P recision measurements of W/Z asymmetry at DØ

1. Precision measurements of W/Z asymmetry at DØ 尹航 Fermi National Accelerator Laboratory USTC Seminar August 30th, 2012

2. Outline • Introduction • W charge asymmetry (W e) • Z Forward-Backward charge asymmetry (AFB) • Conclusions Hang Yin, USTC seminar

3. Tevatron collider Chicago Booster CDF DØ pbarsource Tevatron Main injector & Recycler Hang Yin, USTC seminar

4. The DØ Collaboration 429 physicists from 18nations Institutions: 82 total ~120 students and postdocs Physics: B, EW, QCD, TOP, Higgs September 2011 Collaboration Photo Hang Yin, USTC seminar

5. The DØ detector Uranium Liquid Argon calorimeters Central (CC) and Endcap (EC) Coverage: || < 4.2 Drift chambers and scintillator counters 1.8 T toroids Coverage: || < 2.0 Silicon Microstrip Tracker (SMT) Central Fiber Tracker (CFT) 2 T magnetic field Coverage: || < 3.0 Hang Yin, USTC seminar

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7. W and Z physics • W and Z boson production at Tevatron • Z (ee, ) events are often used for detector calibration • W/Z are backgrounds for many measurements and searches • Make precision measurements of electroweak parameters (sin2W) • Test high-order QED and QCD corrections (AFB) • Constrain parton distribution functions (PDFs) (W charge asymmetry) • Search for physics beyond the SM (AFB for high mass region) l = e, ,  Hang Yin, USTC seminar

8. W and Z events • Z boson: Two high pT charged leptons • Both charged leptons are detected and their momentum measured • W boson: One high pT charged lepton, one high pT neutrino • Charged lepton is detected and momentum measured, Neutrino cannot be detected • pT() is inferred by the “missing ET (MET)” in the detector Hang Yin, USTC seminar

9. Electron Electron Track MET Electron W and Z events in detector W Z Hang Yin, USTC seminar

10. W charge asymmetry Hang Yin, USTC seminar

11. u u d proton W charge asymmetry gluon Proton - antiproton collisions u d Hang Yin, USTC seminar

12. u u u u d d Anti-proton proton Tevatron W productions - - - Hang Yin, USTC seminar

13. W and lepton asymmetry • W charge asymmetry is sensitive to Patron distribution functions (PDFs) • Tevatron is pp collider, u quark intend to carry higher momentum than d quark • Lepton asymmetry is comes from a convolution of W asymmetry and the W V-A decay: A(y)  (V-A) - PTYHIA with CTEQ6.6M Hang Yin, USTC seminar

14. CDF/D0 central jets D0 We e±y W √s CDF/D0 forward jets 14 X-Q2 reach • W asymmetry measurement: • This measurement: • |yW| < 3.2  0.002 < x < 1.0 • Previous measurements: • |yW| < 2.5  0.003 < x < 0.5 • Complementary to central and forward jet measurements at D0 and CDF • CTEQ and MRST groups • Well constrained PDFs are essential for many measurements and searches at hadron colliders • Expect Tevatron Run II WM <15 MeV, currently 11 MeV due to PDFs MW x = momentum fraction of parton Q2 = square of momentum transfer Q2 ≈ MW2, x = Hang Yin, USTC seminar

15. W Asymmetry predictions VRAP generator @ NLO Hang Yin, USTC seminar

16. Solutions of neutrino pZ • W boson asymmetry: • W rapidity: for a massive particle rapidity • To get the solutions for neutrino pZ • Fix the W mass at the world average one: 80.385GeV Hang Yin, USTC seminar

17. Two solutions • With given W mass • Two imaginary solutions: MET is not measured ‘properly’, we will scale the MET to make the imaginary part to be 0 • Two solutions: give each solution a weight factor, according to the W generation properties (W pT, collins) and differential cross section (dσ/dy) Hang Yin, USTC seminar

18. Collins angle MC@NLO with CTEQ6.6M, 90 M Hang Yin, USTC seminar

19. Sea-quark/valence quark ratio MC@NLO with CTEQ6.6M, 90 M Hang Yin, USTC seminar

20. Fitting of W pT vs. rapidity • Fitting function Hang Yin, USTC seminar

21. Rapidity predictions • VRAP + NLO + PDF sets Hang Yin, USTC seminar

22. Closure test of method MC@NLO with CTEQ6.6M, 10 M as faked data All events Lepton pT > 25 GeV Good agreement between predictions and closure test results. Hang Yin, USTC seminar

23. Input bias remove • The NNLO differential cross sections are took as inputs, which may introduce the bias • A iteration method will be performed to remove this bias • Take the measured cross section as inputs … Hang Yin, USTC seminar

24. Results … • Cannot show you final results with full data-set, still working on this Rapidity region extent to 3.2 With 10 fb-1 data Test CT10 or even latest PDFs ET>35 GeV Hang Yin, USTC seminar

25. Forward backward charge asymmetry (AFB) of Z Phys. Rev.D 84, 012007 (2011) Phys. Rev. Lett. 101, 191801 (2008) Hang Yin, USTC seminar

26. Drell-Yan • Coupling of a virtual photon to fermions: vector coupling • Couplings of a Z-boson to fermions: vector and axial-vector couplings • Drell-Yan Procedure: annihilation of and the production of di-lepton or pair via a Z-boson or virtual photon Hang Yin, USTC seminar

