Measurement of Spin Correlations in ttbar events in the dilepton channel at 7 TeV

1. Measurement of Spin Correlations in ttbar events in the dilepton channel at 7 TeV Thomas Speer (Brown University), Felix Hohle (RWTH), Jeremy Andrea (IPHC, Strasbourg), Kelly Beermaert (Ghent), Heiko Geenen (RWTH), Haryo Sumowidadgo (RiverSide), Efe Yazgan(Ghent) D. Barge, C. Campagnari, D. Kovalskyi, V. Krutelyov (UCSB)W. Andrews, G. Cerati, D. Evans, F. Golf, I. MacNeill, S. Padhi, Y. Tu, F. Wuerthwein, A. Yagil, J. Yoo (UCSD)L. Bauerdick, K. Burkett, I. Fisk, Y. Gao, O. Gutsche, B. Hooberman, S. Jindariani, J. Linacre, V. Martinez Outschoorn (FNAL)

2. Analysis Notes

3. PAS and Talks Talks: https://indico.cern.ch/getFile.py/access?contribId=5&resId=0&materialId=slides&confId=187624 https://indico.cern.ch/getFile.py/access?contribId=4&resId=0&materialId=slides&confId=191553 https://indico.cern.ch/getFile.py/access?contribId=1&resId=0&materialId=slides&confId=180584 http://indico.cern.ch/getFile.py/access?contribId=8&sessionId=0&resId=0&materialId=slides&confId=180655 http://indico.cern.ch/getFile.py/access? contribId=1&resId=0&materialId=slides&confId=187624 https://indico.cern.ch/getFile.py/access?contribId=2&resId=0&materialId=slides&confId=190610 https://indico.cern.ch/getFile.py/access?contribId=3&resId=0&materialId=slides&confId=191553

4. Introduction and Motivation • The SM predicts a correlation between the spins of top and anti-top quark in top quark pair production. • Sensitive to new physics in both production and decay mechanism. • Two analyses in dilepton channel: • Measure the spin correlations using dilepton azimuthal opening angle (Deltaphi), which can be compared directly with the ATLAS result (http://arxiv.org/abs/arXiv:1203.4081v1). • Measure asymmetries related to the spin correlation • using Deltaphi. • by full reconstruction of the dilepton event kinematics. • measurements are done in both the inclusive region and the high mass region (M_ttbar > 450 GeV)

5. Event selection (inclusive) • Event cleaning: if >= 10 tracks; at least 25% purity; at least 1 good DA vertex (not isFake, ndf > 4, rho < 2 cm, z < 24 cm) • 2 opposite sign isolated leptons: pT > 20 GeV, |eta| < 2.5 (2.1 or 2.4) for e (µ) • ≥2 pf jets with pT > 30 GeV, |eta| < 2.5 • loose pfjet ID (L1FastL2L3 corrected) • ∆R > 0.4 from all leptons passing analysis selection • ≥1b tags: CSVL orCSVM (tighter requirement which suppresses DY significantly) • MET > 40 GeV (ee and mm) or > 30 GeV in all 3 channels • Z veto: 76<mll<106 GeV veto (for SF leptons) • mll>20 GeV or 12 GeV to veto low mass resonances (SF leptons)

6. Deltaphi Analysis In PAS • The azimuthal opening angle between two leptons from the decay of top quark pair (Mahlon & Parke, PRD 81, 074024). • Fit data to templates made of MC sample with and without spin correlation + template for the background

7. Dataset, MC Sample, and Selection • Use the dataset, trigger, selection, and background estimation as the TOP-11-005 dilepton cross-section / TOP-11-016 AMWT mass analysis. • MC sample with and without spin correlation are generated with MC@NLO. • Simulation is done with FastSim. • Extensive comparison and cross-checks were done between FastSim & FullSim.

8. Fast vs Full Sim The jet multiplicity is not perfectly described by he fastSim. This will be fixed by re-producing the FastSim with a patch given by the fastsim group, or we will add an additional systematics.

9. Analysis Method • Fit deltaphi in 3 channels separately using 3 templates: ttbar with & w/o spin correlation, background. • Extract the fraction on ttbar with spin correlation (SM). 9

10. Fit Procedure • Two fit parameters: • Ratio of ttbar w/ and w/o spin corrrelation • Overall ttbar normalization. • Background normalization is fixed. • Fit using RooFit • Nsig, Nbkg, PDFcorr, PDFnocorr, and PDFbkg are different for each of the 3 channels. • fSM is identical for all 3 channels.

11. Calibration and Linearity Check (i) • Using pseudoexperiment, vary the fraction of ttbar corr. & uncorr. in the pseudodata, and check the extracted fraction from the fit. • No bias is observed.

