Calibration of the CMS Electromagnetic Calorimeter
with first LHC data Maria Margherita Obertino
(Universita’ del Piemonte Orientale – INFN Torino)On behalf of the CMS ECAL Collaboration
IPRD10Siena, June 7-10 2010
OUTLINE
The CMS Electromagnetic Calorimeter (ECAL)
Pre-calibration at the LHC startup
Results with very first collisions
Calibration streams
2010 plans
ECAL Hermetic homogeneous calorimeter made of PbWO4 Crystals
Barrel (EB) [|h|<1.48 ] 36 Supermodules (SM) with 1700 crystals each Photodetectors: APD
Endcaps (EE) [1.48<|h|<3.0 ] 2 Endcap sides, each 7324 crystals Photodetectors: VPT
Preshower (ES) [1.65<|h|<2.6 ] sampling calorimeter (lead, silicon strips)
Energy resolution :
€
σ E
E=a
E⊕b
E⊕c
• Target value: 0.5%
• Crucial dependence on the channelto channel inter-calibration precision.
Supermodule
ES
EE
PRE-CALIBRATIONExtensive10 years long pre-calibration program carried out before ECAL installation in CMS.
EB: Light Yield Measurements Intercalibration precision ~4.5-6.0%
EE: Light Yield + VPT gain Measurements Intercalibration precision ~8%
Laboratory measurements during crystal qualification phase(2000-2006)
PRE-CALIBRATIONExtensive10 years long pre-calibration program carried out before ECAL installation in CMS.
EB: Light Yield Measurements Intercalibration precision ~4.5-6.0%
EE: Light Yield + VPT gain Measurements Intercalibration precision ~8%
36 EB SMs operated on a cosmic ray stand for a period of ~1 weekDeposited energy per crystal ~ 300 MeV
Mean intercalibration precision ~1.5% Channel
intercalibration with cosmic muons (2006-2007)
Cosmic muons
Laboratory measurements during crystal qualification phase(2000-2006)
EB SM h=0 h=1.48
PRE-CALIBRATION
10 EB SMs Intercalibration precision ~0.3%
460 EE crystals Intercalibration precision <1%
Test Beam at CERN
90-120 GeV electrons
(2000-2006)
PRE-CALIBRATION
10 EB SMs Intercalibration precision ~0.3%
460 EE crystals Intercalibration precision <1%
All EE crystals Intercalibration precision ~6%
Test Beam at CERN
90-120 GeV electrons
(2000-2006)
Beam splash:in September 2008 and November 2009 beam was circulated in LHC, stopped in collimators 150 m away from CMS
ACHIEVEMENT WITH PRE-CALIBRATIONIn EB (from 11% to 0.3-2.2%)
LY, cosmic, test beam.
Combination strategy: Select best calibration available Combine when comparable
precision from two sources
In EE (from 27% to 5%)
Test beam (460 crytals) Combination of LY and beam splash
RMS of the constant distribution gives indication of inhomogeneity at the construction
27%
11%
ENERGY SCALE: ADC to GeV
Distribution of 5x5 amplitude sum (S25) with and without intercalibration for EE crystals exposed to 120 GeV electron beam.
(*) presented by F.De Guio
Confirmed later in dE/dx analysis with cosmics(*) and in collisions with p0/h mass peak.
Waiting for higher mass resonances to check the scale in-situ at high energy.
ECAL energy scale fixed by Test Beam data in Barrel and Endcap separately
66.67 MeV/ADC in EE
38.01 MeV/ADC for EB
INTERCALIBRATION
120 GeV
IN-SITU CALIBRATIONSeveral methods to calibrate in-situ:
Laser monitoring: correction for crystal transparency changes.
Due to very low radiation damage, negligible variation is expected at the startup.
IN-SITU CALIBRATIONSeveral methods to calibrate in-situ:
Laser monitoring: correction for crystal transparency changes.
Due to very low radiation damage, negligible variation is expected at the startup.
f-symmetry method: intercalibration for crystals at the same pseudorapidity. Based on invariance around the beam axis of the energy flow in minimum bias events.
Calibration performed comparing the total energy deposited in each crystal with the mean of the distribution of the total energies for all crystals at the same pseudorapidity
IN-SITU CALIBRATIONSeveral methods to calibrate in-situ:
Laser monitoring: correction for crystal transparency changes.
