1. Intercalibration of the CMS Electromagnetic Calorimeter Using Neutral Pion Decays 1 M. Gataullin (California Institute of Technology) on behalf of the CMS ECAL Group PRD08: 11th Topical Seminar On Innovative Particle and Radiation Detectors, 1-4 Oct 2008, Siena, Tuscany

2. 2 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 mm 3 (25.8 X 0 ) Test beams: energy resolution of <0.5% (~100 GeV electrons) Calibration goal: achieve and maintain it in situ at the LHC Barrel Crystals EndCap |η|<1.48 See Paolo Meridiani’s talk later in this session!

3. 3 Main Purpose of the Calibration: Higgs Hunting Crystals Pulse Amplitudes (+clustering algorithm) Particle Energy Calibration Achieving a precise in situ crystal-by-crystal calibration of the CMS ECAL will be crucial for the H  γγ search (discovery channel for M < 140 GeV). Design calibration precision: ~0.5%; achieved in various test beam studies.

4. 4 π 0 Calibration Concept  Level 1 trigger rate dominated by QCD: several π 0 ‘s/event  Useful π 0  γγ decays selected online from such events  Main advantage: high π 0 rate (nominal L1 rate is 100kHz !)  “Design” calibration precision  better than 0.5% Achieving it would be crucial for the H  γγ detection  Studies performed with about four million fully simulated QCD events. Results given for the scenario of L=2x10 33 cm -2 s -1 and L1 rate of 10 kHz. Data after L1 Trigger Online Farm  0 Calibration >10 kHz ~1 kHz

5. π 0  γγ Selection 5 Based on local, ECAL variables — suitable for online filter farm.  Kinematics: P T ( γ ) >1 GeV, P T (pair) > 3.5 GeV and η < 1.4 (barrel).  Photon shower-shape cuts: S 9 /S 25 > 0.9 and S 4 /S 9 > 0.9, where the sums S i are defined with 2x2, 3x3, and 5x5 crystal matrices.  Isolation cut optimized to remove pairs with converted photons: Other P T in ΔR < 0.2 and Δη < 0.05 should be below 60% of P T (pair). η φ Δη γ γ ΔRΔR

6. π 0  γγ Selection Results 6 After selection, π 0 yield is about 0.07 per event accepted by L1 triggers. Signal/Background is 1.9, as calculated in After selection, π 0 yield is about 0.07 per event accepted by L1 triggers. Signal/Background is 1.9 ± 0.1, as calculated in ±2σ window. The selected sample of 300,000 was then used for the calibration exercises. The π 0 yield can be translated into the rate of useful decays. Assuming a L1 rate of 12.5 kHz:

7. Correction for Gaps and Noise Suppression 7 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.

8. “L3” Calibration Algorithm 8 where N is the iteration step number and w i 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.

9. “Fit” Calibration Algorithm 9  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.

10. Performance of the Calibration Algorithms 10 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

11. Calibration Results 11 The calibration precision is fitted to  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 π 0 ‘s/crystal, a calibration precision of 0.5% was obtained.

12. Studies performed with four million fully simulated events. Results given for the scenario of L=2x10 33 cm -2 s -1 and L1 rate of 12.5 kHz. After selection, the π 0 yield is about 0.07 per event accepted by L1 triggers. Using the results of the calibration exercises, the π 0 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 π 0 yield is lower but but still quite useful 3) The online filter for the endcaps is also being developed. Projected Calibration Performance 12

13. 13 Calibration Studies in Test Beams at CERN π 0 decays produced through: π - +Al  π 0 +X (11/2006) Three different π - beam energies: 9, 20, and 50 GeV Consider only 9x8 crystal matrix: about 140 π 0 decays/crystal

14. 14 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. π 0  γγ from upstream scintillators

15. 15 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. Calibration with ~5 GeV photons works well for higher-energy showers!

16. 16 Summary and Outlook Crystal-by-crystal intercalibration to 1% should be possible after a few days at L=2x10 33 cm -2 s -1  Crystal-by-crystal intercalibration to 1% should be possible after a few days at L=2x10 33 cm -2 s -1  Optimistic 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 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 π 0  γγ 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.

17. 17 Backup

18. Using L1 Trigger Objects as Seeds π 0 candidates selected in regions of 20x20 crystals, containing L1 trigger electromagnetic candidates. The same selection approach with two additional cuts N clus <4 and (E tot – E  )/E tot < 0.35. A more realistic approach for the online filter farm environment. The π 0 rate and S/B found to be comparable to the results obtained by selecting π 0 candidates in the entire barrel. Assuming a L1 rate of 12.5 kHz: 18

19. Selection Results 19 After selection, high rapidity regions suffer both in the event rate and Signal/Background (background rate is almost constant with η).

20. Dependence on Signal/Background 20 The dependence on S/B is well described by This can then be used to estimate the calibration performance for different η regions and different LHC running conditions.