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ELECTROMAGNETIC CALORIMETER at CMS

ELECTROMAGNETIC CALORIMETER at CMS. EVANGELOS XAXIRIS June 2005 Experimental Physics Techniques. COMPACT MUON SOLENOID. 4 detectors Tracker, Electromagnetic Calorimeter, Hardronic Calorimeter, Muon Chambers Rapidity Coverage | η | = 5 equivalent to θ = 0.8º Radius R = 7.5 m

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ELECTROMAGNETIC CALORIMETER at CMS

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  1. ELECTROMAGNETIC CALORIMETER at CMS EVANGELOS XAXIRIS June 2005 Experimental Physics Techniques

  2. COMPACT MUON SOLENOID • 4 detectors • Tracker, Electromagnetic Calorimeter, Hardronic Calorimeter, Muon Chambers • Rapidity Coverage • |η|= 5 equivalent to θ = 0.8º • Radius • R = 7.5 m • Weight • 12.500 tons • Magnet • Superconductive solenoid, B = 4 Tesla

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

  4. Compact Calorimeter ECAL: Lead Tungsten Crystals, PbWO4 • Homogeneous Crystals • 50% Lead Oxide PbO - 50% Tungsten Oxide WO3 • Approximately 80,000 Crystals (22X22mm2) • Properties • Small Radiation Length (0.89cm) • Small Moliere Radius (22mm) • Quick Scintillation decay time • Easy production from raw materials • Large radiation hardness

  5. ECAL: Lead Tungsten Crystals, PbWO4

  6. ECAL: Lead Tungsten Crystals, PbWO4 • Optical Properties • Light Emission Spectrum • Gaussian at 440nm (360-570nm) 80% quantum efficiency in APDs at that region Decay time Large slow reducing molybdenum component impurities 5ns 39% 15ns 60% 100ns 1% All light collected in 100ns

  7. ECAL: Lead Tungsten Crystals, PbWO4 Light Yield Thermal 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

  8. ECAL: Lead Tungsten Crystals, PbWO4 Radiation Hardness • Affected • Transparency of crystal (self- absorption from colour centres) • Loss in the amount of collected light • Not Affected • Scintillation mechanism • Longitudinal uniformity Correction by the monitoring system, results in no effect on the energy resolution

  9. Avalanche Photodiodes in barrel Vacuum Phototriodes in endcap ECAL: Photodetectors • Strong axial magnetic field in the barrel • High levels of radiation in the endcap • No photomultiplier to deal with both aspects

  10. 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

  11. 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

  12. 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) Submodule 10 Crystals 2 in φ, 5 in η 17 types

  13. ECAL: Barrel Module 5 submodules in φ 36 modules in barrel 4 types Supermodule 4 modules 36 supermodules in barrel 20 in φ, 85 in η 1 type

  14. 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 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 supercrystals Each supercrystal is made up of an array of 6 X 6 crystals

  15. ECAL: Endcap Preshower • Rapidity Coverage1.653 <|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 (π0γγ)

  16. 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

  17. ECAL: Barrel Preshower Low luminosities vertex known High luminosities spread in interaction vertices in z (5.3cm rms) Knowledge of vertex required for good energy resolution Angular determination (photon angle in η direction) • Preshower section at |η| < 0.9 • Τ = 12ºC η Without preshower: contribution of 1.5 GeV to energy resolution Combining position measurements of ECAL and Preshower gives a 500 MeV/c2 contribution to the energy reconstruction

  18. ECAL: Barrel Preshower • 5mm insulating foam • 4mm Al cover • Electronics • Al-Pb-Al absorber • 4mm Al • Varying thickness of Pb 13.2mm at η = 0 9.0mm at η = 0.9 • Cooling pipes in second Al • Si detectors • Front-end Electronics • 5mm insulating foam

  19. ECAL: Cooling systems • 1st System • Cooling crystals and APDs • Water flow of 50l/sec temperature spread of 0.05ºC 2nd System Prevents heat from very-front-end electronics Water flow of 3l/sec temperature spread of 2.5ºC

  20. ECAL: Cooling systems

  21. ECAL: Calibration • Pre-Calibration • In high energy electron beams (2 energies) • resolution 2% • In Situ Calibration • In 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

  22. 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

  23. 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

  24. ECAL: Energy reconstruction Correction for intermodule gaps in the cluster Algorithms take into account the loss in energy deposition Different functions for gaps on η and on φ Only in regions and Loss of 3.8% of photons which hit the barrel

  25. ECAL: Energy reconstruction Correction of converted photons Photons convert into e+e- in materials 2 types of conversion, visible/invisible electrons

  26. ECAL: Energy reconstruction Loss 4.8% photons in the barrel Loss 9.3% photons in the endcaps

  27. 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

  28. ECAL: Energy reconstruction π0srejection For a π0of 25 GeV the 2 photons have a distance of 15mm when they hit the crystal 1st method Distinguishes the 2 showersusing the lateral shower shape in the crystal

  29. ECAL: Energy reconstruction π0srejection 2nd method Distinguishes the 2 showers using the preshower detector (smaller granularity)

  30. ECAL: Energy reconstruction Single photon reconstruction efficiency

  31. ECAL: Energy resolution • Energy resolution for 25 < mH < 500GeV a: stochastic term b: noise term c: constant term

  32. 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 process • N, number of photoelectrons/GeV, N > 4000/GeV in APDs, VPTs Approximately

  33. ECAL: Energy resolution, Noise term • Pre-amplifier noise • Digitisation noise • Pile-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 barrel 150 MeV/channel in endcap

  34. ECAL: Energy resolution, Constant term

  35. ECAL: Energy resolution, Summary

  36. Photon reconstruction efficiency of 74.5 % Significant signal after 30fb-1 over the entire range 100 < mH < 140 GeV ECAL: Conclusion

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