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Top Physics in ATLAS

Top Physics in ATLAS. M. Cobal On behalf of the ATLAS Collaboration University of Udine & INFN Udine La Thuile , 9 March 2016. Measurements and Searches. SM top pairs strong production. New Physics. Cross section Charge asymmetry Spin correlations Top polarisation Mass Charge

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Top Physics in ATLAS

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  1. Top Physics in ATLAS M. Cobal On behalf of the ATLAS Collaboration University of Udine & INFN Udine La Thuile, 9 March 2016

  2. Measurements and Searches SM top pairs strong production New Physics Cross section Charge asymmetry Spin correlations Top polarisation Mass Charge W helicity Couplings Resonances Vector Like Quarks Scalar tops FCNC Anomalous Wtbvertex SM single top ew production

  3. Top Quark Production Assuming m(t) = 172.5 GeV Ds/s = 3.5% ATLAS-CONF-2014-054 CMS PAS TOP-14-016

  4. s(tt) from em events @ 13 TeV • Measuring pp → tt + jets → e±+µ∓+νe+νµ+ jets. • Very small backgrounds: most precises(tt) measurement • Simultaneously measure σ(tt) and the efficiency to identify and reconstruct a b-jet • Events in 1 and 2 b-tag multiplicity bins: correlated. • Exploit this correlation to constrain b-tagging efficiency. • σ(tt) = 825 ± 49 (stat) ± 60 (syst) ± 83 (lumi) pb • Ds/s = 14% ATLAS-CONF-2015-033

  5. Top Quark Production Assuming m(t) = 172.5 GeV

  6. s(t) in t-channel in l+jets @ 13 TeV • pp→t+jets →b+µ+νµ+2jets (1 b-tagged) • Process is proportional to |fLV · Vtb| • Background yields from theory and fit to orthogonal data. • Neural network discriminant to separate background - signal. Single top • σ(tq) = 133 ± 25 pb • σ(tq) = 96 ± 45 pb • |fLV · Vtb| = 1.03 ± 0.11 • Theory (NLO): • σ(tq) = 136+5.4−4.6pb • σ(tq) = 81+4.1−3.6pb ATLAS-CONF-2015-079

  7. s(t)in the s-channel in l+jets @ 8 TeV Single top • Lepton+jets selection with 2 b-jets and large ETmiss • Build Matrix Element discriminant foreach selected event • s-channel vs t-channel, tt, W+jets • Templatefit in signal and control regions s = 4.8 ±1.1(stat.)−2.0+2.2(syst.+lum) pb Observed (expected) significance: 3.2σ (3.9σ) arXiv:1511.05980 First evidence of single-top s-channel production at the LHC!

  8. Differential cross section • Large statistics makes it possible to evaluate MC generators, parton shower, etc. with differential cross-section vspTt, ytMtt, pTtt, ytt(lepton+jets). • Enhanced sensitivity to BSM physics • Reconstruct variable X from selected events. X is corrected for detector resolution and neutrinos (“unfolding” technique). • Parton level and Particle-level measurement are done. • Parton-level : Easier comparison with theoretical prediction • Particle-level : Less model dependence

  9. s(tt) differential in l+jets@ 8 TeV • Important to correctly describe tt kinematic distributions. • Measurements extrapolated to the fiducial and partonic levels. • High pT distributions constrains NNLO predictions • Angular distributions constrain PDFs. arXiv 1511.04716

  10. s(tt+jets) differential in dilepton @13 TeV • Measuring tt pairs decaying in the di-leptonicchannel as a function of additional jets • (not including the b-jets). • Rates depend on higher order QCD modelling. • Distributions are an important background to Higgs and new physics measurements. • Distributions are unfolded to the fiducial detector acceptance. first direct measurement of jet multiplicity in the ttdilepton channel by ATLAS ATLAS-CONF-2015-065

  11. mtis scheme-dependent mpole: top quark as a free parton Other schemes, e.g. MS scheme, give different values Top Quark Mass • Search • Precision • ‘Direct’ mass measurements • Get mtop(rec), extract mtop(MC) • Experimentally most precise, limited by • (Flavour-dependent) JES uncertainties, final state modellingetc.. • Sub GeV precision (0.36%) reached • Uncertainty O(1 GeV) in interpreting measurements into top quark pole mass • ‘Alternative’ mass measurements • Complementary (experimental or theoretical) uncertainties • Comparison/combination with ‘direct’ mass measurements can reduce uncertainties further • C.Quigg

  12. Top Quark Mass summary • _ World (2014): mt= 173.3±0.8 GeV ATLAS (2015): mt= 173.0±0.9 GeV CMS (2015): mt= 172.4±0.5 GeV “direct” (MC) mass and pole mass (theoretically well defined) agree Uncertainties currently <2 GeV)

  13. Charge Asymmetry arXiv 1207.0331 Central-Forward asymmetry due to interference at NLO Top-antitop asymmetry Lepton asymmetry l+jets and dilepton Need kinematic reconstruction Dilepton only No kinematic reconstruction arXiv:1512.06092, arXiv:1512.06092 JHEP 5 (2015) 061

