ELECTROMAGNETIC CALORIMETER at CMS EVANGELOS XAXIRIS June 2005 Experimental Physics Techniques

Evangelos Xaxiris, June 2005 Experimental Physics Techniques 4 detectors Tracker, Electromagnetic Calorimeter, Hardronic Calorimeter, Muon Chambers Rapidity Coverage |η|= 5 equivalent to θ = 0.8º Radius R = 7.5 m Weight tons Magnet Superconductive solenoid, B = 4 Tesla COMPACT MUON SOLENOID

Evangelos Xaxiris, June 2005 Experimental Physics Techniques ECAL: General Purposes Need for a high-resolution electromagnetic calorimeter comes from the Higgs decay channel H  2γ, for Higgs mass 100 < m H < 140 GeV 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

Evangelos Xaxiris, June 2005 Experimental Physics Techniques Homogeneous Crystals 50% Lead Oxide PbO - 50% Tungsten Oxide WO 3 Approximately 80,000 Crystals (22 X 22mm 2 ) Properties Small Radiation Length (0.89cm) Small Moliere Radius (22mm) Quick Scintillation decay time Easy production from raw materials Large radiation hardness ECAL: Lead Tungsten Crystals, PbWO4 Compact Calorimeter

Evangelos Xaxiris, June 2005 Experimental Physics Techniques ECAL: Lead Tungsten Crystals, PbWO4 Nal (Tl) BGOCSI BAF 2 CeFe 3 PbWO 4 Density [g/cm 3 ] Radiation [cm] Interaction Length [cm] Moliere Radius [cm] Light decay time [ns] (39%) 14 (60%) 100 (1%) Retractive Index Maximum of Emission [nm] Temperature Coefficient [%/ºC] App / Relative Light Output /481.3

Evangelos Xaxiris, June 2005 Experimental Physics Techniques ECAL: Lead Tungsten Crystals, PbWO4 Optical Properties Light Emission Spectrum Gaussian at 440nm ( nm) 5ns39% 15ns60% 100ns1% All light collected in 100ns Decay time Large slow reducing molybdenum component impurities 80% quantum efficiency in APDs at that region

Evangelos Xaxiris, June 2005 Experimental Physics Techniques 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

Evangelos Xaxiris, June 2005 Experimental Physics Techniques 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

Evangelos Xaxiris, June 2005 Experimental Physics Techniques 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

Evangelos Xaxiris, June 2005 Experimental Physics Techniques ECAL: Photodetectors, APDs 2 in each crystal Cover 50mm 2 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 2 X neutrons/cm 2

Evangelos Xaxiris, June 2005 Experimental Physics Techniques ECAL: Photodetectors, VPTs Cover 180mm 2 crystal surface Quantum efficiency 15% Gain approximately 12 (B=0) Insensitive to bias voltage faceplates of C96-1 radiation hard glass

Evangelos Xaxiris, June 2005 Experimental Physics Techniques ECAL: Barrel Rapidity Coverage |η| < 1.48 Τ = 16ºC±0.1ºC No. crystals: Crystal Volume: 8.14 Crystal dimensions: 21.8 X 21.8 X 230mm 3 (25.8 X 0 ) Submodule 10 Crystals 2 in φ, 5 in η 17 types

Evangelos Xaxiris, June 2005 Experimental Physics Techniques 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

Evangelos Xaxiris, June 2005 Experimental Physics Techniques ECAL: Endcaps Rapidity Coverage 1.48 <|η| < 3.00 (precise measurements till n=2.6) Τ = 18ºC±0.1ºC No. crystals: Crystal Volume: 3.04 Crystal dimensions: 24.7 X 24.7 X 220mm 3 (24.7 X 0 ) Each endcap consists of 600 supercrystals Each 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

Evangelos Xaxiris, June 2005 Experimental Physics Techniques ECAL: Endcap Preshower Rapidity Coverage <|η| < 2.60 Τ = -5ºC±0.1ºC At channel Hγγ, 1 photon falls to endcaps and must be separated by high energy π 0, which also give closely spaced decay photons (π 0 γγ)

Evangelos Xaxiris, June 2005 Experimental Physics Techniques ECAL: Endcap Preshower 40mm neutron moderator Thin hitting film 10mm insulating foam Cooling unit 1.75Χ 0 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

