Online Laser Monitoring for the CMS ECAL: 2006 Test Beam Results

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Online Laser Monitoring for the CMS ECAL2006
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

• 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

• Electromagnetic calorimeter
• 76,000 Lead Tungstate (PbWO4) crystals
• ECAL Barrel 36 supermodules

ECAL supermodule, showing individual modules
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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.
• PbWO4 Crystals change transparency under
  radiation
• Correct using the observations of laser
  monitoring system

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
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).
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Laser Monitoring System
?1 440 nm ?2 796 nm

• Lasers at two different wavelengths

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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
Beam line

• 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
• Online monitoring system was implemented to
  reconstruct laser runs and log values

ECAL SM 22
Moveable stand
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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
All channels, all modules Stability 1.4 from
gauss fit to peak.
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

Raw stability
D APD/PN
Overall stability good, even at this basic level
without any further corrections.
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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

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
Xtal 168 SM 22
Normalized laser and electron responses

• For each electron response point an interpolated
  laser response value is calculated

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Example Correlation Plot
Xtal 168 SM 22
Relative electron response
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