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Zγ Generator and Background Studies

Zγ Generator and Background Studies. Lindsey Gray University of Wisconsin at Madison EWK: Multiboson Meeting 9 April, 2009. Zγ Production. “Inner Zγ”. Direct Zγ coupling = zero In Standard Model Two Channels Photon radiated by quark “Outer Zγ” Photon radiated by lepton “Inner Zγ”

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Zγ Generator and Background Studies

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  1. Zγ Generator and Background Studies Lindsey Gray University of Wisconsin at Madison EWK: Multiboson Meeting 9 April, 2009

  2. Zγ Production “Inner Zγ” • Direct Zγ coupling = zero • In Standard Model • Two Channels • Photon radiated by quark • “Outer Zγ” • Photon radiated by lepton • “Inner Zγ” • Useful for calibration • Mz = Mllγ • Rate affected by Trilinear Gauge Coupling • Rate accurately predicted in SM • Look for excess outer Zγ l = e, μ “Outer Zγ” Signal  Lindsey Gray, UW Madison

  3. New Physics Accessible With Zγ Possible causes, if excess is observed: • Composite vector boson models • Could give electric dipole moment to Z • Higgs  Zγ • Any model which adds particles that decay into Zγ. • 4th Generation of Quarks   Leads to increase in cross section. Lindsey Gray, UW Madison

  4. Anomalous Electric Dipole Moment is the center of mass energy. • Standard Model Zγ • 3.3 pb per lepton channel • Shown: anomalous electric dipole moment. (f63 = .08) • Enhanced rate of Zγ • 3.4 pb per lepton channel Λ = 1 TeV • 4.3 pb per lepton channel Λ = 5 TeV ‘O’ is the combination of interacting fields ‘j’ denotes vector or axial vector couplings ‘n’ is the order of the correction ‘f’ is the Anomalous Coupling constant (AC) ‘Λ’ is the scale of new physics interactions Λ = 5 TeV √s =10 TeV Λ = 1 TeV Plots generated with program described in: U. Baur, T. Han, J. Ohnemus Phys. Rev. D 57, 2923 (1998) Standard Model Outer Zγ Lindsey Gray, UW Madison

  5. Search for Excess Energetic Photons from Outer Zγ • Anomalous couplings and new resonances enhance Zγ production cross-section • Enhancement occurs for events with energetic photons • Rare in SM Outer Zγ • Excess of high ET photons compared to standard model indicates new physics √s =10 TeV Λ = 5 TeV Λ = 1 TeV Standard Model Outer Zγ Lindsey Gray, UW Madison

  6. Current Anomalous Coupling Limits • Place limits by modeling Zγ photon ET of various anomalous coupling strengths • Determine what range of coupling strengths is consistent with ET distribution seen in data • Tevatron measured limits on anomalous couplings: • New Physics Scale Λ > 1 TeV • Determined by Tevatron mass reach. • Anomalous Coupling f63 < .083 • At Λ = 1 TeV Anomalous Coupling 2.0 fb-1 Limits on CP Conserving Vector (h3) and Axial Vector (h4) Couplings • Jianrong Deng, Al Goshaw, Thomas Phillips • Jan 31, 2008 • http://www-cdf.fnal.gov/physics/ewk/2008/Zgamma/ Lindsey Gray, UW Madison

  7. High Level Electron Trigger • • Electron HLT • Find groups of energy in ECAL • Reconstruct tracks near deposit • Match energy deposit to tracks • Recover energy losses to bremsstrahlung by extending included calorimeter area in phi direction. • For Zγ study at LHC startup: • Use isolated electron trigger to tag possible Zγ events. • Isolate electrons by summing nearby calorimeter deposits to check for activity. • Electron pT >15 GeV Lindsey Gray, UW Madison

  8. Muon High Level Trigger • • Muon HLT • Find track in muon system • Reconstruct tracks in tracker pointing towards muon system track • Match muon system track to tracker track • For Zγ study at LHC startup: • Zγ events use non-isolated muon trigger to tag possible events. • Muon pT > 5 GeV μ- Lindsey Gray, UW Madison

  9. Photon Reconstruction • Photons reconstructed from collections of associated crystals with energy in ECAL called SuperClusters. • Starts from a “seed” crystal of > 1 GeV • Make 5x5 crystal “seed cluster” if seed crystal is local maximum • Add up to 17 1x5 rows in each direction in phi, keeping rows with energy sum > .1 GeV • All SuperClusters are Photon candidates • ET > 10 GeV • H/E < .2 • Requires no matched pixel detector hit. R9 ■ Crystals in Seed Cluster ■ Other crystals within Supercluster --- Supercluster boundary Crystal f Crystal h Lindsey Gray, UW Madison

