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James Howarth on behalf of the ATLAS Collaboration The University of Manchester

Spin Correlations, Polarisation and Charge Asymmetries in Top Quark Pairs. James Howarth on behalf of the ATLAS Collaboration The University of Manchester. Top Quark Properties. l +. Decay W Polarisation , Colour Flow, Branching Ratios, | V tb |. ν. The Top Quark

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James Howarth on behalf of the ATLAS Collaboration The University of Manchester

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  1. Spin Correlations, Polarisation and Charge Asymmetries in Top Quark Pairs James Howarthon behalf of the ATLAS Collaboration The University of Manchester

  2. Top Quark Properties l+ Decay W Polarisation, Colour Flow, Branching Ratios, |Vtb| ν • The Top Quark • Very heavy (Mtop=173 GeV) • Narrow width short lifetime (10-25s) • Decays before hadronisation • “Bare” quark (almost) b t W+ Properties Mass, Mass difference, Width,Charge, Spin, Polarisation, Spin correlation, Asymmetries t d Production Cross section, generator studies, rare couplings W- u b

  3. Dilepton Events ν l+ W+ b t b t • Dilepton Events: • 2 Leptons (opposite sign, isolated, trigger match) • At least 2 Jets • ET > 60 GeV(ee/μμ) • HT > 130 GeV(eμ) HT = Scalar sum of all lepton & jet pT • mll > 15 GeVand veto 81 GeV < mlll < 101 GeV(ee/μμ) • Optional b-tagging (70% efficiency working point) W- miss l- νl

  4. Single Lepton d W+ u b t b • Single Lepton Events: • 1 Lepton (isolated, trig match) • At least4 Jets with at least1 b-tag t W- l- miss miss • (μ + jets) • ET> 35 (30) GeV • mT > 25 (30) GeV • (e + jets) • ET > 20 GeV • ET + mT > 30 GeV miss W W νl mT = W

  5. Spin Correlation and Polarisation

  6. Introduction – Spin Variables • Polarisation(P)andSpin Correlation (A) closely related. Greg Mahlon, 3rd International Workshop on Top Quark Physics, Bruges, May 2010

  7. Introduction – Spin Variables • Polarisation(P)andSpin Correlation (A) closely related. • cos(θi/j) • Angle between a decay particle from the top and some chosen • spin quantisation axis in top rest frame. • Top Helicityis a well known example.

  8. Introduction – Spin Variables • Polarisation(P)andSpin Correlation (A) closely related. Spin Correlation Spin Correlation &

  9. Introduction – Spin Variables • Polarisation(P)andSpin Correlation (A) closely related. • Looking at the double differential distribution requires full tt reconstruction. • ΔΦ between two spin analysers in lab frame is also sensitive to spin correlations. • Does not require tt reconstruction and sensitive to gluon gluon fusion. • Precision test of the SM. • Ideal variable for LHC!

  10. Introduction – Spin Variables • Polarisation(P)andSpin Correlation (A) closely related. Polarisation

  11. Introduction – Spin Variables • Polarisation(P)andSpin Correlation (A) closely related. • Leptons and down type jets are the best analysers • Hard to distinguish u type jets from dtype jets F. Hubaut, E. Monnier, P. Pralavorio, K. Smolek, V. Simak, Eur.Phys.J. C44S2 (2005) 13-33

  12. Results

  13. Observation of Spin Correlation 2.1 fb-1 7 TeV • Perform likelihood fit with templates A=SM and A=0 and extract fSM. • fSM= 1.30 ± 0.14 (stat.) +0.27/-0.22 (syst.) • Extrapolate AHELICITY = fSM* PredHELICITY • Exclude the A = 0 case with 5.1 • First Observation of Spin Correlation in tt! • Consistent with hypothesis of spin ½ AHELICITY : 0.40 ± 0.04 (stat) +0.08/-0.07 (syst)SM Prediction: 0.31 Phys. Rev. Lett: Volume 108 Issue 21 (Prediciton) W. Bernreuther and Z. G. Si, Nucl. Phys. B 837, 90(2010)

