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The Measurement of A FB in Electron-Positron Pairs

The Measurement of A FB in Electron-Positron Pairs. Gregory Veramendi (Illinois) for the CDF Collaboration. Outline  A Brief History of A FB  Signal and Background  Acceptance  Results. f ’. f ’. f. f. f. f. f ’. f ’. f ’. q. f. f. f ’. A FB.

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The Measurement of A FB in Electron-Positron Pairs

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  1. The Measurement of AFB in Electron-Positron Pairs Gregory Veramendi (Illinois) for the CDF Collaboration Outline A Brief History of AFB Signal and Background Acceptance Results

  2. f ’ f ’ f f f f f ’ f ’ f ’ q f f f ’ AFB • Interference between Z and g* exchanges • q-dependent cross sections ds / dcosq = A(1+cos2q) + Bcosq • AFB = 3B / 8A • Directly probing the couplings • Reduces systematics • Luminosity, efficiencies and acceptance g* Z + f = e, quarks

  3. AFB at e+e- Colliders • CESR • PEP • PETRA • TRISTAN • SLC • measure s L,R using polarized e beams • LEP • Tevatron needs >10 fb-1 to compete

  4. CDF Run I (~110 pb-1) e- q q e+ AFB 2.2s dev. from SM q AFB at Hadron Colliders

  5. AFB beyond LEPII: Z´,etc... 500 GeV/c2 Z’: • Looking for symmetries beyond the SM • Various models predict new neutral, heavy bosons: Z´s • New resonance could interfere with g and Z. • Complementary to cross-section search (see Muge’s talk) Rosner, J.L.: Phys. Rev. D 54, 1078 (1996) AFB Zx Z Mee(GeV/c2)

  6. Z/g* Candidates and CDF • Selection (5211 candidates) • 2 high PT isolated electrons • ET > 20 GeV CDF II Preliminary CDF detector CDF II Preliminary

  7. Backgrounds for Z/g* • Dijet dominant Background Summary CDF II Preliminary CDF II Preliminary

  8. Data-MC Comparisons CDF II Preliminary

  9. Data-MC: Cos(*) CDF II Preliminary

  10. Calculating AFB • Calculating AFB: • cos* in Collin-Soper frame • Minimize ambiguity in the incoming quark Pt • cos*>0 Forward • cos*<0 Backward lab frame Z0/g* PZ =0 Z-Axis Z0/g*

  11. Acceptance:Event Loss • Generator Level: • Acceptance is symmetric, but distribution is not • Grey: • |1 | <1,|1 | <3, • ET > 20 GeV • Tends to shift AFB in a bin Raw Distr. Acceptance Low Mass Pole Region High Mass

  12. Acceptance:Event Migration • Event Migration Matrix • Large correlations between bins near Z pole • Unfolding Problem • Invert Matrix by fitting for AFB • Large variances • Correction factors • Biases result • High Mass O.K. Sum AFB in Rows: 600 GeV/c2 CDF II Preliminary 40GeV/c2 600GeV/c2

  13. Fit with smoothing • Result fitting AFB • Large uncertainties • Tikhonov regularization • Only in bins with correlations CDF II Preliminary CDF II Preliminary

  14. Fitting for Z Couplings • Fit at Raw AFB Level • Test AFB is smeared • c2 show good agreement with SM c2=10.66/11

  15. Couplings Results Quark Couplings Electron Couplings:

  16. Acceptance*Efficiency • Correction Factors: • Energy Resolution • Kinematic and Fiducial cuts • Radiation from FSR and Brems • Electron ID efficiency • Assumes SM • Allow Z couplings to float CDF II Preliminary Couplings fit used as input AFB

  17. SM w/ measured Z couplings • Result with correction factors • Not useful for non-SM physics near Z-pole CDF II Preliminary CDF II Preliminary

  18. Conclusions • Unfolding Problem near Z pole • Fit for couplings using uncorrected AFB • Good agreement with current world ave. • Contribute to Z-quark couplings in future? • High mass not affected by smearing • Nothing new above pole, yet… • PRD on its way…

  19. Backup Slides

  20. Understanding the detector • Radiation Effects: Charge ID • Get Mee dependence from MC • two effects: “tridents”, (pT) • Trident: • Energy Scale and Resolution CDF II Preliminary CDF II Preliminary MisID rate (trident):0.7%

  21. Cross-Check: SS vs OS • SS vs. OS • CC : Find 36 SS events in our sample • Have to worry about Material  Tridents • Subtracting peak from MC • 13 ± 6(stat) ± 4(sys) events • Checks both radiation in Simulation and background estimation CDF II Preliminary CDF II Preliminary

  22. Dijet background • From Data • Mee from jet sample • AFB from background sample • AFB ~ 0 CDF II Preliminary CDF II Preliminary

  23. Acceptance Line: backward | Dashed: forward • Generator Level Studies: • Event Loss: • A: Fiducial Cuts • B: Kinematic Cuts • Event Migration: • C: ET Smearing • D: FSR Corrections scal(E) |1 | <1 |2 | <3 FSR ET > 20 GeV

  24. Correlations in Couplings

  25. Theory Band • Number of MC’s • VPB • g resummation • ZGRad • O() EW corr. • Pythia • Parton shower (QED+QCD) • Nothing w/ NLO QCD & O() EW

  26. Systematics • Energy Scale • Central: 0.5% • Plug: 1% • Energy Resolution • Central: +0.5 GeV • Plug: +1.5 GeV • Material • Central: ±0.015 Xo Copper • Plug: ±1/6 Xo Iron • Background • QCD: ±fake rate • Montecarlo: • Checked(negligible): • AFB(dijet) • AFB(W+X) • Fiducial acceptance • Charge mis-ID • Trigger efficiency • For SM plot: • AFB input from couplings measurement

  27. Fitting for AFB • Parameterization takes f(AFBphysics) AFBraw • Construct -log(likelihood): binomial function • Some bins poorly constrained… • Add smoothing function  • Choose l such that a’=a+0.5

  28. Add Smoothing • Using Tikhonov regularization • We assume parabolic form for • Solve for three adjacent bins • New likelihood: • Choose l such that a’=a+0.5

  29. Parameterizing Acceptance • Define 4 15x15 matrices: • Efficiency for bin i to end up in bin j • Mean # of events in bin i: • Number of raw/detector level events:

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