  10. Photon Identification ■ Crystals in Seed Cluster ■ Other crystals within Supercluster --- 3x3 region --- Supercluster boundary • Photon reconstruction begins with SuperCluster > 10 GeV. • Other particles can create a 10 GeV SuperCluster. • Jets fragmenting primarily to π0 • .001 of jets fake photons (# jets to # photons ~ 1000:1) • Electrons • Photon ID selects reconstructed photons passing various quality cuts. • HCAL < 10 GeV, near reconstructed photon. • ECAL < 10 GeV near reconstructed photon. • Require < 2 tracks near reconstructed photon. • Require that 80% of ECAL energy is within 3x3 crystals. • Electrons appear more spread out in phi than direct photons due to bending in magnetic field. Crystal f Crystal h Lindsey Gray, UW Madison

  11. Electron Reconstruction • • Calorimeter Reconstruction • Create superclusters of ECAL energy to include bremmstrahlung photons. • ET > 4 GeV • H/E < .1 • Tracker Reconstruction • Require calorimeter deposit matched to reconstructed track, ΔR < .15 • pT > 3 GeV ET γ e- Tracker Strips pT Pixels Lindsey Gray, UW Madison

  12. Muon Reconstruction • Standalone Reconstruction • Muon system only • Tracker Reconstruction • Match tracks to regions in the calorimeter consistent with a minimum ionizing particle. • Match within • Global Reconstruction • Match tracker tracks to muon system tracks by minimizing a ‘quality’ variable. • ‘Δd’ is distance between end of tracker track and beginning of muon track Inner Detector Track Track Quality Standalone Muon Track Lindsey Gray, UW Madison

  13. Zγ & Z+Jets Event Simulation • Zγ generated with Pythia 6.409 • LO matrix element cross section calculation • Higher order initial (final) state radiation is approximated • Z+jets background generated with MadGraph • Matrix element cross section calculation for Z + N ≤ 4 Jets • Detector simulated using Full Simulation (GEANT) for signal and FastSim for background. • GEANT simulates passage of particles through matter. • FastSim is a parameterization of GEANT CMS simulation with faster execution time. Detector simulation GEANT 4 FastSim Hard scattering Pythia MadGraph Hadronization, showers, IFSR PYTHIA Reconstruction of event CMSSW Lindsey Gray, UW Madison

  14. Zγ Generator Comparison • Baur Zγ Generator • Developed by Dr. Ulrich Baur (U. Buffalo) et al. • Calculates NLO Zγ cross section using Monte Carlo • Tunable anomalous couplings & new physics scale Λ • Accurately models photon ET for outer Zγ Number of Zγ Events vs. Photon ET Baur SM Outer Zγ Pythia SM Outer Zγ Events Lindsey Gray, UW Madison

  15. Comparing Baur to Tevatron Data Baur • CDF measures inner & outer Zγ • = 4.6 ± 0.2 (stat) ± 0.3 (sys) pb • = 1.2 ± 0.1 (stat) ± .17 (sys) pb • D0 measures inner & outer Zγ as well • = 4.4 ± .27 (stat) ± .27 (sys) pb • All measurements agree with Baur MC predictions: • = 4.5 ± 0.4 pb (Inner + Outer Zγ) • = 1.21 ± 0.1 pb (Outer Zγ Only) 2.0 fb-1 Baur Anomalous Coupling 1.1 fb-1 Lindsey Gray, UW Madison

  16. Z+Jets Background to Outer Zγ • Z+jets • 1 in 1000 jets fragment primarily to π0 • σZ+Jets = 251 pb @ Tevatron (to leptons) • σZ+Jets = 3700 pb @ LHC (to leptons) • Similar kinematics to Outer Zγ • CDF Z+Jets pT measurement matches NLO MCFM well. • MCFM: Monte Carlo for Femtobarn Measurement (dev. by CDF Collab.) • Give accurate background prediction for CDF Zγ measurement • CMS Z+Jets will be measured in 200 pb-1 • Expect more Z + multiple jets Lindsey Gray, UW Madison

