Intercalibration of the CMS Electromagnetic Calorimeter Using Neutral Pion Decays

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M. Gataullin (California Institute of Technology)M. Gataullin (California Institute of Technology)on behalf of the CMS ECAL Groupon behalf of the CMS ECAL Group

PRD08: 11th Topical Seminar On Innovative Particle and PRD08: 11th Topical Seminar On Innovative Particle and Radiation Detectors, 1-4 Oct 2008, Siena, TuscanyRadiation Detectors, 1-4 Oct 2008, Siena, Tuscany

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CMS ECAL: 76K Crystals, 90 Tons

SSSSSshhh

Only Barrel considered: 61,200 crystals – mass 67.4 tons

170 φ-rings of 360 crystals, each ~ 25 x 25 x 230 mm3 (25.8 X0)

Test beams: energy resolution of <0.5% (~100 GeV electrons)

Calibration goal: achieve and maintain it in situ at the LHC

Barrel Barrel CrystalsCrystals

EndCaEndCapp

|η|<1.48

See Paolo Meridiani’s talk later in this session!

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Main Purpose of the Calibration: Higgs Hunting

Crystals Pulse Amplitudes(+clustering algorithm)

Particle EnergyCalibration

• Achieving a precise in situ crystal-by-crystal calibration of the CMS ECAL

will be crucial for the Hγγ search (discovery channel for M < 140 GeV).• Design calibration precision: ~0.5%; achieved in various test beam studies.

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π0 Calibration Concept

Level 1 trigger rate dominated by QCD: several π0‘s/event Useful π0γγ 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 Hγγ detection 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.

Data after L1 Trigger Online Farm 0 Calibration

>10 kHz~1 kHz

π0γγ Selection

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Based on local, ECAL variables — suitable for online filter farm. Kinematics: PT (γ) >1 GeV, PT (pair) > 3.5 GeV and η < 1.4 (barrel).

Photon shower-shape cuts: S9/S25 > 0.9 and S4/S9 > 0.9, where

the sums Si are defined with 2x2, 3x3, and 5x5 crystal matrices. Isolation cut optimized to remove pairs with converted photons: Other PT in ΔR < 0.2 and Δη < 0.05 should be below 60% of PT (pair).

η

φ

Δη

γγ

ΔR

π0γγ Selection Results

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After selection, After selection, ππ00 yield is about 0.07 per event accepted by L1 triggers. yield is about 0.07 per event accepted by L1 triggers. Signal/Background is 1.9Signal/Background is 1.9 ± 0.1, as calculated in , as calculated in ±2σ window.

The selected sample of 300,000was then used for the calibration exercises.

The π0 yield can be translated into the rate of useful decays.

Assuming a L1 rate of 12.5 kHz:

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Rtotal( 1)L =0.87 ±0.03 kHz

Correction for Gaps and Noise Suppression

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Corrections were derived both eta and phi directions. The dots represent the values before correction, red line – after.

Slightly bigger gaps between baskets/supermodules lead to −1.3-1.5% shifts. Selective readout: a -0.4% shift with a period of 5 crystals.

“L3” Calibration Algorithm

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where N is the iteration step number and wi is the fraction of shower energy

in this crystal for the ith shower containing this crystal in the 3x3 matrix.

Only pairs in the peak (±2σ window) are used. Both photon energy and direction reconstructed using crystal level information (as in the selection). After each iteration events are re-selected with new constants. Calibration precision defined as R.M.S. of the products of the final and initial mis-calibration constant. Limited number of events: entire barrel folded onto a 10x10 matrix.

“Fit” Calibration Algorithm

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For each crystal, a histogram is filled with invariant masses of pairs for which this crystal is

central (highest energy) for one of the two photons in the pair. The distributions obtained are then fitted to a gaussian+bkgd.; several iterations are required. Works because 70% of shower energy is in the central crystal. Performs slightly better than the L3 algorithm since the background shape is determined from the fit.

Performance of the Calibration Algorithms

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Several other algorithms were investigated and found to converge to about the same final calibration precision. Final precision also does not depend on the initial mis-calibration.

4% miscalibration

no miscalibration

Calibration Results

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The calibration precision is fitted to

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σC

C=

a2

N

+ b2 , where N is the number of 0 per crystal

At the present level of statistical precision, we see no significant limits to improving calibration accuracy with increasing the calibration sample. Using the full sample of

3000 π0‘s/crystal, a calibration precision of 0.5% was obtained.

Studies performed with four million fully simulated events. Results given for the scenario of L=2x1033cm-2s-1 and L1 rate of 12.5 kHz. After selection, the π0 yield is about 0.07 per event accepted by L1 triggers. Using the results of the calibration exercises, the π0 rate is translated into time needed to achieve a 1%(0.5%) precision: 10 to 35 (25 to 100) hours of continuous running needed to calibrate 95% of the barrel to these levels of accuracy. Additional remarks:1) Available data throughput and CPU on the on-line filter farm will be limited. We are able to stay within the imposed constraints. 2) Situation is a bit more complex at the startup: at 10 TeV (and even

at 900 GeV) the π0 yield is lower but but still quite useful 3) The online filter for the endcaps is

also being developed.

Projected Calibration Performance

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Calibration Studies in Test Beams at CERN

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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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.

π0γγ from upstream scintillators

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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. Calibrationwith ~5 GeV photons works well for higher-energy showers!

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Summary and Outlook

Crystal-by-crystal intercalibration to 1% should be Crystal-by-crystal intercalibration to 1% should be possible after a few days at L=2x10possible after a few days at L=2x103333cmcm-2-2ss-1-1

Optimistic outlook for achieving and maintaining aOptimistic outlook for achieving and maintaining a ~0.5% precision. Many months of work on understanding ~0.5% precision. Many months of work on understanding the ECAL performance and non-uniformity at lower the ECAL performance and non-uniformity at lower energies (work of ~15 physicists from 4 teams). energies (work of ~15 physicists from 4 teams). Test beam study demonstrated a 1% calibration precision Test beam study demonstrated a 1% calibration precision with ~5 GeV photons: successfully used to reconstruct with ~5 GeV photons: successfully used to reconstruct 50 GeV electrons, without noticeable systematic effects.50 GeV electrons, without noticeable systematic effects. Currently a lot of work is being done on finalizing filter

farm tools for collecting π0γγ in situ at the LHC. Calibration of the endcaps is also under development.

For more information on ECAL see Paolo Meridiani’s talk later in this session!The overall ECAL calibration strategy will be presented in the S. Argiro’s poster.A very important aspect of the CMS ECAL calibration&monitoring (laser monitoring system) will be presented tomorrow afternoon by A. Bornheim.

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Backup

Using L1 Trigger Objects as Seeds

π0 candidates selected in regions of 20x20 crystals, containing L1 trigger electromagnetic candidates. The same selection approach with two additional cuts Nclus<4 and (Etot – E)/Etot< 0.35. A more realistic approach for the online filter farm environment. The π0 rate and S/B found to be comparable to the results obtained by selecting π0 candidates in the entire barrel. Assuming a L1 rate of 12.5 kHz:

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Rtotal( 1)L =0.49 ±0.02 kHz < /S B > = 1.6 ±0.1

Selection Results

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After selection, high rapidity regions suffer both in the event rate andSignal/Background (background rate is almost constant with η).

Dependence on Signal/Background

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The dependence on S/B is well described by

This can then be used to estimate the calibration performance for differentη regions and different LHC running conditions.

SBa /1~ +

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σC

C=

a2

N

+ b2