Due to very low radiation damage, negligible variation is expected at the startup.
f-symmetry method: intercalibration for crystals at the same pseudorapidity. Based on invariance around the beam axis of the energy flow in minimum bias events.
Calibration performed comparing the total energy deposited in each crystal with the mean of the distribution of the total energies for all crystals at the same pseudorapidity
p0 and h method: single crystal intercalibration Constants derived using unconverted g’s from p0 and h decay reconstructed in 3x3 matrices
The reconstructed mass is used to determine the photon energy
IN-SITU CALIBRATIONSeveral methods to calibrate in-situ:
Laser monitoring: correction for crystal transparency changes.
Due to very low radiation damage, negligible variation is expected at the startup.
f-symmetry method: intercalibration for crystals at the same pseudorapidity. Based on invariance around the beam axis of the energy flow in minimum bias events.
Calibration performed comparing the total energy deposited in each crystal with the mean of the distribution of the total energies for all crystals at the same pseudorapidity
p0 and h method: single crystal intercalibration Constants derived using unconverted g’s from p0 and h decay reconstructed in 3x3 matrices
The reconstructed mass is used as a constraint on photon energy
High energy electrons from W and Z decays (J/y under test) High luminosity required. Helpful at the startup only for energy scale.
SUPERMODULE RELATIVE SCALEThe relative energy scale of the barrel SMs is the first calibration with collisions.
First results in 2009 with ~0.01 nb-1 collected at
Scale derived in 2010 with ~0.4 nb-1 collected at Systematic error: 0.5% (see next slide) Statistical error negligible w.r.t. systematic one
Tested with beam
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s = 900GeV
€
s = 7TeV
CMS Preliminary CMS Preliminary
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s = 7TeV
• f-symmetry data
• p0 data
SUPERMODULE RELATIVE SCALE
Distribution of the relative scale of 10 SMs calibrated with test beam electrons.
Distribution of the relative scale of 26 SMs calibrated with cosmic rays.
The RMS of 0.5%±0.1% estimates the current precision of the combined in-situ regional scale calibration.
The RMS is 1.2%±0.2% is consistent with the expected precision of cosmic ray scale.
In-situ SM relative scale defined as the weighted average of p0 and f-symmetry results.
CMS Preliminary CMS Preliminary
RECONSTRUCTED p0 gg PEAK L = 0.43 nb-1
PT (g) > 0.4 GeV, PT(ggpair) > 1.0 GeV (pure ECAL selection)
Good agreement observed for the fitted peak width and Signal/Background. The fitted mass agrees with the expectation to within 1%. Number of p0 from the fit: 1.46 106
DATA MC
RECONSTRUCTED h gg PEAK L = 0.43 nb-1
PT (g) > 0.5 GeV, PT(ggpair) > 2.5 GeV (pure ECAL selection)
Good agreement observed for the fitted peak width and Signal/Background Number of h from the fit: 2.55 104
DATAMC
CALIBRATION STREAMIn CMS dedicated streams are implemented for calibration.
Use events accepted by Level 1 triggers in order to accumulate millions of events for a quick channel intercalibration.
In p0 and f-symmetry streams, only few tens of “useful”crystal hits are stored for each accepted event.
The output rate is close to 1kHz per stream.
Calibration stream commissioned with collision data.
PLANS FOR 2010
Improve crystal calibration down to 1% in the whole EB with few pb-1.
Reach the goal of 0.5% precision in EB by the end of the year with ~10pb-1
L=21030 cm-2s-1
Improve EE crystal calibration down from 5% to 1-2%.
Set the absolute scale to few permille.
SUMMARY
First calibration result with collision data is SM relative scale in EB. p0and f-symmetry in agreement. 3 SMs to be rescaled.
Collected statistics (~10 nb-1 at 26th May) good for regional intercalibration (5x5 matrix).
Calibration streams commissioned.
2010 goals: reach the 0.5% calibration precision in EB and improve EE calibration.
ECAL crystal intercalibration in very good shape.
Method Calibrated crystal
Precision
Lab. measurement
All ~ 5% in EB ~ 8% in EE
Cosmic All EB ~1.5% in average
Test Beam 10 SMs in EB~400 xtal in EE
~0.3%
Beam splash All EE ~0.6%
0.3%-2% in EB
5% in EE
backup
CALIBRATION STREAMCalibration stream commissioned with collision data.
Higher transverse energy cuts in calibration stream, better S/B. ~1000 p0/(crystal/pb-1) at L=21030cm-2s-1
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