  14. Charge Asymm. with boosted tops @ 8 Tev Single lepton channel Inclusive and differential measurements in the fiducial phase-space mtt > 0.75 TeV, -2<Δ|y|<2 Ac = 4.2 ± 3.2%, Hadronic top decay reconstructed from substructure of large-R jet (top-tagging) Unfolded to partonlevel All results compatible with theoretical predictions in SM Comparison with BSM models W’ models strongly disfavoured arXiv:1512.06092

  15. New Physics could enhance FCNC couplings by many orders of magnitude Flavour Changing Neutral Currents Highly suppressed in SM Can have strong enhancement in BSM • ACTA Phys. Pol. B 35 (2004)

  16. Best FCNC limit tcX t uX JHEP 12 (2015) 061 , Eur. Phys. J. C76 (2016) 12

  17. Getting close to BSM scenarios FCNC summary

  18. Top couplings with EW bosons • tWb coupling measured already at the Tevatron • At the LHC ttgand ttZ became accessible too ISR JHEP11 (2015) 172 FSR

  19. ttV @ 8 TeV summary JHEP11 (2015) 172 Simultaneous fit performed to all channels to extract s(ttW) and s(ttZ) at 8 TeV σ(ttZ) = 215 ± 30 fb (aMC@NLO) σ(ttW) = 232 ± 32 fb (aMC@NLO)

  20. ttH: the top Yukawa coupling • In SM fermion masses are proportional to Higgs (fermion Yukawa coupling) • Top quark is heavy • Strong Yukawa coupling yt ~ 1 • New physics? • s(ttH) two orders of magnitude smaller • than s(ggF) • Without ttH unable to simultaneously • constrain yt and New Physics in ggF loop

  21. ttH: the top Yukawa coupling • In SM fermion masses are proportional to Higgs (fermion Yukawa coupling) • Top quark is heavy • Strong Yukawa coupling yt ~ 1 • New physics? • s(ttH) two orders of magnitude smaller • than s(ggF) • Without ttH unable to simultaneously • constrain yt and New Physics in ggF loop 58% 30% 0.2%

  22. ttH Phys. Lett. B 749 (2015) • Several decay modes investigated • bbbar: larger rate but complicated final state (4b) with combinatoric background and theoretically difficult t-tbar + hf • γγ: cleaner but lower statistics • WW/ZZ: multi lepton final state (2 same sign or 3 ). Best sensitivity. Need to control ttV and instrumental background • tt multilepton V. Dao Eur. Phys. J. C(2015) 75:349 – 8 TeV Phys. Letters B 740 (2015) 222 – 7 + 8 TeV Physics Letters B 749 (2015) 519-541 – 8 TeV

  23. arXiv:1507.04548 ttH summary • Observed (expected) signal significance: 2.5 s (1.5 s) • Sensitivity of searches below 2xsttHSM • Observed excess in multi-lepton final states (< than 2s from SM) • Combined signal strength: • μttH=1.81±0.80 • Negative top Yukawa disfavored at 1s without assumptions on BSM contributions in Higgs loops and decay

  24. Top quark physics: key to QCD, electro-weak and New Physics Most precise inclusive top quark pair production cross-section measurement @7/8 TeVconsistent with NLO+NNLL prediction Differential cross-sections and ttX measurement provide more information (7/8 TeV) Several processes observed for the first time (tt̅+Z/W/γ) Single top production starts to be utilized for property measurements Some new measurements coming soon Run-II: 100 fb-1expected by 2018 higher energy (e.g. tt̅+Z/W/γ/H) higher statistics (differential property measurements) Conclusions • For more: https://twiki.cern.ch/twiki/bin/view/AtlasPublic/TopPublicResults

  25. Run-II: ~100fb-1/experiment: 80 million tt events and 20 million single top events (>10 Hz at 1034) 80,000 tt+Z and t+Z events each Statistics → systematics and reach Beat down systematics using statistics and combination of methods Ultimate precision esp. for top mass (also through theory advances) • 300 fb-1: 3σ-evidence if B(t→Zq)>0.03% Run 2 expectations • Mahlon, Parke PRD81:074024,2010 • FTR-13-016 • Campbell, Ellis, Roentsch arXiv:1302.3856

  26. Charge Asymmetry summary l+jets @ 8 TeV Inclusive and differential measurements, uncertainty dominated by statistics Full event reconstruction with kinematic fit, unfolded to partonlevel Dilepton@ 7 TeV Inclusive ttbar and leptonic asymmetry measurements. Uncertainty dominated by statistics.Eventreconstruction with neutrino weighting technique Comparison with BSM models in agreement with Tevatron W’ models strongly disfavoured arXiv:1512.06092 JHEP 5 (2015) 061 arXiv:1509.02358

  27. ttH: the top Yukawa coupling • In SM fermion masses are proportional to Higgs (fermion Yukawa coupling) • Top quark is heavy • Strong Yukawa coupling yt ~ 1 • New physics? • s(ttH) two orders of magnitude smaller • than s(ggF) • Without ttH unable to simultaneously • constrain yt and New Physics in ggF loop 58% 30% 0.2%

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