Calibration of the CMS Electromagnetic Calorimeter with first LHC data IPRD10 Siena, June 7-10 2010 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

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

PRE-CALIBRATION Extensive10 years long pre-calibration program carried out before ECAL installation in CMS. Laboratory measurements during crystal qualification phase (2000-2006) EB: Light Yield Measurements Intercalibration precision ~4.5-6.0% 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))

Mean intercalibration PRE-CALIBRATION Extensive10 years long pre-calibration program carried out before ECAL installation in CMS. Laboratory measurements during crystal qualification phase (2000-2006) 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 week Deposited energy per crystal ~ 300 MeV Cosmic muons Channel intercalibration with cosmic muons (2006-2007) Mean intercalibration precision ~1.5% EB SM h=0 h=1.48

PRE-CALIBRATION 10 EB SMs 460 EE crystals ~0.3% <1% Test Beam at CERN 90-120 GeV electrons (2000-2006) 10 EB SMs Intercalibration precision ~0.3% 460 EE crystals <1%

PRE-CALIBRATION 10 EB SMs 460 EE crystals All EE crystals ~0.3% <1% Test Beam at CERN 90-120 GeV electrons (2000-2006) 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%

ACHIEVEMENT WITH PRE-CALIBRATION In 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 27% 11% RMS of the constant distribution gives indication of inhomogeneity at the construction

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

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.

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

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

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.

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

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.

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 DATA MC

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.

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.

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.

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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