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Asymmetries in W± and Z0/g* Production at the TeVatron

This paper discusses various asymmetries in the production of W± and Z0 bosons at the TeVatron, including the W charge asymmetry, lepton charge asymmetry, and the AFB asymmetry in Z0/g* decays. It also explores the impact on quark PDF constraints and tests QCD predictions.

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Asymmetries in W± and Z0/g* Production at the TeVatron

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  1. Asymmetries in W ± and Z0/ g* Production at the TeVatron ICHEP ‘04 August 17, 2004 Eva Halkiadakis University of Rochester For the CDF and D0 Collaborations

  2. W Charge Asymmetry EWK “Spectroscopy” • Lots of interesting physics in production of W ± and Z0 bosons at the Tevatron! • Rapidity Spectrum: ds/dy • quark PDF constraints • Direct impact on MW • Polar Angle Spectrum: ds/dcosq • In Z’s vs. Mee • AFB, sin2qW • quark, lepton couplings • In W’s vs. pT • Tests QCD predictions • Boson pT Spectrum: ds/dpT • Tests QCD predictions • Direct impact on MW Asymmetries in W and Z/Drell-Yan Production at the Tevatron E. Halkiadakis

  3. u d – d – u W ± Charge Asymmetry xf(x,Q2) log(x) u quark carries higher fraction of p momentum! [http://durpdg.dur.ac.uk/hepdata/pdf3.html] Measurement of the W charge asymmetry constrains PDF’s of the proton. yW ←anti-proton direction proton direction→ Asymmetries in W and Z/Drell-Yan Production at the Tevatron E. Halkiadakis

  4. Lepton Charge Asymmetry CDF Run I: PRL 81, 5754, (1998) • Leptonic W decay involves n → pzn is unmeasured. • Use experimentally more direct l ± direction to measure A(hl). • This convolves W production asymmetry with V-A decay distribution. • Sensitivity to the ratio of PDFs for u and d quarks, u(x)/d(x). Least constrained at high h! Asymmetries in W and Z/Drell-Yan Production at the Tevatron E. Halkiadakis

  5. Event Selection hl Out to ~2.8! • W ± →e ±n Candidates [ET, MET > 25GeV, 50 GeV < MT < 100 GeV] • Central: 49214 events • Forward: 28806 events • Charge ID in forward region is key! • Use new silicon tracker and forward calorimeters. • Align forward calorimeters with tracks from central tracker. • Global offsets, rotations. • Internal misalignments. Residuals after alignment look very good. Global Df (mRad) Asymmetries in W and Z/Drell-Yan Production at the Tevatron E. Halkiadakis

  6. Raw Asymmetry Shape is convolution ofA(yW) and V-A Sign switch @ |h| > 2 • Corrections to extract true asymmetry: • Charge misidentification rate. • Background subtraction. • Both bias the asymmetry low → dilution. • Measured in each h bin. • Uncertainties in corrections go directly in A. |h|< 1 ~ linear Curve is just to guide the eye. Asymmetries in W and Z/Drell-Yan Production at the Tevatron E. Halkiadakis

  7. Corrected W Charge Asymmetry Data lower than existing CTEQ prediction. • Gain sensitivity to W production asymmetry A(yW) with ET dependence. • Higher ET: electron direction closer to W direction. Production asymmetry enhanced. • Lower ET: decay asymmetry enhanced Asymmetries in W and Z/Drell-Yan Production at the Tevatron E. Halkiadakis

  8. f ’ q f f ’ f f f ’ AFB f f ’ AFB in Z0 / g* → e+e- Decays Z0 / g* f = e, quarks • Vector (V) and Axial-vector (A) couplings give rise to AFB. • Interference between Z0 and g* exchanges. • Different combinations of V and A couplings contribute to ds/dcosq dMee. • AFB direct probe of relative strengths of coupling between Z0 and quarks. • Mass dependence is sensitive to u and d quarks separately. Asymmetries in W and Z/Drell-Yan Production at the Tevatron E. Halkiadakis

  9. AFB Beyond LEPII • Interesting for Mee above LEP II energies. • New interactions → deviations of AFB and ds/dM from SM predictions. • Various models predict new neutral, heavy bosons: Z´s • New resonance could interfere with g and Z. • Complementary to direct searches → excess in total cross section. CDF Run I (~110 pb-1) 2.2 s deviation from SM Rosner, J.L.: Phys. Rev. D 54, 1078 (1996) AFB 500 GeV/c2 Z’ Zx Z Mee(GeV/c2) Asymmetries in W and Z/Drell-Yan Production at the Tevatron E. Halkiadakis

  10. Calculating AFB • 5211 Candidate Z0→ e+e- events • 2 isolated electrons ET>20 GeV • 1892 Central-Central • 3319 Central-Forward • Backgrounds: • Central-Central: 1% • Central-Forward: 5% • Dijet background dominant Low mass CDF II Preliminary Pole Backward Forward High mass AFB is largest Asymmetries in W and Z/Drell-Yan Production at the Tevatron E. Halkiadakis

