1 / 30

Top Quark Pair Production Cross Section using the ATLAS Detector at the LHC

Top Quark Pair Production Cross Section using the ATLAS Detector at the LHC. P. Skubic (On behalf of the ATLAS collaboration) April 29, 2014. Outline. Introduction: Top Production and decay Production Cross section inclusive cross sections 7 TeV 8 TeV

evelyn
Download Presentation

Top Quark Pair Production Cross Section using the ATLAS Detector at the LHC

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Top Quark Pair Production Cross Section using the ATLAS Detector at the LHC • P. Skubic • (On behalf of the ATLAS collaboration) • April 29, 2014 P. Skubic - OU , ATLAS

  2. Outline • Introduction: • Top Production and decay • Production Cross section • inclusive cross sections • 7 TeV • 8 TeV • differential cross section at 7 TeV • Summary P. Skubic - OU , ATLAS

  3. Top quark pair production at LHC Theory: Full NNLO+NNLL with contributions from: Baernreuther, Czakon, Mitov arXiv:1204.5201 Czakon, Mitov arXiv:1207.0236 Czakon, Mitov arXiv:1210.6832 Czakon, Fiedler, Mitov arXiv:1303.6254 ≈ 15/13/10% @ 7/8/14 TeV The large mass of the top quark results in large coupling to the Higgs and possibly to new physics processes. ≈ 85/87/90% @ 7/8/14 TeV P. Skubic - OU , ATLAS

  4. Finding top quark pairevents • Decay: weak interaction : t  wb (~ 100 %) • Final state: from the W decays • b-tagging performance : characterized by b-tagging efficiency (probability to identify a b-jet as such) and rejection of non-b-jets. A typical working point at 70% efficiency using the MV1 tagger has a rejection factor of about 140. All jets 46% τ’s 14% e/μ jets 34% Dilepton (e/μ) 6% • Gives several handles for identification • e/μ/τ from W decays • b-jets • Missing transverse energy from neutrino Each must be understood with high precision P. Skubic - OU , ATLAS

  5. Typical top pair e-µ dilepton candidate with two b-jets Typical event selection Requirements: pTe> 25 GeV pTµ> 25 GeV Etmiss> 45 GeV |ηcl|< 2.47 P. Skubic - OU , ATLAS

  6. 7 TeVdilepton cross section • Measurement with/with • out b-tagging • JHEP 1205 (2012) 059 • Simple counting analysis • Since comparatively clean signal • Stat. Error: ~3% • Sys. Error: ~8% (JES, lepton SF, fakes) • Lumi. Error: ~5% P. Skubic - OU , ATLAS

  7. 8 TeV single lepton crosssection 8 TeV Lepton(e/μ) + jets ATLAS-CONF-2012-149 Multivariate technique used with b-tagging to separate tt signal from backgrounds Variables used in Likelihood are lepton ηand aplanarity transform (exp(-8A)) Dominant systematics include: MC modeling of signal (11%) and Jet/MET reconstruction and calibration (~6%) Good agreement with theory. P. Skubic - OU , ATLAS

  8. Inclusive dilepton cross section 8 TeV ATLAS-CONF-2013-097 Require opposite sign (OS) eµ with exactly 1 or 2 b-tagged jets Events with at least two jets P. Skubic - OU , ATLAS 8 tt purity: 89% 96%

  9. Inclusive dilepton cross section (con’t) Bkg: Single top (Wt) (from simul.), Data-driven fake leptons (extrapolated from same sign lepton sample), Z + jets (extrapolated from Z µµ sample) Good data-MC agreement and signal/background ratio e µ η pT P. Skubic - OU , ATLAS

  10. Inclusive dilepton cross section (con’t) Simultaneous fit for cross section and efficiency to select, reconstruct, and b-tag a jet in 1-b-tag and 2-b-tag samples in order tominimize jet and b-tag syst Primary systematic errors: Lumi~3.1%, Ebeam~ 1.7%, ttmodelling~ 1.5%, Electron ID/isol~ 1.4% Good agreement with theory. P. Skubic - OU , ATLAS