12. Calibration and Linearity Check (ii) • A pull of 0.91 is observed. The fit uncertainty thus is adjusted, taken into account the pull.

13. Fit Result • fSM is measured to be 0.74 ± 0.07 (stat only). • Spin correlation coefficient isA = ASM × fSM= 0.23 ± 0.02 • Shown is the data and fit results for the combined channel. The fit results results for separate channels are in the next page. In PAS

15. Systematic Uncertainties Considered • Lepton isolation, identification. • Lepton energy scale. • Jet energy scale and resolution. • Btagging • Background normalization. • Generator: Compare MC@NLO & POWHEG • Hadronization & Fragmentation: Compare POWHEG+Herwig & POWHEG+Pythia. • Top quark mass. • Parton Distribution Function. For the spin correlation measurement, 3000 pseudo experiments are performed, where the pseudo data are constructed using templates with variation of the systematic. Difference of the mean of the pseudo-data distribution with the nominal fit value is taken as as systematic

16. Table of Systematic Uncertainties

17. Measure Asymmetries • We measure the following asymmetry variables in the dilepton final states, inspired by this theory paper: http://arxiv.org/abs/1105.3743 by D. Krohn, T. Liu, J. Shelton, L.T. Wang • lepton azimuthal angle asymmetry (does not require top reco) • top spin correlation • Any significant differences from the SM expectation could be a signal of new physics

18. Reco Asymmetries in the Inclusive Region Statistical uncertainty only Pythia-powheg ttbar sample is used and the ttbar yields is normalized such that the total MC yields equal data yields. The reconstructed asymmetries are consistent with the SM prediction.

19. Unfolded Asymmetries in the Inclusive Region PAS PAS PAS The unfolded asymmetries are consistent with the SM prediction

20. Reco Asymmetries in High Mass Region (M_tt>450 GeV) PAS PAS PAS No significant discrepancy is observed compared to the SM prediction in the high mass region

21. Systematics PAS

22. Conclusions • We performed the analyses which measure the spin correlation and the asymmetries related to the spin correlation. • The spin correlation measured using Deltaphi in the helicity basis is found to be 0.23±0.02±X. • The asymmetries related to the spin correlation using Deltaphi or by full reconstruction of the dilepton event kinematics are measured in both the inclusive region and the high mass region (M_ttbar > 450 GeV). The results are consistent with the SM prediction. No evidence for NP is found.

23. Back up

24. Data-MC comparison • We compare data to sum of MC in the preselection region, using the 3 different ttbar samples for the ttbar->dilepton component • TTJets_TuneZ2_7TeV-madgraph-tauola:madgraph sample, no spin correlations between top and tbar • TTTo2L2Nu2B_7TeV-powheg-pythia6: powheg dilepton sample, with spin correlations • TT_TuneZ2_7TeV-mcatnlo: MC@NLO sample, with spin correlations • All other background samples (including tt->other) are kept the same • All distributions are normalized to unity (shape comparison only) • asymmetry measurements are only sensitive to the shape • K-S calculated using bins of 0.2* plotted width

25. Shape Comparison

26. Summary for the Shape Comparison • Asymmetry values and K-S vs data, for preselection region • for variables requiring top reconstruction, only events with solution are used • powheg dilepton ttbar sample seems to best represent the data for these variables

27. Reconstruction of Top kinematics • In dilepton channel, each event yields two neutrinos. • There is an ambiguity of combining b-jets and leptons from the same top quark. • It is a challenge to reconstruct top mass in the dilepton channel. • We currently use the analytical matrix weighting technique (AMWT) described in http://arxiv.org/abs/arXiv:1105.5661 • Each events is reconstructed using a series of input M_top values between 100-300 GeV with one GeV step. • In order to determine a preferred value of M_top, a weight is assigned to each solution. For each event, the M_top hypothesis with the maximum averaged weight (over solutions) is taken as the reconstructed top mass. • Events without solutions are discarded.

28. Reconstructed Top Mass About 17% events without solutions. Looks reasonably well

29. Other distributions in preselection region

30. Data-driven BG estimates: DY • Estimate for high region: 47.6 ± 10.6 events (stat. uncertainty only) • consistent with MC predictions of 39.9 ± 4.8 events • Estimate for inclusive region: 142.4 ± 15.0 events (stat. uncertainty only) • consistent with MC prediction 126.7 ± 8.7 events

31. Data-driven BG estimates: Fakes • Estimate for high mass region: 45.9 +108.5-45.9 events (stat. uncertainty only) • consistent with MC prediction of 47.1 ± 5.4 events • Estimate for inclusive region: 138 + 281-138 events (stat. uncertainty only) • consistent with MC prediction 100.1 ± 6.7 events