% photons in the barrelLoss 9.3% photons in the endcaps

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ECAL: Energy reconstruction

Correction with isolation cuts

Pile up events and underlying events excluded with the isolation of the particle

Cuts on the summed transverse energy within a region around and behind the particle (PT thresholds)

Loss of approximately 5% of photons due to isolation cuts

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ECAL: Energy reconstruction

π0s rejectionFor a π0 of 25 GeV the 2 photons have a distance of 15mm when they hit the crystal

1st methodDistinguishes the 2 showers using the lateral shower shape in the crystal

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ECAL: Energy reconstruction

2nd methodDistinguishes the 2 showers using the preshower detector (smaller granularity)

π0s rejection

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ECAL: Energy reconstruction

Fiducial area cuts Fiducial area cuts within within |η| < 2.50

92.5 %92.5 %

Unrecoverable Unrecoverable conventionsconventions

94 %94 %

Isolation cutsIsolation cuts 95 %95 %

ππ00 rejection rejection algorithmsalgorithms

90 %90 %

Total Total reconstruction reconstruction

efficiencyefficiency

74.5 %74.5 %

Single photon reconstruction efficiency

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ECAL: Energy resolution

• Energy resolution for 25 < mH < 500GeV

22 22a bc

E

a: stochastic term

b: noise term

c: constant term

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ECAL: Energy resolution, Stochastic term

• Shower containment 1.5% • Photostatistics 2.3% • Fluctuations in energy deposited in preshower 5%

F ~2, due to event fluctuations in the gain processN, number of photoelectrons/GeV, N > 4000/GeV in APDs, VPTs

Fa

N

Approximately 4.2% / E

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ECAL: Energy resolution, Noise term

Pre-amplifier noise

Digitisation noisePile-up noise

30 MeV for low luminosities 95 MeV for high luminosities

Low luminosities first 2 are significant

High luminosities only pile up noise significant

30 MeV/channel in barrel150 MeV/channel in endcap

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ECAL: Energy resolution, Constant term

FactorFactor Gaussian Gaussian SmearingSmearing

Non-uniformity of longitudinal light collection

0.3%0.3%

Crystal-to-crystal inter-calibration errors

0.4%0.4%

Leakage of energy from the back of the crystal

< 0.1%< 0.1%

Uncorrected and imperfectly corrected geometrical effects

< 0.1%< 0.1%

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ECAL: Energy resolution, Summary

ContributionContribution Barrel Barrel (η=(η=0)0) Endcap (Endcap (η = η = 2)2)

Stochastic term 2.70%2.70% 5.7%5.7%

Constant term 0.55%0.55% 0.0.5555%%

Noise term

155 MeV155 MeV

(low luminosity)(low luminosity)205 MeV205 MeV

(low luminosity)(low luminosity)

210 MeV 210 MeV

(high luminosity)(high luminosity)245 MeV245 MeV

(high luminosity)(high luminosity)

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ECAL: Conclusion

Simulations Simulations for for

mH=100 GeV

Low Low luminosityluminosity

High High luminosityluminosity

σ σ (MeV)(MeV) 650650 690690

Significant signal after 30fb-1 over the entire range 100 < mH < 140 GeV

Photon reconstruction efficiency of 74.5 %