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Calibration from π 0 with a converted photon

Calibration from π 0 with a converted photon. Calo Calibration Meeting 29/04/2009 Plamen Hopchev , LAPP. Contents. Description of the calibration method Short overview; more details in my presentation in the previous “Calorimeter Calibration Exercise”:

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Calibration from π 0 with a converted photon

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  1. Calibration from π0 with a converted photon Calo Calibration Meeting 29/04/2009 PlamenHopchev, LAPP

  2. Contents • Description of the calibration method • Short overview; more details in my presentation in the previous “Calorimeter Calibration Exercise”: http://indico.cern.ch/getFile.py/access?contribId=13&sessionId=3&resId=0&materialId=slides&confId=51076 • Preview of the selection criteria • Results • Conclusion 1

  3. Overview of the method • Aim: Calibrate ECAL for high energy photons ( > few GeV ) • The method: • Use π0 → γ (e+e–)γ(one of the photons converts) • Reconstruct the Converted Photon and predict the energy of the non-converted photon, using • For a given pair of reconstructed Conv Photon and Non-Conv Photon we have • Build the distribution of • We expect it to be: • Flat for the random CP + NCP combinations • Gaussian, centered at 0 for the photon pairs from π0 2

  4. Overview of the exercise • Analyzed ~ 5.4 M MinBias L0-passed events (DC06/L0-v1-lumi2) • Used DaVinci v22r2 • Procedure: • Make all possible pairs of StdTightElectrons (2 Long or 2 Downstream tracks; get also same sign pairs for background estimation) • Form Converted Photon (CP) and extrapolate it to ECAL • Simply sum the momenta of the electrons • Start extrapolating from the mean position of the electrons before the magnet • Look for a brem in a small region around the projection point • Combine the CP with the nearby Photons (Non-Converted Photon, NCP) • Require that the NCP is outside the brem region and inside a larger circle around the CP projection point • The entity of interest is the triplet of the 2 electrons and the NCP • Preselect potential signal triplets and store in a Tree for analysis • The distributions shown later and called “ALL…” correspond to these preselected triplets and not to the real spectrum • List of the preselection cuts and results are shown in the Backup slides 3

  5. Signal Selection Criteria – Conv Photon (1) • Shown on the plot: Invariant mass distribution for opposite sign electron pairs • Cuts used for the selection: • Long Tracks: Minv < 15 MeV • Downstream Tracks: Minv< 70 MeV 4

  6. Signal Selection Criteria – Conv Photon (2) • Shown on the plots: x & y distance between the two electrons before the Magnet • Cuts used for the selection ( used only for Downstream tracks ): • Long Tracks Downstream Tracks: |dx|< 6 mm && |dy| < 4 mm 5

  7. Signal Selection Criteria – Conv Photon (3) • Shown on the plots: Correlation of the VeloChargesof the 2 electrons (LL) • Cuts used for the selection ( used only for Long tracks ): • VeloCharge1 > 45 • VeloCharge2 > 45 • | VeloCharge1 – VeloCharge2) | < 5 6

  8. Signal Selection Criteria – Conv Photon (4) • Shown on the plots: projected converted photon x vs y position at ECAL • Cuts used for the selection: • abs( xEcal ) < 3000 mm • abs( yEcal ) < 3000 mm • Exclude inner hole of 380 x 380 mm 7

  9. Signal Selection Criteria – Brems • Shown on the plots on the right: 1-D chi2 match of Projected Conv Photon and BestRecBrem and MCBrem ( for x and y ) • Cuts used for the selection: • chi2x < 50 • chi2y < 20 8

  10. Signal Selection Criteria – Non-Conv Photon • Shown on the plots on the right: 1-D chi2 match of Projected Conv Photon and Non-Converted Photon Candidate • Cuts used for the selection: • 50 < chi2x < 1.e6 • 20 < chi2y < 1.e6 9

