1. B. Resende Top WG 28/10/05 Polarization studies in ttbar events 1 Polarization studies in tt events with full simulation 1.Physics motivations 2.Full simulation objects (leptons, jets, E T miss) 3.tt event reconstruction: masses, angles 4.W and top polarization measurements 5.Conclusions F. Hubaut, E. Monnier, P. Pralavorio, B. Resende – CPPM C. Zhu – CPPM/SDU

2. B. Resende Top WG 28/10/05 Polarization studies in ttbar events 2 Motivations: test top production and decay LHC σ ~300 pb Weak Interaction single top Tevatron σ ~3 pb Top polarization  BR(t  Wb)~100%  No top hadronization tWb vertex Top production Top decay W polarization  Full hadronic (3.7M) : jets  Dileptonic (0.4M) : 2 l + 2 b + 2   Semileptonic (2.5M) : l + + 2b + jets tt final states (10 fb -1 ) Test SM, Search for anomalous couplings LHC σ ~850 pb Tevatron σ ~7 pb Strong Interaction gg,qq tt pairs  ATLAS sensitivity to polarization with Atlfast in SN-ATLAS-2005-052  Accepted for publication in EPJC

3. B. Resende Top WG 28/10/05 Polarization studies in ttbar events 3  Angle between: lepton in W rest frame and W in top rest frame Standard Model (M top =175 GeV) 0.7030.2970.000 W polarization in top decay  Test the top decay (in fully reconstructed tt) with W polarization... Longitudinal W + (F 0 )Left-handed W + (F L ) Right-handed W + (F R ) NLO0.6950.3040.001 ...by measuring angular distribution of charged lepton in W rest frame Sensitive to EWSBTest of V-A structure 1/21 cos  1/N dN/dcos  W+W+ b l+l+ t spin

4. B. Resende Top WG 28/10/05 Polarization studies in ttbar events 4 tt spin correlation  Test the top production …  t and t not polarised in tt pairs, but correlations between spins of t and t  … by measuring angular distributions of daughter particles in top rest frames A D =-0.29 M tt <550 GeV A D =-0.24 Angle between spin analysers Angle between t(t) and spin analysers top spin ≠1/2, anomalous coupling, t  H + b A=0.42 A=0.33

5. B. Resende Top WG 28/10/05 Polarization studies in ttbar events 5 Measurement method apply weights event by event Reconstruction and cuts Correction function : parton level  Selection cuts and reconstruction distort the parton level distribution  Use an independent sample to parametrize these effects Apply this method for W polarization and spin correlation cos  1/weight cos  1/N dN/dcos 

6. B. Resende Top WG 28/10/05 Polarization studies in ttbar events 6 Scope of full simulation studies SN-ATLAS-2005-052: with 10 fb -1 and combining semilep+dilep (Atlfast): Spin correlationW polarization AADAD F0F0 FRFR SM0.422-0.2900.7030.000 ± Stat± 0.014± 0.008± 0.004± 0.003 ± Syst± 0.023± 0.010± 0.015± 0.024  F 0, F R driven by semileptonic and A, A D by dileptonic  Precision between 1% and 5% dominated by systematics. Challenging ! Perform the same study in full simulation

7. B. Resende Top WG 28/10/05 Polarization studies in ttbar events 7 Full simulation Rome samples Signal samples – semileptonic tt, lepton = e,  : TopReX+PYTHIAAOD/CBNT1500 pb -1 375k# 4520, 4521 MC@NLO+HERWIGAOD/CBNT 500 pb -1 130k# T1 Background samples (most important with Atlfast): W+4jetsAOD 75 pb -1 180k# A7 Single TopAOD 950 pb -1 1800 pb -1 9200 pb -1 90k 70k 19k # 4511 # 4530, 4531 # 4540  Understand performance for each object: extensive comparison AOD/CBNT very useful  Quantify NLO effects on polarization measurements (angle resolution): extensive comparison MC@NLO/TopReX

8. B. Resende Top WG 28/10/05 Polarization studies in ttbar events 8 Understand objects in tt events  Electron  Muon  Jets (light and b)  E T miss Reconstruct tt events  Masses  Background tt(   ), W+4jets, single top Reconstruct angles in W and top rest frames  Understand p T, ,  resolutions and spectra of W and tops   lep  jet  and  angle resolutions  Results on W and top polarizations Topics of the presentation Refine selection (isolation, calibration,...) Understand p T, ,  resolutions and spectra