  14. Top Quark Polarisation 4.7 fb-1 7 TeV Events • Template fit similar to Spin Correlation, hypotheses αP = +1 andαP = -1 • Dominant Systematic comes from jet reconstruction, jet resolution and signal modeling. • All results consistent with SM. αP: -0.060 ± 0.018 (stat) +0.046/-0.064 (syst) ATLAS-CONF-2012-133

  15. Charge Asymmetry

  16. Introduction – Charge Asymmetry Asymmetry measurements at the LHC andTevatron: y AFB • Small Asymmetry expected at Tevatron (~5%) • Large deviation seen (~15%) • LHC tt is pp collider and charge symmetric. • Asymmetry observable but expected to be very small • |Δyl|=|yl+|-|yl-|: Doesn’t Require Reconstruction • |Δyt|=|yt+|-|yt-|: Easier to compare AC

  17. Single Lepton Charge Asymmetry 1.04 fb-1 7 TeV • Events reconstructed using Kinematic Likelihood Fitter (KL) • Rapidity distributions unfolded to parton level. • Inclusive and differential measurements in mtt AC = -0.019 ± 0.028 (stat.) ± 0.024 (syst) inclusiveAC = -0.052 ± 0.070 (stat.) ± 0.054 (syst) mtt > 450 GeV AC = -0.008 ± 0.035 (stat.) ± 0.032 (syst) mtt < 450 GeV high low MC@NLO(inclusive) Prediction AC = 0.006 ± 0.002 Eur. Phys. J. C (2012) 72:2039: http://arxiv.org/abs/1203.4211

  18. Single Lepton Charge Asymmetry D0 AC = -0.019 ± 0.028 (stat.) ± 0.024 (syst) inclusiveAC = -0.052 ± 0.070 (stat.) ± 0.054 (syst) mtt > 450 GeV AC = -0.008 ± 0.035 (stat.) ± 0.032 (syst) mtt < 450 GeV high low mtt > 450 GeV INCLUSIVE Note: Does not include most recent results from all colliders Eur. Phys. J. C (2012) 72:2039: http://arxiv.org/abs/1203.4211

  19. Dilepton Charge Asymmetry 4.7 fb-1 7 TeV • Asymmetry measured at parton level • Background subtracted data corrected for detector acceptance. • Measured both lepton and top asymmetry. • Results consistent with SM (predictions from MC@NLO). COMB WITH L+JETS RESULT ATLAS-CONF-2012-57

  20. Current ATLAS Asymmetry Results prediction ATLAS (combined) ATLAS (dilep) ATLAS (single lepton) AC • Current Status of the 7 TeV Asymmetry measurements at ATLAS • Everything consistent with SM

  21. tt + HF NEW! • tt + HF (HF = bb, cc, b, c) • Flavour coming from gluon splitting or proton • main irreducible background to ttH • Many BSM models also predict additional HF in tt events. Heavy Flavor Ratio At least 3 b-tagged jets • Many experimental uncertainties cancel in ratio • Cross sections measured in fiducial region (with a factor to correct for detector acceptance) • Correction factor is 0.093 ± 0.004(stat.) in HF, 0.129 ± 0.001(stat.) in tt + j At least 2 b-tagged jets

  22. tt + HF NEW! 4.7 fb-1 7 TeV • perform a binned maximum likelihood fit to extract σfid (tt + HF) • σfid (tt + j) taken from observed yield in data. • Use the displaced vertex mass in bins of b-tag purity (improves LF-c jet discrimination). RHF= [7.1 ± 1.3 (stat.) +5.3/-2.0 (syst.)] % RHF (LO) = [3.4 ± 1.1 (syst.)] %RHF (NLO) = [5.2 ± 1.7 (syst.)] % • Dominant systematic is flavor composition To be submitted to Physical Review D

  23. Summary and Outlook • LHC delivering precision measurements. • First observation of non vanishing Spin correlations, measurements on Polarisation and Charge Asymmetry. • 7 TeV Analyses for ATLAS Top Properties almost completed (one or two more to look forward to) • Start to see 8 TeV properties results soon! • Charge Asymmetry results so far statistics limited. • Long Shutdown 1 will be a very exciting time for top properties and plenty to look forward to with 13 TeV!

