1. Calibration of the CMS Electromagnetic Calorimeter with first LHC data
IPRD10
Siena, June
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

2. OUTLINE The CMS Electromagnetic Calorimeter (ECAL)
Pre-calibration at the LHC startup
Results with very first collisions
Calibration streams
2010 plans

3. 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)
Supermodule ES
Energy resolution : EE
Target value: 0.5%
Crucial dependence on the channel
to channel inter-calibration precision.

4. PRE-CALIBRATION
Extensive10 years long pre-calibration program carried out
before ECAL installation in CMS.
Laboratory measurements during crystal qualification phase
( )
EB: Light Yield Measurements
Intercalibration precision ~ %
EE: Light Yield + VPT gain Measurements
Intercalibration precision ~8%
76000 different objects (purity of the process, forni taglio polishing costruzione meccanica crystals with different charact. ) Per avere 0.5% abbiamo bisogno di correggere per qualunque cosa.
Cosa fa ATLAS (10/sqrt(E))

5. Mean intercalibration
PRE-CALIBRATION
Extensive10 years long pre-calibration program carried out
before ECAL installation in CMS.
Laboratory measurements during crystal qualification phase
( )
EB: Light Yield Measurements
Intercalibration precision ~ %
EE: Light Yield + VPT gain Measurements
Intercalibration precision ~8%
36 EB SMs operated on a cosmic ray stand for a period of ~1 week
Deposited energy per crystal ~ 300 MeV
Cosmic muons
Channel intercalibration with cosmic muons
( )
Mean intercalibration
precision ~1.5%
EB SM
h=0
h=1.48

6. PRE-CALIBRATION 10 EB SMs 460 EE crystals ~0.3% <1%
Test Beam at CERN
GeV electrons
( )
10 EB SMs
Intercalibration precision
~0.3%
460 EE crystals
<1%

7. PRE-CALIBRATION 10 EB SMs 460 EE crystals All EE crystals ~0.3% <1%
Test Beam at CERN
GeV electrons
( )
10 EB SMs
Intercalibration precision
~0.3%
460 EE crystals
<1%
Beam splash:
in September 2008 and November 2009 beam was circulated in LHC, stopped in collimators 150 m away from CMS
Limiting factor locan non univormity in muon flux
All EE crystals
Intercalibration
precision ~6%

8. ACHIEVEMENT WITH PRE-CALIBRATION
In EB (from 11% to %)
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
27%
11%
RMS of the constant distribution gives indication of inhomogeneity at the construction

9. ENERGY SCALE: ADC to GeV
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
Distribution of 5x5 amplitude sum (S25) with and without intercalibration for EE crystals exposed to 120 GeV electron beam.
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.
INTERCALIBRATION
120 GeV
(*) presented by F.De Guio

10. IN-SITU CALIBRATION Several 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.

11. IN-SITU CALIBRATION Several 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

12. IN-SITU CALIBRATION Several 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

13. IN-SITU CALIBRATION Several 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.

14. SUPERMODULE RELATIVE SCALE
The 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
CMS Preliminary
CMS Preliminary
Tested with beam
f-symmetry data
p0 data

15. SUPERMODULE RELATIVE SCALE
In-situ SM relative scale defined as the weighted average of p0 and f-symmetry results.
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.
CMS Preliminary
CMS Preliminary
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.

16. 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:
DATA MC

17. 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:
DATA MC

18. CALIBRATION STREAM
In 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.

19. 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=21030 cm-2s-1
Improve EE crystal calibration down from 5% to 1-2%.
Set the absolute scale to few permille.

20. SUMMARY 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
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.

21. backup

22. CALIBRATION STREAM
Calibration stream commissioned with collision data.
Higher transverse energy cuts in calibration stream, better S/B.
~1000 p0/(crystal/pb-1) at L=21030cm-2s-1