Evangelos Xaxiris, June 2005 Experimental Physics Techniques 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 Combining position measurements of ECAL and Preshower gives a 500 MeV/c 2 contribution to the energy reconstruction Without preshower: contribution of 1.5 GeV to energy resolution

Evangelos Xaxiris, June 2005 Experimental Physics Techniques 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

Evangelos Xaxiris, June 2005 Experimental Physics Techniques ECAL: Cooling systems 1 st System Cooling crystals and APDs Water flow of 50l/sec 2 nd System Prevents heat from very-front-end electronics Water flow of 3l/sec temperature spread of 0.05 ºC temperature spread of 2.5 ºC

Evangelos Xaxiris, June 2005 Experimental Physics Techniques ECAL: Cooling systems

Evangelos Xaxiris, June 2005 Experimental Physics Techniques ECAL: Calibration 1) Pre-Calibration In high energy electron beams (2 energies) resolution 2% 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

Evangelos Xaxiris, June 2005 Experimental Physics Techniques ECAL: Monitoring System Injects light pulses into each individual PbWO 4 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 PbWO 4 crystals from radiation damage

Evangelos Xaxiris, June 2005 Experimental Physics Techniques 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

Evangelos Xaxiris, June 2005 Experimental Physics Techniques 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

Evangelos Xaxiris, June 2005 Experimental Physics Techniques ECAL: Energy reconstruction Correction of converted photons ECAL Region UnconvertedConverted(Invisible)Converted(Visible) Barrel 76.2 % 5.0 % 18.8 % Endcap 65.1 % 8.7 % 26.2 % Photons convert into e + e - in materials 2 types of conversion, visible/invisible electrons

Evangelos Xaxiris, June 2005 Experimental Physics Techniques ECAL: Energy reconstruction Loss 4.8% photons in the barrel Loss 9.3% photons in the endcaps

Evangelos Xaxiris, June 2005 Experimental Physics Techniques 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 (P T thresholds) Loss of approximately 5% of photons due to isolation cuts

Evangelos Xaxiris, June 2005 Experimental Physics Techniques ECAL: Energy reconstruction π 0 s rejection For a π 0 of 25 GeV the 2 photons have a distance of 15mm when they hit the crystal 1st method Distinguishes the 2 showers using the lateral shower shape in the crystal

Evangelos Xaxiris, June 2005 Experimental Physics Techniques ECAL: Energy reconstruction 2 nd method Distinguishes the 2 showers using the preshower detector (smaller granularity) π 0 s rejection

Evangelos Xaxiris, June 2005 Experimental Physics Techniques ECAL: Energy reconstruction Fiducial area cuts within Fiducial area cuts within |η| < % Unrecoverable conventions 94 % Isolation cuts 95 % π 0 rejection algorithms 90 % Total reconstruction efficiency 74.5 % Single photon reconstruction efficiency

Evangelos Xaxiris, June 2005 Experimental Physics Techniques ECAL: Energy resolution Energy resolution for 25 < m H < 500GeV a: stochastic term b: noise term c: constant term

Evangelos Xaxiris, June 2005 Experimental Physics Techniques 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

Evangelos Xaxiris, June 2005 Experimental Physics Techniques 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

Evangelos Xaxiris, June 2005 Experimental Physics Techniques ECAL: Energy resolution, Constant term Factor Gaussian Smearing Non-uniformity of longitudinal light collection0.3% Crystal-to-crystal inter-calibration errors0.4% Leakage of energy from the back of the crystal < 0.1% Uncorrected and imperfectly corrected geometrical effects < 0.1%

Evangelos Xaxiris, June 2005 Experimental Physics Techniques ECAL: Energy resolution, Summary Contribution Barrel (η=0) Endcap (η = 2) Stochastic term 2.70%5.7% Constant term 0.55% 0.55% Noise term 155 MeV (low luminosity) 205 MeV (low luminosity) 210 MeV (high luminosity) 245 MeV (high luminosity)

Evangelos Xaxiris, June 2005 Experimental Physics Techniques ECAL: Conclusion Simulations for m H =100 GeV Low luminosity High luminosity σ (MeV) σ (MeV) Significant signal after 30fb -1 over the entire range 100 < m H < 140 GeV Photon reconstruction efficiency of 74.5 %