  11. Inclusive cross-section summary Measurements are in good agreement with predictions 7 TeV summary/history 8 TeV summary P. Skubic - OU , ATLAS

  12. Top quark pair production vs center of mass energy Good agreement with predictions at several values of Ecm P. Skubic - OU , ATLAS

  13. Top quark pair production – differential cross-section • Require 1 isolated e, µ; ETmiss > 30 GeV, mTW > 35 GeV, ≥4 jets, ≥ 1 b-tag • Reconstruct with kinematic likelihood fit: (mt, mW constraint) with cut on quality of fit • Unfold d(N-Nbkg)/dX to full phase space: (regularized unfolding, linearity tests), scale with L and ATLAS-CONF-2013-099 Combine (e,µ) + jets channels with minimal covariance estimator including correlations Propagate syst. Uncertainties through unfolding: Modify migration matrix and acceptances, correct data Compare to MC simulations and selected theoretical calculations. P. Skubic - OU , ATLAS

  14. (con’t) • Backgrounds: W + jets ( data-driven: normalize pre-tag with W+/W- asymmetry;extrapolate b-tag prob. from 2-jet-bin); fake leptons (data-driven method); Single top, dibosons (from MC) ATLAS-CONF-2013-099 dN/dpT,top P. Skubic - OU , ATLAS

  15. (con’t) Compare with MC, NLO & approx NNLO pT,top spectrum is softer than most predictions for pT,top > 200 GeV ATLAS-CONF-2013-099 P. Skubic - OU , ATLAS

  16. (con’t) Compare data with NLO QCD using FCFM with different PDF sets Data show sensitivity to PDF with Some preference for HeraPDF P. Skubic - OU , ATLAS

  17. Summary • Top productionmeasurements are in precision era - Pair production cross section uncertainty O(5%) level at LHC compared to ~4% prediction uncertainty (NNLO+NNLL) - Differential cross-sections now measured with 10%-20% relative uncertainties • Most top physics measurements systematics dominated - Work is on going for full run-1 LHC samples • Run 2 @:new kinematic phase space to be explored with ~ factor 3 enhanced cross section • Higher statistics inclusive, exclusive, and differential cross section measurements • Fiducial measurements • New physics decaying into top quark (pairs) not yet seen -Large machinery developed looking into many signatures, reusable in 2015 - P. Skubic - OU , ATLAS

  18. Backup Slides P. Skubic - OU , ATLAS

  19. Lepton + Jets: Analysis (No Btag) • Variables chosen (based on the • optimization w.r.t. stat+JES error) : • lepton η: ttbar more central • lepton q : ttbar symmetric, • W+jets asymmetric • aplanarity : ttbar more isotropictransformed to e-8xAfor uniformity; • Aplanarity defined: 1.5x smallest eigenvalue of momentum tensor P. Skubic - OU , ATLAS

  20. Lepton + Jets: Analysis Strategy • Measurement strategy (multivariate) : • exploits the difference in kinematic distributions of signal and background events. • Projective Likelihood (LH) is used: • to separate signal from bkg(both • analysis with and without b-tag) • Discriminant constructed from • multiple variables • MC signal and background models these • variables for building LH discriminant • Fit the likelihood discriminant distribution in data by sum of two “templates”, signal and bkg, and get the 20 P. Skubic - OU , ATLAS

  21. Lepton + Jets: Background Estimate • Backgrounds: • W+jets backgrounds • Shape is determined by MC • Normalization from fit • Small Bkgd (Z+jets , diboson , single top) • Shape from MC • Normalization from NLO calculation • QCD multijet (Fake lepton) • Due to mis-ID of lepton, not well modeled in simulation • Used (for example) matrix method for μ channel • anti-electron for e channel P. Skubic - OU , ATLAS 21

