Measurement of the Forward-Backward Charge Asymmetry in Top-Quark Pair Production at DØ

1. - SUSY Conference, Seoul 2008 - Measurement of the Forward-Backward Charge Asymmetry in Top-Quark Pair Production at DØ

2. Overview µ+ νµ b W+ t - t - b W- q q’ • Introduction • Analysis • Event Selection • Sample Composition and Asymmetry • Acceptance and Reconstruction Effects • Sensitivity to New Physics • Summary

3. Introduction - - p p p C p • Top-quark pair production at the Tevatron: • Initial pp state not a charge conjugation C eigenstate: • Final state not expected to be symmetric under C - 85% qq annihilation 15% gluon fusion -

4. Forward-Backward Asymmetry • Not enough statistics for differential measurement => measure total forward-backward asymmetry Afb: CP symmetry, not at LHC Charge asymmetry Forward-backward asymmetry Nf = # “forward” events (cos >0 ~> y>0) Nb = # “backward” events (cos <0 ~> y<0) (LO; invariant to boosts)

5. Theoretical Predictions • At LO top-quark pair production is charge symmetric • At NLO asymmetry arises from interferences between diagrams • Total asymmetry of (4-5)% (NLO) expected [Bowen et al.; Kühn et al.] • Different for 2->2 and 2->3 processes: • Interesting to measure separately Negative contribution to Afb Positive contribution to Afb “≥5 jets” “4 jets” -(7-8)% (NLO) (-3-0)% (NNLO)[Dittmaier et al.] 6.4% (NLO)

6. Analysis Strategy • Select sample enriched in top pair events • Reconstruct full event using kinematic fitter • Fit for sample composition and asymmetry simultaneously • Acceptance effects:no correction: simple description of accepted phase space • Reconstruction effects:no correction: dilution given to be used with any model

7. Final State and Selection µ+ νµ b W+ t - t - b W- q q’ Primary Vertex of good quality Isolated lepton with track match, electron pT>15 GeV, |h|<1.1 or muon pT>18 GeV, |h|<2 • Decay channels classified by • W decay: • All jets • Lepton + Jets • Dilepton Missing transverse energy MET>15 GeV ≥1 b-tags ≥4 jets in |h|<2.5, pT>20 GeV, Leading jet pT>35 GeV

8. Kinematic Fitter µ+ νµ b W+ t - t - b W- q q’ • Varies 4-momenta of objects within their resolutions • Minimizes 2 statistic with: { constrain to W mass 80.4 GeV Lepton charge determines t or t - constrain both to top quark mass 170 GeV use b-tag information to reduce jet combinatorics { constrain to W mass 80.4 GeV • Gives twice the sensitivity than lepton charged-signed direction

9. Background Estimation • Largest backgrounds: W+jets and multijet production • W+jets: Define discriminant L based on variables, which • well-modeled • good separation between signal and W+jets • do not bias |y| • Asymmetry of W+jets is suppressed by kinematic fit, estimated by simulation (ALPGEN+PYTHIA) • Size and asymmetry of multijet background from data (sample with non-isolated lepton) • Other background sources in systematic uncertainties • Maximum likelihood fit of L distributions;simultaneously for sample composition and sign of y

10. Asymmetry • Fitted distribution for ≥4 jets in 0.9 fb-1 data and simulation: yreco > 0 yreco < 0 L L +15 - 17

11. Acceptance Effects • Integrated asymmetry strongly depends on region of phase space probed • Strongest effect: requiring ≥4 jets above pT threshold => generated asymmetry as function of 4th highest particle-jet pT MC@NLO • Exact structure in SM unknown => no correction of acceptance • Analysis designed so that selection can be described by simple particle-level cuts (change asymmetry <2%)

12. Reconstruction Effects • Misreconstruction of sign of y dilutes the observed asymmetry • Misidentification of lepton charge (negligible) • Misreconstructed event geometry • Probability p for correct sign of y=> fraction of asymmetry visible (“dilution factor”) • Correction for reconstruction effects highly model-dependent => provide parameterization of dilution instead: - tt PYTHIA

13. Predicted Asymmetry • To compare predicted asymmetry from MC@NLO with observed • MC@NLO generated Afb has to be folded with acceptance and reconstruction effects: probability density within geometrical acceptance dilution particle-level Afb (MC@NLO here) • Any model can be compared to observed asymmetry • Dilution is provided for ≥4, =4, ≥5 jets • Particle level cuts: jet pT>21 GeV, ||<2.5, leading jet pT>35 GeV, lepton requirements

14. Sensitivity to New Physics • Resonance production of tt can change asymmetry • Axigluons predicted to give negative asymmetries • Z' production of tt can give large positive asymmetries • Sensitive to narrow and wide resonances • Complementary to direct Z' search (P6 [168] by T. Schliephake) • Leptophobic Z' with Z-like coupling • Predict distribution of Afb as a function of the fraction f of tt events produced via Z' - - -

15. Limits on Z' • Ensembles of simulated data sets • Upper limits on fraction f of tt events produced via Z' at 95% confidence (Feldman/Cousins) for narrow resonances: - Observed limits Expected limits Band of ±1 sd expected limits Band of ±2 sd expected limits

16. Summary • First measurement of integrated forward-backward charge asymmetry in top pair production • Measured asymmetries consistent with MC@NLO predictions • Expected behavior for events with =4 jet and ≥5 jet observed • Parameterization of acceptance and dilution allows comparison to any model • Limits on tt production via Z' • Published in Phys. Rev. Lett. 100 142002 (2008) arXiv:0712.0851 -

17. BACKUP SLIDES

18. Tevatron at Fermilab Chicago p CDF DØ _ Tevatron p Main Injector • pp collisions at center of mass energy √s = 1.96 TeV - 4.23 fb-1 ~900 pb-1 analyzed 3.66 fb-1

19. DØ Detector • Multi-Purpose Detector • Acceptance: • Electrons |h| < 3.0 • Muons |h| < 2.0 • Jets |h| < 4.2 • Tracking System • Tracks • Vertices • Muon System • Muons • Calorimeter • Electrons • Photons • Jets • Missing ET