Calibration of CMS Electromagnetic Calorimeter at LHC startup

Riccardo Paramatti (INFN Rome & Cern)on behalf of CMS ECAL Collaboration

CALOR2010 Conference

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OutlineThe CMS Electromagnetic CalorimeterPre-calibration at the LHC startupResults with very first collisionsCalibration streams2010 plans

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CMS Electromagnetic Calorimeter

Excellent energy (and position) resolution for photons and electrons (H→γγ, H→ZZ →4e)Lead Tungstate (PbWO4) homogenous crystalBarrel (EB):

36 Supermodules (SM), each 1700 crystals |η|

Test Beam: Cern electron beams.

From 15 GeVto 250 GeV.(2004-2007)

Pre-calibration Campaign

Laboratory measurements during crystal qualification phase.(2000-2006)

Beam Splash:In September 2008

and November 2009, beam was circulated in LHC, stopped in collimators 150m away from CMS red = ECAL, green=ES, blue=HCAL

beam

Channel intercalibrationwith cosmic muons (only Barrel SMs)

(2006-2007)

cosmic muons

A very intense 10 years long pre-calibration campaign. Several orders of magnitude in energy: from 1 MeV of Co60 source to 120 GeV electron beam.

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Barrel Pre-calibration

Test Beam at Cern10 Supermodules on electron beam (intercalibration accuracy ~0.3%)

Cosmics Calibration36 SMs (~1.4-2.2%)

Light Yield Measurements36 SMs (~4.5-6.0%)

Combination strategy:Select best calibration availableCombine when comparable precision from two sources

Cosmics Calibration

η=0 η=1.48Supermodule

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Endcap Pre-calibration

Test Beam at Cern460 crystals (

Achievement with pre-calibrationIn the Barrel:

from inhomogeneity at the construction of 11.2% (RMS of constant distribution)to a pre-calibration precision of 0.3%-2.2%

In the Endcaps:from inhomogeneity at the construction of 27.2% (RMS of constant distribution)to a pre-calibration precision of ~5%

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Energy Scale

ECAL energy scale (the ADC to GeV conversion factor) fixed by Test Beam data in Barrel and Endcap separately.Confirmed later in dE/dxanalysis with cosmics (*) and in collisions with π0/η mass peak.Waiting for higher mass resonances to check the scale in-situ at high energy.

(*) presented by Andrea Benaglia

Distribution of 5x5 amplitude sum (S25) with and without intercalib. for EE crystals exposed to 120 GeV electron beam.

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In-situ CalibrationIntercalibration precision goal is 0.5%. Main contribution to the constant term of energy resolution.Several methods to calibrate (and follow-up) in-situ:

Laser monitoring: correction for crystal transparency changes (*). Due to very low radiation damage, negligible variation is expected at the startup.φ-symmetry calibration: invariance around the beam axis of energy flow in minimum bias events. Intercalibrate crystals at the same pseudorapidity, other methods are needed to intercalibrate regions at different pseudorapidity.π0 and η calibration: mass constraint on photon energy,use unconverted γ’s reconstructed in 3x3 matrices of crystals.High energy electron from W and Z decays (E/p with single electrons and invariant mass with double electrons). High luminosity required. Helpful at the startup only for energy scale. Testing also J/ψ.

(*) presented by Federico Ferri

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Supermodule relative scaleThe relative energy scale of the Barrel Supermodules is the first calibration with collisions. Results already with ~0.01 nb-1. With ~0.4 nb-1 stat. error negligible w.r.t. syst. (= 0.5% see next slide)

CMS Preliminary

φ-symmetry results for 2009 and 2010 collisions

CMS 2010 Preliminary

φ-symmetry and π0 results for 2010 collisions. Arrows point SMs tested with beam.

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Supermodule relative scaleDistribution of the relative scale of 10 Supermodules calibrated with test beam electrons.

The rms of 0.5%±0.1% estimates the current precision of the combined in-situ regional scale calibration.

CMS Preliminary

Distribution of the relative scale of 26 Supermodules calibrated only with cosmic rays.

The rms is 1.2%±0.2% with all SMs. Consistent with the expected precision of cosmic ray scale.

CMS Preliminary

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Reconstructed π0→γγ peakL = 0.43 nb-1. P⊥(γ) > 0.4 GeV, P⊥(γγ pair) > 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 π0 from the fit: 1.46▪106

DATA MC

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Reconstructed η→γγ peakL = 0.43 nb-1. P⊥(γ) > 0.5 GeV, P⊥(γγ pair) > 2.5 GeV (pure ECAL selection)Good agreement observed for the fitted peak width and mass Number of η from the fit: 2.55▪104

DATA MC

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Calibration streamsIn 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 π0/η and φ-symmetry streams, only few tens of “useful” crystal hits are stored for each accepted event.The output rate is close to 1kHz per stream.

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Performance of Calibration streamCalibration stream commissioned with collision data.Higher transverse energy cuts in calibration stream, better S/B.~1000 π0/(crystal▪pb-1) at L=2▪1030 cm-2 s-1

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2010 plans

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 ~10 pb-1Improve EE crystal calibration down from 5% to 1-2%.Set the absolute scale to few permille.

Integrated luminosity required to achieve a crystal intercalibration precision of 1% and 0.5% for Barrel crystals with different pseudorapidity, assuming L=2▪1030 cm-2 s-1

η=0 η=1.48Barrel

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ECAL crystal intercalibration in very good shape.0.3%-2% in EB, 5% in EEZ width already almost insensitive to residual mis-calibration even in EE. ~3.5 (3.0) GeV with 5% (0.5%) precision in EE.

First calibration result with collision data is SM relative scale in EB. π0 and φ-symmetry in agreement.Collected statistics (~3 nb-1 at 12th May) good for regional intercalibration (e.g. matrices of 5x5 crystals).Calibration streams commissioned.2010 goals: reach the 0.5% calibration precision in EB and improve EE calibration.

Summary

Backup

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EB calibration at the startupIntercalibration constants spread: 11.2%Intercalibration constants precision: 0.3%-2.2%

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EE calibration at the startupIntercalibration constants spread: 27.2%Intercalibration constants precision: 5%

Calibration of CMS Electromagnetic Calorimeter at LHC startupOutlineCMS Electromagnetic CalorimeterPre-calibration CampaignBarrel Pre-calibrationEndcap Pre-calibrationAchievement with pre-calibrationEnergy ScaleIn-situ CalibrationSupermodule relative scaleSupermodule relative scaleReconstructed p0 → peakReconstructed h → peakCalibration streamsPerformance of Calibration stream2010 plansSummaryBackupEB calibration at the startupEE calibration at the startup