  11. Acceptance Corrections Low mass High mass ok! Pole Large correlations at pole 600 GeV/c2 • Unconstrained unfolding analysis • Acceptance and event migration parameterized to transform AFBphys to AFBraw. • Use maximum log likelihood method to compare to data at the detector level. High mass 40GeV/c2 600GeV/c2 Kin. and geom. cuts sculpt the cosq* distrib., esp. in forward region. Detector resolution and radiative effects → event migration. Asymmetries in W and Z/Drell-Yan Production at the Tevatron E. Halkiadakis

  12. Unconstrained Fit with Smoothing • Result fitting AFB • Large statistical uncertainties • No SM assumptions about AFB! • Systematic uncertainties small compared to statistical: • Energy Scale, Resolution • Detector Material • Backgrounds CDF II Preliminary CDF II Preliminary • All results consistent with SM. • Not useful for non-SM physics near Z pole. • Nothing new above the pole yet. Asymmetries in W and Z/Drell-Yan Production at the Tevatron E. Halkiadakis

  13. Couplings Results Quark Couplings: Electron Couplings: Asymmetries in W and Z/Drell-Yan Production at the Tevatron E. Halkiadakis

  14. Summary Future • ds/dy measurements in Z’s will further constrain PDF uncertainties. • ds/dpT in both W’s and Z’s will further test QCD predictions. • CDF has a new measurement of the lepton charge asymmetry in W→en decays. • Look at data at large ET and large h. • Uncertainty on PDFs could be reduced by inclusion of this data in global fits. • First Run II measurement of W charge asymmetry! • We also measure AFB vs. Mee in Z→ee events. • Unfolded AFB with out SM assumptions. • Fit for Z couplings. • Nothing new above the Z pole yet. Important for MW! Asymmetries in W and Z/Drell-Yan Production at the Tevatron E. Halkiadakis

  15. Backup Asymmetries in W and Z/Drell-Yan Production at the Tevatron E. Halkiadakis

  16. Phoenix (Calorimeter-Seeded) Tracking Two points and a curvature define a helix: • Primary collision vertex position. • Fitted position of calorimeter shower maximum. Use both central and forward electrons! |h| < 2.8 Zoom into Silicon: Phoenix SiTrack Cal. Seed Tracks Asymmetries in W and Z/Drell-Yan Production at the Tevatron E. Halkiadakis

  17. Charge Misidentification Rate • Charge mis-ID rate, fQ: • Residual misalignments in silicon and plug shower maximum detectors. • Measured in data using Z0→ e+e- events, with one leg in the central tracker (COT). • Measured in each h bin. • Uncertainties in fQ directly go in A. • Monte Carlo predicts fQ<1% even at h=2 → naïve. Asymmetries in W and Z/Drell-Yan Production at the Tevatron E. Halkiadakis

  18. Background Corrections • We correct A for backgrounds: • W →t n → e nn , Z → e e • Measured from Monte Carlo • Asymmetric • QCD Jets • Measured from data • Use Calorimeter Isolation (e) and MET distributions (n) projected into signal region. • Isolation correlated with other selection criteria. • Estimate is upper limit. • Biases A toward 0. QCD background is upper limit → use 0.5% ± 0.25% → gives full coverage. Asymmetries in W and Z/Drell-Yan Production at the Tevatron E. Halkiadakis

  19. Past Results from e+e- • Previous experiments have done very precise measurements (LEP, SLC, etc.) • Need > 10 fb-1 to compete on sin2(W) • Difficult to compete with LEPII • 120<s<207 GeV Asymmetries in W and Z/Drell-Yan Production at the Tevatron E. Halkiadakis

  20. Calculating AFB • cos* in Collins-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* Correct AFBraw to obtain AFBphys →compare to theory: Sculpt cosq* dist. Mee bin migration Asymmetries in W and Z/Drell-Yan Production at the Tevatron E. Halkiadakis

  21. Mee and h for Z→ee Asymmetries in W and Z/Drell-Yan Production at the Tevatron E. Halkiadakis

  22. Backgrounds for Z/g* • Dijet dominant Background Summary CDF II Preliminary CDF II Preliminary Asymmetries in W and Z/Drell-Yan Production at the Tevatron E. Halkiadakis

  23. Fitting for Z Couplings • Fit at Raw AFB Level • Test AFB is smeared • c2 show good agreement with SM c2=10.66/11 Asymmetries in W and Z/Drell-Yan Production at the Tevatron E. Halkiadakis

  24. 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 Asymmetries in W and Z/Drell-Yan Production at the Tevatron E. Halkiadakis

  25. Theory Band • Different MC’s: • VPB • g resummation • ZGRad • O() EW corr. • Pythia • Parton shower (QED+QCD) • Nothing w/ NLO QCD & O() EW • Indicates size of effects • PDF’s, ISR… Asymmetries in W and Z/Drell-Yan Production at the Tevatron E. Halkiadakis

  26. SM with Measured Z0 couplings Using couplings fit as input AFB gives: • All results consistent with SM. • Not useful for non-SM physics near Z pole • Nothing new above the pole yet. Asymmetries in W and Z/Drell-Yan Production at the Tevatron E. Halkiadakis

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