  11. Signal Selection Criteria – summary • Selection of Brems • Look for Neutral PP near the projected CP ( two 1-D matches ): • chi2x < 50 • chi2y < 20 • Potential additional cuts: • Cluster Energy < 20 GeV • Cluster size < 15 cells • PhotonID > X • Selection of Non-ConvPhot: • 50 < chi2x < 1.e6 • 20 < chi2y < 1.e6 • We use exactly the same cuts to select background from same sign electron pairs • Selection of electron pairs • 2 Long Tracks: • Inv Mass < 15 MeV • Velo Charges > 45 && abs( VeloCharge1 – VeloCharge2) < 5 • 2 Downstream Tracks: • Inv Mass < 70 MeV • x- and y- distance between electrons measured at TT less than 6 and 4 mm respectively • Collect pairs with el. charges which are: opposite - possible signal same - to estimate the background • Project ConvPhoton to ECAL: • abs( xEcal ) < 3000 mm • abs( yEcal ) < 3000 mm • Exclude inner hole of 380 x 380 mm 10

  12. Fitting procedure • Parametrize the Background with double sigmoid: • Example double sigmoids • We use only part of the curve • Also tried Argus function (defined in RooFit), but its drop-off is not steep enough • Compare 2 different ways of fitting: Directly fit the Signal + Background distribution • Use double sigmoid + gaussian • All 5 + 3 parameters are variable (not fixed) • The parameters of the gaussian are of primary interest Alternative approach : • Fit the background obtained from “same sign electron pairs” with a double sigmoid • Fix all but one of the parameters of the sigmoid – leave parameter Cvariable • Fit the signal + background distribution with gaussian + the double sigmoid • Results in comparison with the “direct” fit: • sensibly worse χ2 of the fit • in some of the cases the fitted gaussians have sensibly different parameters 11

  13. Long Tracks && No Brem - plots 1 GREEN –”direct” fit • RED – fit “same sign el.“ bkg and fit sig+bkg 12

  14. Long Tracks && No Brem - plots 2 c = 534 μ = 9.5 % σ = 12.7 % c = 565 μ = 8.9 % σ = 14.0 % 13

  15. Long Tracks && Yes Brem - plots 1 GREEN –”direct” fit • RED – fit “same sign el.“ bkg and fit sig+bkg 14

  16. Long Tracks && Yes Brem - plots 2 c = 96 μ = 4.4 % σ = 13.0 % c = 103.6 μ = 4.1 % σ = 14.9 % 15

  17. Downstream Tracks && No Brem - plots 1 GREEN –”direct” fit • RED – fit “same sign el.“ bkg and fit sig+bkg 16

  18. Downstream Tracks && No Brem - plots 2 c = 1.9 e3 μ = 2.8 % σ = 16.9 % c = 1.9 e3 μ = 3.1 % σ = 17.4 % 17

  19. Downstream Tracks && Yes Brem - plots 1 GREEN –”direct” fit • RED – fit “same sign el.“ bkg and fit sig+bkg 18

  20. Downstream Tracks && Yes Brem - plots 2 c = 223.5 μ = 2.4 % σ = 22.4 % c = 230.1 μ = 0.6 % σ = 16.6 % 19

  21. Summary of the Fits • For the fit results: • The upper line describes the “direct” fit • The lower line describes the “2-step” fit • To be added: A column with a info about the energy resolution on the ConvPhot 20

  22. Expected Rates • Rate of preselected triplets: 10.5/event (1.34x10-3/ event are MCConf) • The preselection is described in the backup slides • Rate of triplets passing final cuts: 3x10-3/event (0.4x10-3 / event are MCConf)  Eff ≈ 34 %  Bkg Retention ≈ 3x10-4 • Distribution of signal events over CALO Zones: Inner : Middle : Outer = 5.1 : 2.2 : 1.0 21

  23. Conclusion • The results keep improving

  24. BAKUP SLIDES

  25. Preselection cuts for triplets • Triplet = 2 electrons and a photon • Electron pair: • Track Types: Long/Long or Downstream/Downstream • Minv < 120 MeV • Distance between projected Conv Photon and the Non-Converted Photon < 1200 mm • Other “sanity” cuts: • e.g. for long electrons we require existence of State at BegRich1 • … • RESULTS OF THE PRESELECTION • Number of preselected electrons • opposite el charge: 2.0/event ( 0.19/event are MCConf ) • same el charge: 2.5/event ( 2x10-4/event are MCConf ) • Number of preselected triplets: 10.5/event (1.34x10-3 / event are MCConf )

  26. More Plots to be done: • DELTA as function of the ECAL zone • DELTA as function of the NCP energy • To Fix: • The x/y distance of the electrons before the magnet: only for opposite sign electrons !!! • For LL try distance cut like DD, instead of VeloCharge

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