Transcript Slide 1

CMS ECAL 2006 Test Beams Effort
Caltech HEP Seminar
Christopher Rogan
California Institute of Technology
May 1, 2007
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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State of the Higgs: 2007
 Electroweak fit (w/ quantum corrections) to mH :
depends on mW, mTOP
Low MH < 150 GeV
 Best-fit value (2007): mH = 76+34–23 GeV
using mTOP = 170.9 ± 1.8, mW = 80.396 ± .025 GeV
 Direct search limit:
mH > 114.4 GeV
 95% CL upper limit:
mH < 144 GeV
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ECAL layout
PWO: PbWO4
barrel cystals
Pb/Si preshower
barrel
Super Module
(1700 crystals)
Barrel: || < 1.48
36 Super Modules
61200 crystals (2x2x23cm3)
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endcap
supercystals
(5x5 crystals)
EndCap “Dee”
3662 crystals
EndCaps: 1.48 < || < 3.0
4 Dees
14648 crystals (3x3x22cm3)
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CMS ECAL Test Beams 2006
H4
 H4 ECAL Test Beam
 10 SM calibrated (1 twice, 13600 xtals)
 Detailed studies of E,  behaviour
 Irradiation studies
 Energy linearity studies
H2
H2 ECAL+HCAL Test Beam
 1 ECAL SM
 Two subdetector DAQ
 Wide beam calibration
 0 data
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CMS ECAL Test Beams 2006
 A wide array of important studies were completed:
 Electron, 0 and cosmic muon inter-calibrations
 Energy linearity studies
 Crystal containment corrections
 Energy resolution studies
 Amplitude reconstruction optimization
 Noise studies
 DAQ, Monte Carlo and software studies
 Online laser monitoring
 Crystal irradiation
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Cluster Containment Corrections
1
Measurement in fixed size matrix of NxN crystals  position dependence of EREC
Example: 3x3 matrix
683
703
723
684
704
724
685
705
725
Containment effect decreases with the matrix size
5x5
3x3
3%
e
Hodoscope
Resolution:
 Uniform impact  containment corrections needed
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Energy Resolution
Central impact
“Uniform” impact
0.5%
0.5%
• Energy resolution ≤ 0.5% at 120 GeV for any electron impact.
• Same shower containment correction applied (for all E and all Xtals).
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Caltech CMS @ ECAL test beams
 Caltech leadership in two important test beam tasks:
 Operation of the online laser monitoring system
 Improving π0 inter-calibration technique using test beam data
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ECAL Laser Monitoring 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%
Calibrating and maintaining the calibration of this device will be very
challenging. Hadronic environment makes physics calibration more
challenging
PbWO4 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
 Lasers at two different wavelengths:
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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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Laser Monitoring @ H4
 Test Beam at CERN from June to November
Beam line
2006
ECAL SM 22
 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
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.)
 10 ECAL supermodules examined
 Over 1,600 laser runs processed
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Online Laser Data Analysis
~15 min. to process each
laser run
Plots of various
distributions are available
online immediately after
processing. APD/PN
values (among other
things) logged in database
for higher level analysis
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Consecutive run monitoring
Comparison plots between consecutive runs for the APD/PN and APD
values are used to monitor short term stability and inter-run changes
For example, this plot shows the relative difference in the APD/PN values, for
each channel, between two consecutive runs. Almost all channels are stable
to within .5 per mille between consecutive runs
00013061-00013064
.001
0.0
-.003
Runs 13061->13064
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SM16
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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
Raw stability
Normalize average APD/PN to 1
for each SM
Fit gauss to normalized APD/PN
for each channel
D APD/PN
Sigma of these fits is the stability
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 %
Peak before and after correction : ~0.170 %
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0.088 %
~0.05 %
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Raw Monitoring Stability at H2
Black : APD/PN, averaged over 100 channels.
Red : DT/20+1
APD/PN vs. Time, 100 Channels (1040 – 1140, center Module 3).
Anti-correlation between temperature and APD/PN – as expected.
Hardware intervention around t=2150 h, stability reasonable.
Temperature correction based on thermistors
Raw APD/PN stability at reasonable level
APD/PN shows ~ -2%/C0 temperature dependences – as expected.
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Laser Pulse Width Correction
 Reconstructed APD/PN ratio sensitive to laser pulse width
 For normalized APD/PN ratio, ~2%/ns
Long-term pulse width stability ~1-2 ns
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Pulse Width Measurement
All slope for one SM
Example
error bars blown up
by a factor of 10
normalizatio
n value



Linear fit of the APD/PN-width dependence for each
channel of each SM
Normalize APD/PN by the fit value at width = 30 ns
Distributions and crystal maps for the slope,
intercept, chi2, etc. of the linear fits for the
normalized APD/PN values
Sigma / |Mean| = 6.9(1)%
A total of 6 SMs have been measured.
