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W + jets events with CMS at the LHC

W + jets events with CMS at the LHC. Kira Grogg University of Wisconsin - Madison Preliminary examination. Outline. Standard Model Importance of W+jets Large Hadron Collider Compact Muon Solenoid W+jets events Event Simulation Next Steps. The Standard Model. Fundamental particles:

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W + jets events with CMS at the LHC

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  1. W + jets events with CMS at the LHC Kira Grogg University of Wisconsin - Madison Preliminary examination Kira Grogg, U. Wisconsin

  2. Outline • Standard Model • Importance of W+jets • Large Hadron Collider • Compact Muon Solenoid • W+jets events • Event Simulation • Next Steps Kira Grogg, U. Wisconsin

  3. The Standard Model • Fundamental particles: • Fermions (matter) • Electron, muon, tau, corresponding neutrinos • up, down, charm, strange, top, bottom quarks • Bosons (force carriers) • Photon (EM) • Gluon (Strong) • W, Z (EW) Kira Grogg, U. Wisconsin

  4. Importance of W+jets • Background for • Higgs • top production • SUSY • High cross section makes it useful • as a precise luminosity monitor • as a high-statistics detector calibration tool • to demonstrate the performance of the CMS experiment • for verification of theoretical cross-section and structure functions Kira Grogg, U. Wisconsin

  5. W+jets production • Feynman Diagrams of W + jet production at the parton level • [Add higher level diagrams with more jets] Kira Grogg, U. Wisconsin

  6. The Large Hadron Collider • 14 TeV proton-proton collider • Circumference of 27 km • Luminosity up to 1034/cm-2s-1 • Protons travel the circumference in ~90s • 8T Magnets dump clean clean Kira Grogg, U. Wisconsin

  7. The LHC Goals Goals for the 14 TeV p-p accelerator: • More precise measurement of known particles • Produce and find the Higgs boson • Reach energy scales of SUSY • Gain insight about the beginning of the universe • New physics (unpredicted?) • Explore heavy ion collisions Kira Grogg, U. Wisconsin

  8. The Acceleration Process • Linac2, generates 50 MeV protons • Proton Synchrotron Booster (PSB) increases energy to 1.4 GeV • Proton Synchrotron (PS) increases energy to 24 GeV • Super Proton Synchrotron (SPS) increases energy up to 450 GeV Kira Grogg, U. Wisconsin Starts here

  9. Proton-Proton interaction at the LHC @start-up 1028 - 1031 cm-2s-1 Luminosity L = particle flux/time Interaction rate Cross section  = “effective” area of interacting particles Kira Grogg, U. Wisconsin

  10. Compact Muon Solenoid (CMS) Weight: 12,500 T Diameter: 15.0 m Length: 21.5 m Kira Grogg, U. Wisconsin

  11. Detector Parts Tracker ECAL Solenoid HCAL Muon Chambers Kira Grogg, U. Wisconsin

  12. Tracker • Measures the path and pT of charged objects • Will help identify the electron from the W decay, measure the pT, and eliminate photons Barrel and endcaps have near interaction region Tracker coverage extends to ||<2.5, with maximum analyzing power in ||<1.6 Kira Grogg, U. Wisconsin

  13. Electron Calorimeter • Measures e/ energy within || < 3 using 76,000 lead tungstate (PbWO4) crystals • Will measure energy of the electron from W decay • Resolution: Kira Grogg, U. Wisconsin

  14. Hadron Calorimeter • Measures shower energy and location • Will measure energy of the jets that are formed along with the W boson • Barrel resolution: HF resolution: Uses steel plates/quartz fiber in forward region (HF, include || < 5) Uses brass/scintillator layers in barrel (||<3) Kira Grogg, U. Wisconsin

  15. Muon System • Measures energy and location of muons • not used for W+jets events with We Kira Grogg, U. Wisconsin

  16. Particles in the CMS Detector ppW+jetse+jets Kira Grogg, U. Wisconsin

  17. Selects “good” events Trigger Kira Grogg, U. Wisconsin

  18. Searching for W+jets at the LHC • Look for one electron in barrel, missing energy, and several jets • Cross section of ~10 nb • Detectable soon after LHC start up • Trigger on single electron Kira Grogg, U. Wisconsin