  17. Zγ Signal and Z+Jets Background • Require electrons, muons and photons to be within the tracker and to pass trigger. (-2.5 < η < 2.5) • Require e±: ET > 15 GeV & μ±: pT > 5 GeV • Removes poorly reconstructed e± and μ±. Starting With: 105 Signal 28k Bkg [200pb-1] Zγ -> eeγ MC Z+jets MC Zγ->μμγ MC Z+jets MC 200pb-1 200pb-1 e μ Lindsey Gray, UW Madison

  18. Cut on Dilepton Invariant Mass • Require dilepton mass near Z peak (70 < Mll < 100) • Majority of signal Zs are on shell • Suppresses Inner Zγ What’s Left: 85 Signal, 81% 21k Background, 75% Reject Reject Reject Reject e μ Zγ->μμγ MC Z+jets MC Zγ -> eeγ MC Z+jets MC Lindsey Gray, UW Madison

  19. Selecting Signal Photons: H/E • Hcal-to-Ecal energy ratio of a reconstructed photon. • Jets have a larger hadronic energy fraction. • Hence, so do many jets that fake photons. • Cut at H/E = .025 What’s Left: 75 Signal, 74% 9k Background, 33% Z+jets Zγ EM Supercluster ECAL g jet ECAL HCAL reject Supercluster Lindsey Gray, UW Madison

  20. Selecting Signal Photons: R9 • Cut on ratio of E3x3 to Esupercluster ( “R9”) • EM deposits from Jets will be more spread out. • Except energetic π0’s • Cut at r9 = .90 What’s Left: 45 Signal, 43% 2k Background, 7.9% Z+jets Zγ 200pb-1 f h reject Lindsey Gray, UW Madison

  21. Selecting Signal Photons: Track Isolation • Count number of reconstructed tracks in a cone near the photon with pT > .5 GeV • Faked photons have more tracks in .4 ΔR cone. • Cut at Number of Tracks = 2 What’s Left: 41 Signal, 39% 1.5k Background, 5.3% Z+jets Zγ γ Isolation Cone ΔR=0.4 reject Lindsey Gray, UW Madison

  22. Selecting Signal Photons: ETIsolation • Σ (Hcal ET + Ecal ET + Track pT)/ET, Supercluster in annulus around reconstructed photon. • Faked photons have more energy and tracks in the .06 < ΔR < .4 annulus. • Cut at (Isolation Sum)/ET = .4 What’s Left: 25 Signal, 24% 480 Background, 1.5% Z+jets Zγ γ Signal Cone ΔR=0.06 Isolation Cone ΔR=0.4 reject Lindsey Gray, UW Madison

  23. Selecting Signal Photons: Phi Width • Since π0 -> γγ, faked photons will appear wider in phi due to the opening angle between the photons. • Cut at Phi Width < .015 What’s Left: 17 Signal, 16% 300 Background, 1.0% Z+jets Zγ f h reject Lindsey Gray, UW Madison

  24. Selecting Signal Photons:Minimum ΔRlγ • ΔRlγ > 1.3 cut applied after previous photon cuts. • Further rejects Z+Jets background and Inner Zγ • Added advantage of avoiding singularity in the Zγ cross section from photon collinearity • Improves cross section prediction What’s Left: 9 Signal, 8.5% 38 Background .13% e reject reject Z+jets Zγ Z+jets Zγ μ Lindsey Gray, UW Madison

  25. Cut on llγ Invariant Mass • Inner Zγ events with large photon ET can pass ΔRlγ cut. • Dilepton+photon invariant mass will be near Z mass. • Majority of outer Zγ will be outside of Z peak. • Cut at Mllγ > 105 GeV What’s Left: 8 Signal, 7.6% 10 Background .035% Z+jets Zγ Z+jets Zγ e μ reject reject Lindsey Gray, UW Madison

  26. Summary of Signal & Background Signal Background 200pb-1 Lindsey Gray, UW Madison

  27. Conclusion and Next Steps • Signal to background is roughly 1:1 on Z peak. • 8 (11, with Anom. Coup.) Events & 10 Background • Next Steps: • Improve Signal-to-Background to 2:1 • Measure Z+Jets background in data in tandem with Zγ measurement and apply fake rate • 200 pb-1 analysis allows SM Zγ measurement • Sensitive to new physics • Assuming maximum allowed anomalous coupling, 3 events & 1 background (NLO prediction) • Photon ET > 100 GeV Lindsey Gray, UW Madison

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