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CMS ECAL performance and upgrade

CMS ECAL performance and upgrade. Anton Karneyeu (INR, Moscow) CMS Collaboration. INSTR14, Novosibirsk, Russia, 27 February 2014. CMS experiment. Tracker: charged-particle tracks and momentum measurements. ECAL: photon and electron energy and position. Solenoid: 3.8 T.

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CMS ECAL performance and upgrade

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  1. CMS ECAL performance and upgrade Anton Karneyeu (INR, Moscow) CMS Collaboration INSTR14, Novosibirsk, Russia, 27 February 2014

  2. CMS experiment Tracker: charged-particle tracks and momentum measurements ECAL: photon and electron energy and position Solenoid: 3.8 T • Compact Muon Solenoid • A General purpose experiment at LHC for Higgs boson and New Physics searches • Weight: 14000 tonnes • External diameter: 15.0 m • Length: 21.5 m • Magnetic field: 3.8 T • ECAL inside the coil • Tracker coverage |η|<2.5 HCAL: hadronic calorimeter Muon chambers (white) and return yoke INSTR14, 27/02/2014, Anton Karneyeu

  3. Electromagnetic calorimeter Weight: 90 t Length: 7.8 m Diameter: 3.5 m Endcaps (EE) 1.48 < || < 3.0 14648 crystals Barrel (EB) || < 1.48 61200 crystals ENERGY RESOLUTION (BARREL, test-beam results) Pb/Si preshower 1.65 < | | < 2.6 The energy resolution for photons from H→γγin EB is 1.1% to 2.6% and in EE 2.2% to 5%. Timing resolution is 190 ps and 280 ps in EB and EE. INSTR14, 27/02/2014, Anton Karneyeu

  4. Crystals (PbWO4) • Reasons for choice: • Homogeneous medium 8.3 g/cm3 • Fast light emission ~ 80% in 25 ns • Short radiation length 0.89 cm • Small Molière radius 2.20 cm • Emission peak 420 nm • Caveats: • Low light yield (LY~100γ/MeV ) Need photodetectors with gain in magnetic field • LY temperature dependence ~ −2.2%/C Need to stabilise to few 0.05 C • Obtained stability: ~ 0.03 C • Formation/decay of colour centres under irradiation altering crystal transparencyNeed precise monitoring system • LY spread between crystals ~ 10% Need intercalibration • Producers: • Bogoroditsk Techno-Chemical Plant, Russia (> 95% of crystals) • Shanghai Institute of Ceramic, China INSTR14, 27/02/2014, Anton Karneyeu

  5. Photodetectors Barrel: Avalanche photo-diodes (APD, Hamamatsu) Two 5x5 mm2 APDs/crystal, ~ 4.5 p.e./MeV Gain 50 QE ~ 75% at 420 nm Temperature dependence 1/G ΔG/ΔT = −2.4%/C High-Voltage dependence 1/G ΔG/ΔV = 3.1%/V Need to stabilize HV at 30 mV Measured HV fluctuation: ~30 mV Endcaps: Vacuum photo-triodes (VPT, Research Institute “Electron”, Russia) More radiation resistant than Si diodes UV glass window Active area ~ 280 mm2/crystal, ~ 4.5 p.e./MeV Gain 8 -10 (B=4T) Q.E. ~ 20% at 420 nm Gain spread among VPTs ~ 25% Need intercalibration INSTR14, 27/02/2014, Anton Karneyeu

  6. Laser Response monitoring system • Sources of response variations under irradiation: • Crystal transparency loss and recovery • VPT ageing (not yet disentangle from transparency loss) Monitoring with LASER: • Laser: 447 nm (main laser), green and infra-red • Laser light injection in each crystal every ~40 minutes • Light also injected in PN diodes • ECAL signals compared event by event to PN reference • Energy corrections derived • Also LED (blue/orange) in endcaps. INSTR14, 27/02/2014, Anton Karneyeu

  7. Response monitoring system Observed response change is from 6% (barrel) to 70% (endcap). These measurements are used to correct the physics data. INSTR14, 27/02/2014, Anton Karneyeu

  8. Calibration • π0/η → γγ method: equalizes measured π0/η peaks for individual crystals. • φ-symmetry: invariance around the beam axis of the energy flow in zero-bias events. • single-electrons from W decays: use E/p ratio where p is measured in the tracker and E in the ECAL. In addition to single-crystal intercalibration, this method also intercalibrates the average response of 248 φ-rings. • di-electrons from Z decays: use measured invariant mass to obtain the global scale corrections and study the ECAL resolution. • Precalibration in 2000-2009 performed using test beams, cosmic rays, radiation source and "beam splashes" during the first LHC runs. • ~30% of the Barrel and 400 crystals in the endcaps were calibrated in the test beams to the design-goal single-crystal precision of 0.5%. INSTR14, 27/02/2014, Anton Karneyeu

  9. Calibration The precision of the phi-symmetry and photon calibrations is at the level of the systematic errors. The precision of the electron calibration is still dominated by the statistical errors for |η|>1. INSTR14, 27/02/2014, Anton Karneyeu

  10. ECAL energy calibration Impact on the Z → e+e- energy scale and resolution of corrections which account for the intrinsic spread in crystal and photo-detector response, and compensate crystal response lost. INSTR14, 27/02/2014, Anton Karneyeu

  11. ECAL supercluster energy Impact on the Z → e+e- energy scale and resolution from the incorporation of more sophisticated clustering and cluster correction algorithms. INSTR14, 27/02/2014, Anton Karneyeu