9. B. Resende Top WG 28/10/05 Polarization studies in ttbar events 9  p T (GeV) Electron selection  Algorithm: author = 1 (high p T electron : egamma)  Identification with simple cuts: IsEM&2047=0 + hasTrack  Isolation: E T (EM+HAD in  R=0.3*) < 10 GeV around electron (loose cut)  Rec. Eff. = 81.4%  Selection: 1 e p T >20 GeV, |  <2.5  Efficiency = 60.5%  Purity = 98.9%  AOD = CBNT *:0.2 preferred but is not filled in AODs

10. B. Resende Top WG 28/10/05 Polarization studies in ttbar events 10  Algorithm: author = 1 (high p T muon)  AOD (Moore) ≠ CBNT (MuonBoy)  Identification (combined information): hasCombinedTrack=1 with  2 /ndof<20  Reject muons from  and K decays (bad  2 )  Isolation: E T (EM in  R=0.2) <10 GeV around muon track (loose cut)  Reject muons from b and c-jets  Rec. Eff. (MuID) = 85.2%  Selection: 1  p T >20 GeV, |  <2.5  Efficiency = 58.9%  Purity = 99.2%  p T (GeV) Muon selection

11. B. Resende Top WG 28/10/05 Polarization studies in ttbar events 11 Selected lepton resolution in tt e EM p T scale is correct to 0.4%  =2.0 % e ID  =3.2 %  comb  =2.6 % e EM  =20.10 -3 e ID  =0.6.10 -3  =0.13 mrad  =0.6.10 -3  =0.21 mrad No correction of track curvature  comb p T scale is correct to 0.4%  Electron: E in EM calorimeter (brem. recovery) and ,  in ID (Not AOD default !)  Comparison with Atlfast : p T OK,  (  ) not gaussian in Atlfast  Same resolutions with MC@NLO  p T /p T resolution  resolution  resolution  =1 mrad e EM

12. B. Resende Top WG 28/10/05 Polarization studies in ttbar events 12 NTRTHits Electron : problem with # of TRTHits ?  10% of electron tracks with no TRT hits ? Muons OK  Those tracks degrade  p T )/p T by 40%   and  resolutions degraded by 10% and 20%  e + 2 times more affected than e -  NTRTHits  

13. B. Resende Top WG 28/10/05 Polarization studies in ttbar events 13 Jets  Algorithm : cone with  R size of 0.7  Electron/muon in jets :  if  R(isolated el. - jet)<0.2 remove the jet (80%)  if 0.2<  R(isolated el. - jet)<0.7 subtract electron 4-momentum from jet (15%)  Add muon 4-momentum if  R(muon-jet)<0.2  In situ calibration : the 2 jets coming from W  Reduce combinatorial background  Selection : ≥ 4jets with p T >30 GeV, |  <2.5  Efficiency = 37.6%  AOD ≠ CBNT : same ,  but p T (slightly) different... ?

14. B. Resende Top WG 28/10/05 Polarization studies in ttbar events 14 b-tagging, p T miss  CBNT:  SV2 weight > 3  Efficiency = 60%  R uds = 84  R c = 8  AOD:  SV1 + IP3D weight > 3  Efficiency = 53%  R uds = 93  R c = 11  B-tagging selection : ≥ 2 b-jets  p T miss : >20 GeV  Efficiency = 91.2%

15. B. Resende Top WG 28/10/05 Polarization studies in ttbar events 15 Jet/p T miss resolution in tt events light jet p T scale correct to 1%  = 9.8 % b-jet  = 8.6 %  = 18 %  = 0.03  = 22 mrad  = 0.27  = 0.22 rad  = 0.03  = 18 mrad  Large negative shift for p T miss?? Resolutions as expected.  Same resolutions (and p T miss shift !) with MC@NLO  resolution  resolution  pT/pT resolution p T scale correct to 1% shifted by 9% b-jet light jet