  19. W+jets characteristics • Electron selection: • ET > 20 GeV for an EM cluster, • |cluster| < 1.4 for barrel electrons • 1.6 < |cluster|< 2.4 for endcap electrons • Small energy deposit in HCAL • EHad/EEm < 0.05 • Nearby track (details later) • Isolated Kira Grogg, U. Wisconsin

  20. W+jets characteristics (2) • We selection: Kira Grogg, U. Wisconsin

  21. The jets • Quarks to jets Kira Grogg, U. Wisconsin

  22. Produced Observed The jets (2) Kira Grogg, U. Wisconsin

  23. Previous W+jets studies • Tevatron info: • p -pbar collisions • s = 1.96 TeV • L = 108 pb-1 to 320 pb-1 • Backgrounds to W+jets at Tevatron: • Top • QCD • W • Ze+e- • Measurements: • Select events with electron ET > 20 GeV and || < 1.2 ; missET > 30 GeV • Require high PT (> 13 GeV) track near EM deposit • N jets, found using R =0.4 cone algorithm. ET < 15 GeV, |  | < 2.4  Selection efficiency ~20% Kira Grogg, U. Wisconsin

  24. CDF W+ n jets Results • ET v. theory Mismatch at higher ET Kira Grogg, U. Wisconsin

  25. Cross section measurements Dijet mass and ∆R jets CDF W+ n jets Results (2) Kira Grogg, U. Wisconsin

  26. Event Simulation Kira Grogg, U. Wisconsin

  27. Generation of SignalW+jets Kira Grogg, U. Wisconsin

  28. Signal Identification Cuts • Require: • One lepton, 30< pt < 120 GeV (no near jets) • Njets  0, Et > 20 GeV • Missing Et > 30 GeV • Leptonic W -- reconstruct mass using lepton and missET •  is the azimuthal angle between the electron and the neutrino. • 60< mT < 100 Kira Grogg, U. Wisconsin

  29. QCD top W->taunu WW Z->ee Backgrounds Kira Grogg, U. Wisconsin

  30. Electron PT Pt of Electron Kira Grogg, U. Wisconsin

  31. Electron PT for n = 1 & 2 Kira Grogg, U. Wisconsin

  32. Separating jets and electrons Not a jet if r < 1.5 and e Pt / jet Pt > .85 Kira Grogg, U. Wisconsin

  33. Pt of 1st and 2nd Jet Kira Grogg, U. Wisconsin

  34. Pt of 3rd and 4th Jet Kira Grogg, U. Wisconsin

  35. Missing Et Kira Grogg, U. Wisconsin

  36. Transverse W mass mT of W calculated based on electron and missing ET information Seems too high… Kira Grogg, U. Wisconsin

  37. Future activities/next steps Kira Grogg, U. Wisconsin

  38. Backup slides Kira Grogg, U. Wisconsin

  39. Explanation of W+jet uses • Luminosity Monitor • Lots of W+jets events, even at low L • Clean signal because mW > QCD scale • Use cross section in ratio with other cross sections • / ~ 1% • Theoretical cross section and Structure functions • Provide better measurement of structure functions • better understanding of QCD Kira Grogg, U. Wisconsin

  40. The Higgs Particle • The Higgs particle is the missing piece of the Standard Model • Is needed to explain the mass of elementary particles • Cannot simply add a mass term to the weak lagrangian, but W and Z must be massive • Introducing a scalar field (Higgs) with hidden symmetry breaking allows a mass term to appear • The coupling strength of a particle to the Higgs determines its mass • The mass of the Higgs itself cannot be determined theoretically: • The scale factor  is unknown, v = 246 GeV (measured) • The production and decays are known, so that with sufficient mH the Higgs can be found from qqWW →qqH→qqWW→qqejj • W+ jets are a major background to this signal  Kira Grogg, U. Wisconsin

  41. Electron Selection • Look at energy in clusters of EM calorimeter towers • Number of events with generated electrons: • Require minimum energy (__% efficiency) • Require no nearby jets of significant energy (__% efficiency) • Require a nearby track of sufficient energy (__% efficiency) • “Nearby” is defined by a cone: • Phi is around detector, eta is a function of angle form the beam path Kira Grogg, U. Wisconsin

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