  24. Backup

  25. References Figures “Spin Correlations: TEVATRON vs. LHC”, Greg Mahlon, 3rd International Workshop on Top Quark Physics, Bruges, May 2010 https://agenda.irmp.ucl.ac.be/conferenceTimeTable.py?confId=538#20100530 “ttbar asymmetries in the SM”, Germán Rodrigo, LHC Workshop “From Charge Asymmetry to the Boosted Regime” 02-04 May 2012 https://indico.cern.ch/conferenceOtherViews.py?view=standard&confId=175916 “Top Charge Asymmetry at ATLAS”, KerimSuruliz, LHC Workshop “From Charge Asymmetry to the Boosted Regime” 02-04 May 2012 https://indico.cern.ch/conferenceOtherViews.py?view=standard&confId=175916 Papers “ATLAS sensitivity to top quark and W boson polarization in tt̄ events”, F. Hubaut, E. Monnier, P. Pralavorio, K. Smolek, V. Simak, Eur.Phys.J. C44S2 (2005) 13-33 http://arxiv.org/abs/hep-ex/0508061 “Distributions and correlations for top quark pair production and decay at the Tevatron and LHC” W. Bernreuther and Z. G. Si, Nucl. Phys. B 837, 90(2010), and private communication.

  26. Systematics: Dilep Charge Asymm

  27. tt + HF – b tag purity Medium Purity High Purity Low Purity

  28. Asymmetries Cartoon http://cds.cern.ch/record/1493476/files/TOP-12-010-pas.pdf CMS Dilepton http://cds.cern.ch/record/1458955/files/plb.717.129.pdf CMS l+jets http://cds.cern.ch/record/1453785/files/ATLAS-CONF-2012-057.pdf ATLAS Dilep + Combination http://link.springer.com/article/10.1140%2Fepjc%2Fs10052-012-2039-5 ATLAS l + jets

  29. Introduction – Spin Variables • Polarisation(P)andSpin Correlation (A) closely related. [a] Double Differential DistributionP= 0, A= SM

  30. Physics Objects at ATLAS • ELECTRONS: • Central energy deposit combined with Inner Detector track • Isolated from other objects • |ηcluster| < 2.47, • 1.37 < |ηcluster | < 1.52 • ET > 25 GeV • JETS: • Anti-kT jets (R=0.4) • Corrected to EM Scale • |η| < 2.5, ET > 25 GeV • 75% tracks point to vertex • B-Tagged using Neural Net based tagger, eff 70%. • MUONS: • Muon Spectrometer • track combined with Inner • Detector track • Isolated from other objects • |η| < 2.5 • PT > 20 GeV • MISSING TRANSVERSE ENERGY: • Sum of transverse energies of all reconstructed physics objects. • Accounts for unassociated calorimeter cells. ?

  31. Event Model - Backgrounds • Single Top: • MC@NLO + HERWIG & JIMMY • Dilepton Wt channel only. • Drell-Yan: ee, μμ, ττ • ALPGEN + HERWIG & JIMMY • Z + (0-5 jets inclusive) • DiBoson: • HERWIG (ALPGEN) & JIMMY • WW, WZ and ZZ • W + jets: • ALPGEN + HERWIG & JIMMY • W + (0-5 jets inclusive) “lepton” • Fakes (mismeasured leptons) • Data Driven • Matrix Method

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