16. B. Resende Top WG 28/10/05 Polarization studies in ttbar events 16 Reconstructed masses, Background  Kinematic cuts efficiency: 6.5 %. With  M tops <35 GeV: 5.0 %  Agreement with Atlfast and MC@NLO  Non tt background (W+4jets, single top) increases by a factor 2-3 wrt Atlfast (still preliminary)  S/B~10 ; still dominated by internal background tt   + X M=175.5 ± 0.3 GeV  =7.5 ± 0.4 GeV M=172.7 ± 0.3 GeV  =7.2 ± 0.6 GeV hadronic top leptonic top

17. B. Resende Top WG 28/10/05 Polarization studies in ttbar events 17 Ws resolution: Full vs fast sim  reco –  true  reco –  true (p Treco –p Ttrue )/p Ttrue  Good overall agreement between TopReX Full simulation and Atlfast  Difference in leptonic W p T resolution explained by p T miss miscalibration W had. W lept. W had. W lept. 1. 1.1. 1. 1.1.

18. B. Resende Top WG 28/10/05 Polarization studies in ttbar events 18 Resolution on  angle in W rest frame Cos  Cos  true  Full sim ATLfast TopReX MC@NLO  Very good agreement between TopReX Full simulation and Atlfast... ... and between TopReX and MC@NLO Cos  Cos  true   Angle between: lepton in W rest frame and W in top rest frame 1/21 W+W+ b l+l+ t spin

19. B. Resende Top WG 28/10/05 Polarization studies in ttbar events 19 Results: W polarization TopReX Atlfast 10 fb -1 TopReX full sim 1.5 fb -1 F 0 =0.699 ± 0.005 F L =0.299 ± 0.003 F R =0.002 ± 0.003 F 0 =0.677 ± 0.015 F L =0.309 ± 0.009 F R =0.014 ± 0.009 MC@NLO full sim 0.5 fb -1  Values compatible with SM expectations in each case F 0 =0.684 ± 0.021 F L =0.307 ± 0.011 F R =0.009 ± 0.011

20. B. Resende Top WG 28/10/05 Polarization studies in ttbar events 20 Top resolution: Full vs fast sim  Good overall agreement between TopReX Full simulation and Atlfast  Difference in leptonic top p T resolution explained by p T miss miscalibration top had. top lept. top had. top lept.  reco –  true  reco –  true (p Treco –p Ttrue )/p Ttrue

21. B. Resende Top WG 28/10/05 Polarization studies in ttbar events 21 Resolution on angles in top rest frame Full sim ATLfast TopReX MC@NLO  cos  lep-lej  cos  lep  cos  jet   Very good agreement between TopReX Full simulation and Atlfast  Small differences between TopReX and MC@NLO

22. B. Resende Top WG 28/10/05 Polarization studies in ttbar events 22 Results: top spin correlation A (±stat)A D (±stat) SM values0.42-0.29 TopReX Atlfast 0.42  0.02-0.29  0.01 TopReX Full sim 0.45  0.06-0.29  0.03 MC@NLO Full sim 0.02  0.090.00  0.05  Toprex values compatible with SM expectations  MC@NLO have no spin correlation included... Check only it gives 0

23. B. Resende Top WG 28/10/05 Polarization studies in ttbar events 23 Conclusions  Fast simulation (SN-ATLAS-2005-052) shown that ATLAS can measure precisely W polarization ~1-2% and top spin correlation ~5% after 10fb -1  This precision requires a deep understanding of all objects:  Electrons: OK except N(TRT). Combine E from calo and angles from tracker  Muons: switch to more recent version of Moore/MuID  Jets: switch to cone size 0.4 for jets  Shift in p T miss scale : ?  Good agreement between Fast/Full sim. and TopReX/MC@NLO  Next step: systematics update with full simulation  light/b-jet miscalibration, b-tagging efficiency, …

24. B. Resende Top WG 28/10/05 Polarization studies in ttbar events 24 Spare: W, t performances : TopReX vs MC@NLO  Good overall agreement between TopReX and MC@NLO for the Ws  Difference in tops under invetsigation (b-jet association?) top had. W had. W lept. top lept. W had. W lept.  p T /p T

25. B. Resende Top WG 28/10/05 Polarization studies in ttbar events 25 Spare: Correction functions tt spin correlation : A D tt spin correlation : A W polarization  Good agreement within statistics with Atlfast  Agrees also with MC@NLO Full sim ATLfast