W + jets events with CMS at the LHC

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. 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

4. W+jets production • Feynman Diagrams of W + jet production at the parton level • Higher orders: Kira Grogg, U. Wisconsin

5. Selects “interesting” events Trigger Kira Grogg, U. Wisconsin

6. W+jets characteristics • Cross section of ~10 nb • Detectable soon after LHC start up • 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

7. The jets • Look in a cone of = 0.5 Produced Kira Grogg, U. Wisconsin

8. 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

9. CDF W + n jets Results • ET v. theory Mismatch at higher ET LHC will have higher statistics and larger range ??? Kira Grogg, U. Wisconsin

10. Cross section measurements by jet number CDF W + n jets Results (2) Kira Grogg, U. Wisconsin

11. Electron Missing ET CDF W + n jets (3) Kira Grogg, U. Wisconsin

12. Dijet mass and ∆R jets CDF W + n jets Results (4) Kira Grogg, U. Wisconsin

13. LHC has much more extensive kinematic realm y is rapidity (close to ) Kinematics Kira Grogg, U. Wisconsin

14. Event Simulation Kira Grogg, U. Wisconsin

15. CMSSW_1_4_5 to generate particles and simulate the detector Generated with pythia and alpgen CMSSW_1_6_0 to create digis and reconstruct particles Simulation Kira Grogg, U. Wisconsin

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

17. Signal Identification Cuts • Require: • One lepton, 15< pT < 120 GeV • Isolated: rjet-elec > 0.15 & elec pt / jet pt > 0.85 • Njets  0, Et > 20 GeV • Missing Et > 30 GeV • Leptonic W -- reconstruct mass using lepton and missET • should find 60< mT < 100 • Because: • Electron PT should on average be half the W mass • Jets should have higher energy than those from QCD radiation • Missing Et should be roughly half the W mass • The reconstructed mWT from the decay products should help identify events with a W Kira Grogg, U. Wisconsin

18. QCD produces many jets, some of which can be misidentified as an electron top Decays into W+b W ??? tau decays to electron or jets? WW (W e) (W jj) Zee Misidentify one of the electrons as a jet Backgrounds Kira Grogg, U. Wisconsin

19. Electron PT Kira Grogg, U. Wisconsin

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

21. Isolated Non-isolated L1 Em Objects Automatically trigger on electrons pt > 63 GeV Kira Grogg, U. Wisconsin

22. Jets identified as electrons are removed from jet list Electrons chosen are those within r = .15 of nearest jet & with pT between 85% and 115% of jet pT Isolating Electrons Kira Grogg, U. Wisconsin

23. PT of highest pT Jet Some jets (such as all those in W+0 jets event) are underlying QCD radiation Kira Grogg, U. Wisconsin

24. Pt of 2nd and 3rd Jet Kira Grogg, U. Wisconsin

25. Missing Et Missing ET is roughly 1/2 mW Kira Grogg, U. Wisconsin

26. Transverse W mass mT of W calculated based on electron and missing ET information cuts: elec pT >15 GeV miss Et > 30 GeV Kira Grogg, U. Wisconsin

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

28. Backup slides Kira Grogg, U. Wisconsin

29. 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 • Don’t know much about PDFs at LHC scale - need to be sure they are understood before assuming a new discovery has been made Kira Grogg, U. Wisconsin

30. 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

31. 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

32. Fragmentation Kira Grogg, U. Wisconsin