Transcript Slide 1
Online Laser Monitoring for the CMS ECAL: 2006 Test Beam Results
APS Meeting - Jacksonville, FL Christopher Rogan California Institute of Technology On behalf of the CMS ECAL Collaboration April 14, 2006
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CMS Detector
Crystal ECAL
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General purpose detector
p-p collision at CM energy of 14 TeV
Goals: Discover the Higgs, new physics beyond standard model, …
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CMS ECAL
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ECAL supermodule, showing individual modules
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Electromagnetic calorimeter
~76,000 Lead Tungstate (PbWO 4 ) crystals
ECAL Barrel: 36 supermodules
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Introduction
CMS is building a high resolution Crystal Calorimeter (ECAL) to be operated at LHC in a very harsh radiation environment.
Resolution design goal: ~0.5% constant term Calibrating and maintaining the calibration of this device will be very challenging. Hadronic environment makes physics calibration more challenging
PbWO 4 Crystals change transparency under radiation The damage is significant (few % - up to ~5 % for CMS ECAL barrel radiation levels) at high luminosity The dynamics of the transparency change is fast (few hours) compared to the time scale needed for a calibration with physics events (weeks - month).
Correct using the observations of laser monitoring system
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Laser Monitoring System
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Lasers at two different wavelengths:
1 = 440 nm
2 = 796 nm
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Laser Monitoring System
Laser light is injected into the crystals via fiber-optic cables
Avalanche photodiode response is measured (APD)
Light is also injected in reference PN diodes
Ratio of APD and PN responses is used to monitor crystal transparency changes
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Irradiation Crystal Response
Monte Carlo with a ~12 hour LHC fill cycle
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Irradiation Crystal Response
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Test Beam 2006
Test Beam at CERN from June to November 2006
One ECAL supermodule in beam at time
15-250 GeV electrons
Intensity: Up to 50K events / 60s, Approx. 15 rad/hour ECAL SM 22
Online monitoring system was implemented to reconstruct laser runs and log values Moveable stand Beam line
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Online Laser Monitoring
For each laser run:
APD and PN pulses reconstructed
APD, APD/PN and PN distributions for each channel (1700 per SM) are fit and used to extract mean values
Similar distributions are monitored in geometric groupings (half SM, light modules); used for potential corrections
Correlations between different values (APD - APD/PN - timing, Chi2, etc.)
9 ECAL supermodules examined
Over 1,600 laser runs processed
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Online Monitoring Stability
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All channels, all modules : Stability 1.4 % from gauss fit to peak.
Raw stability
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APD/PN Overall stability good, even at this basic level without any further corrections.
APD/PNStability:
Get APD/PN ratios for each channel, each SM
Normalize average APD/PN to 1 for each SM
Fit gauss to normalized APD/PN for each channel
Sigma of these fits is the stability
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Offline Monitoring Stability Example for one SM (22)
Small systematic change in reconstructed APD value related to Peak timing.
Correct APD/PN ratios with a simple linear function of peak timing
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Mean before and after correction : 0.180 % 0.088 % Peak before and after correction : ~0.170 % ~0.05 %
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Example Irradiation Cycle
Normalized laser and electron responses Xtal 168 SM 22
For each electron response point an interpolated laser response value is calculated
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Xtal 168 SM 22 Relative electron response
Example Correlation Plot
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Relative Laser Response
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Example Corrected Resolution
120 GeV electrons, 3x3 crystal matrix Xtal 168 SM 22
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Outlook
Measured the APD/PN stability for individual channels on a large scale
Demonstrated reasonable online APD/PN stability; could be used for online electron response corrections
Achieved offline APD/PN stability for majority of channels with simple corrections. Further corrections are currently being studied
Demonstrated the ability to maintain resolution during irradiation
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