27. AFB • The presence of both vector and axial vector couplings gives rise • to non-zero AFB. Forward Backward • The differential cross section: • Forward-backward asymmetry : Hang Yin, USTC seminar

28. Asymmetry prediction _ uu  e+e- Z/* interference Pure Z exchange Primarily pure * exchange Hang Yin, USTC seminar

29. Tevatron prediction PTYHIA prediction Hang Yin, USTC seminar

30. Weak mixing angle • AFBis sensitive to sin2ϴW • SMparameters: α, Gf, MZ, MWand sin2ϴW Also depends on quark couplings Only depends on lepton couplings Hang Yin, USTC seminar

31. Standard model parameters (sin2Weff includes higher order corrections) Dzero 1.1 fb-1 publication. Phys. Rev. Lett. 101, 191801 (2008) And CDF 72 pb-1 PRD publication LEP and SLD most precise results are off by 3 in opposite direction Hang Yin, USTC seminar sin2W(N)‏ 0.2277  0.0016

32. Z quark couplings LEP/SLD: Phys. Rep. 427, 5 (2006) Hang Yin, USTC seminar

33. New resonance search AFB Zx Rosner, PRD 54, 1078 (1996) Z 500 GeV Z’ boson Mee (GeV) • New resonance (Z’, LED etc) can interfere with Z and * • AFB measurement is complementary to bump search Hang Yin, USTC seminar

34. Event selection • RunII, L = 5.0 fb-1, with all diem triggers • Primary vertex |Z|< 40 cm • Electron selections: • Transverse momentum > 25 GeV, isolated and shower shape cuts • in calorimeter fiducial region • Only look at CCCC and CCEC events • For CCCC events, require two opposite track matches • For CCEC events, require CC electron must has a track match(use the charge of CC cluster to determine forward/backward event) • Track matching requirements: Hang Yin, USTC seminar

35. MC tuning • PythiaGeant MC samples: • Z/γ*->eesamples : 40-60 GeV, 60-130 GeV, 130-250 GeV, 250-500 GeV and > 500 GeV • At generate level: • applying QCD NNLO corrections on Z-pT rapidity and Z Mass reweighting • At reconstructed level: • EM scale and smearing • Electron selection efficiencies corrections • Trigger efficiencies • VtxZ, Phi-mod and luminosity correction • F/B efficiencies difference vs. InvMass Before sample reweight After sample reweight Hang Yin, USTC seminar

36. Background modeling • EW: Z ττ, W+X, WW, WZ, γγ, ttbar • QCD background: jets are misidentified as electrons, estimated from data • Shape(Mee) of QCD background measured from real data: inverting electron shower shape requirement in CC and in EC • Fraction of QCD background: using minimum χ2 fitting Hang Yin, USTC seminar

37. Data and MC comparison Hang Yin, USTC seminar

38. Extraction of weak mixing angle • Pythia templates generated with 40sin2ϴefflept inputs: • From 0.22552 to 0.23722 with step size 0.0003 • Using minimum χ2 fitting method • Measured value： • sin2ϴefflept= 0.2304±0.0008(Stat.)±0.0006(Syst.) Hang Yin, USTC seminar

39. Corrections and uncertainties • Systematic uncertainties: • Dominant systematic is from PDF CTEQ6.1M • High order correction • Measured from ZGrad2 • Correction: +0.0005 • Final results: Hang Yin, USTC seminar

40. Weak mixing angle comparisons Hang Yin, USTC seminar

41. Unfolding: events migration • Due to the finite detector resolution, events migrate into other mass bins • At generate level: and • At selected level: and Hang Yin, USTC seminar

42. Unfolding: acceptance*eff • Acc*eff for CCCC and CCEC events • To remove geometric and kinematic cuts effects Hang Yin, USTC seminar

43. Unfolding: charge mis-ID • fQ: Charge mis-identification rate • For CCCC events, we have If fQ = 50%, cannot measure charge asymmetry • For CCEC events, we have • Measured fQ vs. Mee in analysis Hang Yin, USTC seminar

44. Unfolded AFB Hang Yin, USTC seminar

45. Z-quark couplings • Use ResBosgenerate pure γ*, pure Z boson, and Z/γ* interference: 20M*44 (PDFs) events for each sample • Performed both 4-D, and 2-D fit of Z to light quark couplings • Fix weak mixing angle into world average one (0.23153) - ddee Hang Yin, USTC seminar

46. 4-D fits Z-u quark couplings Z-d quark couplings Hang Yin, USTC seminar

47. Compared with LEP, H1 Z-u quark couplings Z-d quark couplings Hang Yin, USTC seminar

48. Conclusion • DØ collected ~10 fb-1ppbar data • W charge asymmetry • Measure both W asymmetry and lepton asymmetry • Useful for future global PDFs fit • Z charge asymmetry • Legacy measurement of weak mixing angle • Z to light quark couplings • Full dataset analyses are on going: with better understanding of the detector, more high precision electroweak measurement expected!! Hang Yin, USTC seminar

49. Ongoing/future topics • Z Rapidity measurement • Z angular coefficient • Z pT measurement • W pT measurement • Cross section of W and Z • …… Hang Yin, USTC seminar

50. Backup Hang Yin, USTC seminar