  12. Electron energy resolution Relative energy resolution of electron (from Z→e+e- decays) => Affected by the amount of material in front of the ECAL => Degraded near eta cracks between ECAL modules (vertical lines in the plot) => Dedicated calibration improves resolution significantly INSTR14, 27/02/2014, Anton Karneyeu

  13. Z → e+e- mass resolution The mass resolution of the Z → e+e- as a function of time. => Mass resolution is stable at the per mill level. INSTR14, 27/02/2014, Anton Karneyeu

  14. Physics results: H →γγ High energy resolution of the ECAL was critical for the discovery of the Higgs => Latest results on Moriond 2014 conference (March 15th - 22nd) INSTR14, 27/02/2014, Anton Karneyeu

  15. LHC schedule Phase2 Phase1 LS1 LS2 LS3 HL-LHC: L=5 ·1034 cm-2s-1 250 fb-1 per year ~140 events per bunch-crossing ECM=13 TeV L=1 ·1034 cm-2s-1 50 fb-1 per year 3 years L=2 ·1034 cm-2s-1 ≥50 fb-1 per year 3 years ~ 300 fb-1 ~ 3000 fb-1 • A new machine, for high luminosity, to measure the H couplings, H rare decays, HH, Vector boson scattering, other searches and difficult SUSY benchmarks, measure properties of other particles eventually discovered in Phase1. INSTR14, 27/02/2014, Anton Karneyeu

  16. Detector challenges at HL-LHC • Number of events per bunch crossing (pile-up): ~ 140 • Radiation levels will be 6 times higher than for the nominal LHC design.Strong η dependence in the endcaps (at η=2.6 hadron fluence 2·1014/cm2, gamma radiation: 30 Gy/h, total: 300kGy) MARS calculations, P.C. Bhat et al., CERN-CMS-NOTE-2013-001 INSTR14, 27/02/2014, Anton Karneyeu

  17. Crystal radiation damage Gamma irradiation damage is spontaneously recovered at room temperature. Hadron damage creates clusters of defects which cause light transmission loss. The damage is permanent and cumulative at room temperature. Plot shows “external” optical transmission of crystal after diffent irradiation doses. - - - PbWO4 emission spectrum INSTR14, 27/02/2014, Anton Karneyeu

  18. ECAL endcap response evolution • Extensive test-beam studies with proton-irradiated crystals • Reduction of light output causes: • Worsening of stochastic term • Amplification of the noise • light collection non-uniformity (impact on the constant term) Simulation 50 GeV e- Fraction of ECAL response Progressive deterioration of energy resolution and trigger efficiency, with strong η dependence Performance for e/y is acceptable up to 500 fb-1 ECAL endcaps should be replaced during LS3 INSTR14, 27/02/2014, Anton Karneyeu

  19. Endcap calorimetry options for HL-LHC Two possible scenarios: 1. HE absorber is left, only active material is replaced. New EE will be a standalone calorimeter 2. HE is fully replaced. This opens the possibility of a more coherent redesign of the endcap calorimeters. Aim: radiation hardness, high granularity, precision timing HE EE CMS plan is to replace the Endcap calorimeters in LS3 INSTR14, 27/02/2014, Anton Karneyeu

  20. Scenario 1: standalone EE Shashlik Calorimeter in a sampling configuration with inorganic scintillator (LYSO or CeF3) and W as absorber. Light readout via wavelength shifting fibers (WLS) Transverse dimensions are 14x14mm → fine grained Light path is short → rad-hard Challenges: rad-hard fibers, photo-detectors, mechanical mounting (tolerances) INSTR14, 27/02/2014, Anton Karneyeu

  21. Scenario 2: Combined Forward Calorimeter Scintillation fiber Brass absorber • Challenges: rad-hard fibers, photo-detectors, mechanical design, triggering Cherenkov fiber Dual Readout: simultaneous measurements of Cherenkov and scintillation signal (2 types of fibers) enables event-by-event compensation resulting in superior hadron and jet measurement (DREAM/RD52 collaboration) INSTR14, 27/02/2014, Anton Karneyeu

  22. Scenario 2: High Granularity Calorimeter Layers of silicon “pads” (1cm2) separated by absorber (lead/Cu) Measure charged particle momentum with the inner tracker, and neutrals in the calorimeter. Key point: resolving/separating showers through a finely granulated and longitudinally segmented calorimeter. Challenges: number of channels, compact and inexpensive electronics, trigger, cooling, performance in high pile-up, linearity INSTR14, 27/02/2014, Anton Karneyeu

  23. Pile-up Mitigation HL-LHC expected luminosity region: Gaussian width of ~ 10 cm Number of events per bunch crossing (pile-up) ~ 140 Within tracker acceptance, charged particles are well reconstructed (Zvertex resolution ~ 0.1 mm) Pile-up is critical in the forward region: 1. Increased granularity and segmentation. 2. High precision (pico second) timing. Studies are ongoing for pile-up mitigation through time-of-flight. Desired resolution is 20-30 ps. INSTR14, 27/02/2014, Anton Karneyeu

  24. Conclusions • The excellent ECAL performance was crucial for the Higgs boson discovery by CMS • ECAL will continue to be highly performant throughout all Phase 1 • The HL-LHC poses severe requirements to detectors in terms of performance and radiation hardness. • ECAL endcaps should be replaced at the end of the LHC phase1 (after 500 fb-1). • New calorimeter options are being studied. Key points are radiation hardness, granularity and segmentation. • Timing resolution may add important information for pile-up mitigation. INSTR14, 27/02/2014, Anton Karneyeu

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