Pulse Width Non-Linearity has little channel to channel variation !
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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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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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Continuing Irradiation Studies
Hodoscope hits - entire irradiation period
Beam events
distributed throughout
crystal
Sufficient statistics to
explore variations in
electron response within
crystal
Xtal 168
SM 22
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Continuing Irradiation Studies
Hodoscope hits - entire irradiation period
 Reconstruct
electron data for 25
different bins
 Generate R-plot
for each bin
Xtal 168
SM 22
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C. Rogan
Continuing Irradiation Studies
Xtal 168
SM 22
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Continuing Irradiation Studies
Still statistics limited in outer
bins
Can potentially be used for
precision offline corrections
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Laser Monitoring 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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π0 Calibration Concept
Data after L1 Trigger
0 Calibration
Online Farm
~1 kHz
>10 kHz
 Level 1 trigger rate dominated by QCD: several π0‘s/event
 Useful π0γγ decays selected online from such events
 Main advantage: high π0 rate (nominal L1 rate is 100kHz !)
 “Design” calibration precision  better than 0.5%
Achieving it would be crucial for the Hγγ detection
 Reporting on studies performed with about four million
fully simulated QCD events. Results given for the scenario
of L=2x1033cm-2s-1 and L1 rate of 10 kHz (LHC start-up).
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π0 Selection
Based on local, crystal-level variables — suitable for online filter farm.
 Kinematics: PT () >1 GeV, PT (pair) > 3.5 GeV and η < 1.48 (barrel)
 Photon shower-shape cuts: S9/S25 > 0.9 and S4/S9 > 0.9 defined with
2x2, 3x3, and 5x5 crystal matrices (S9 is chosen as photon energy)
 Additional isolation cut optimized to remove showers with significant
bremsstrahlung radiation: want to select mainly unconverted photons
Trigger Tower (5x5 crystals)
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Selection Results
π0 rate of 0.9 kHz or 1,250 π0/crystal/day with S/B ≈ 2.0
High-rapidity regions suffer both in rate and S/B (31)
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A Calibration Algorithm (of many)
Simple iterative algorithm (L3/RFQ Calibration)
(wi  fraction of shower energy deposited in this crystal)
 Both photon energy and direction reconstructed using
crystal level information (same as during selection).
 After each iteration pairs are re-selected with new constants
(typically 10-15 iterations to converge).
 Miscalibration is done before selecting events (4%).
 Calibration precision defined as R.M.S. of the product
of the final and initial miscalibration constant.
 Use only pairs from ±2σ window around fitted π0 mass
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Calibration Performance
Precision is then fitted to
a=27±1% and b=0.20±0.25%
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N is the number
C
a2
=
+ b2 of π0/crystal
C
N
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Calibration Studies in Test Beams
π0 decays produced through: π-+Al  π0+X (11/2006)
Three different π- beam energies: 9, 20, and 50 GeV
Consider only 9x8 crystal matrix: about 140 π0 decays/crystal
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Reconstruction of π0
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Selection of π0 using S1, S2 ADC
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First Resonance Observed by CMS
Clear improvement over the uncalibrated peak (L3 algorithm).
For a precise estimate of the calibration precision:
use the 50 GeV electron test beam data.
π0 from upstream scintillators
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50 GeV e- peaks with TBS1 9 GeV constants
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Calibration Precision with
50 GeV Electrons
For each crystal, electron energy spectra were fitted to a Gaussian.
Distributions of the obtained peak positions for 9x8 crystal matrix:
Precision: 1.0±0.1% with 0.9±0.1% expected. Calibration
with ~5 GeV photon works well for higher-energy showers!
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π0 Conclusions and Outlook
 Proof-of-principle was achieved with full detector
simulation: crystal-by-crystal intercalibration to 1%
should be possible after a few days at L=2x1033cm-2s-1
Other methods are much slower and tracker dependent.
 Optimistic outlook for achieving and maintaining a
~0.5% precision. Many months of work on understanding
the ECAL performance and non-uniformity at lower
energies (work of ~15 physicists from 4 teams).
 Test beam study demonstrated a 1% calibration precision
with ~5 GeV photons: successfully used to reconstruct
50 GeV electrons. No noticeable systematics.
(Many thanks to the entire H2 test beam team).
 Currently a lot of work is being done on developing filter
farm tools for collecting π0 in situ at the LHC.
Calibration of the endcaps is also being considered.
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Test Beam 2006 Summary
 Two successful ECAL test beam efforts (H4, H2)
 Recorded invaluable data for upcoming LHC startup while
demonstrating viability of ECAL performance expectations
 Caltech continues its leadership roles in hardware/software
development of the 0 inter-calibration and laser monitoring
 Credit is due to the hard work of entire ECAL community
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