  22. QCD MULTIJET : MUON CHANNEL • Dominated by b-jets or c-jets producing muons • Background in signal region can be estimated by using matrix method : • : from data – Z decay • : Control regions (loose the standard criteria) • These are applied to the signal region • Uncertainty : 30 % Isolated muons from W decays QCD muon from jets εreal εfake εreal εfake Standard muon selection • Apply b-tagging to get the estimate after b-tagging P.Skubic – OU, ATLAS P. Skubic - OU , ATLAS

  23. Top pair production cross-section – 35 pb-1 • Measurement without use of b-tag [Phys. Lett. B711(2012) 244-263] • Multivariate analysis in e/μ + 3,≥4 jets • Background: W+ jets, Multijet, WW/WZ/ZZ, single top • Lepton charge, lepton η, aplanarity • Stat. Error ~10 % • Syst Error ~11% (JES 5%, bkgmodelling ~ 4.0%, IFSR 6%) P. Skubic - OU , ATLAS

  24. Top pair production cross-section – 35 pb-1 • Measurement with use of b-tag [Phys. Lett. B711(2012) 244-263] • Multivariate analysis in e/μ + 3,4,≥5 jets • Lepton charge, lepton η, aplanarity transform (exp(-8A)), b-tag weight • Stat. Error ~6 % • Syst Error ~9.7% (JES 4%, bkg modeling ~ 4.0%, IFSR 5%) P. Skubic - OU , ATLAS

  25. Top pair production cross-section – 0.70 fb-1 • Measurementwithout use of b-tag [ATLAS-CONF-2011-121] • Multivariate analysis in e/μ + 3,4,≥5 jets • Background: W+ jets, Multijet, WW/WZ/ZZ, single top • lepton η, aplanarity transform (exp(-8A)), leading jet pT, and HT,3p transform (exp(-4HT,3p) • Stat. Error ~4 % • Syst Error ~9.0% (JES 4%, signal modeling ~ 5.0%, IFSR 3.0%) P. Skubic - OU , ATLAS

  26. Top quark pair production –  lepton channels  + jets 7 TeV  + lepton 7 TeV EPJC 73 3 (2012) 2328 Phys. Lett. B 717 (2012) 89-108 signal ntrackfor τhad candidates after all selection cuts P. Skubic - OU , ATLAS

  27. LHC: A Top producer ~23 fb-1 @ 8 TeV Cumulative LumiVs time 5.6 fb-1 @7 TeV 0.048 fb-1 @7 TeV • Run (2010 – 2011) • 2x1032 cm-2 sec-1(instantaneous lumi) • 3.6x1033 cm-2 sec-1 • Run (2012) • 7.7x1033cm-2 sec-1 P. Skubic - OU , ATLAS

  28. THE ATLAS EXPERIMENT Tile Calorimeter LAr Calorimeter Muon Detector Vertex & Tracker Toroid Magnets Trigger system to record online interesting events(collisions every 25/50 ns) P. Skubic - OU , ATLAS

  29. PIXEL DETECTOR This is high-granularity silicon detector Layout (Oklahoma group was involved in the Pixel detector construction) : 1744 modules located on 3 layers with both barrel and end cap disk geometry 80 million channels Low occupancy (1,000 trk/event at LHC design luminosity) Pixel is close to the intense LHC collision, it is radiation hard, and has an excellent spatial resolution (10 μm * 115 μm ) Because of its fantastic spatial resolution, pixel detector plays a unique role in the identification of b-quark jets or b-tagging. b-quark jet identification plays a central role in many searches of new physics and top quark physics Commissioning : Large testing activity during Integration : Connectivity test : Last chance to repair before Installation inside ATLAS ! P. Skubic - OU , ATLAS

  30. B-TAGGING PRINCIPLES • b-tagging : identification of b-jets (jets originating from b-quarks) • crucial for many physics channels (top quarks, SM and MSSM Higgs, SUSY) • b-tagging algorithms in ATLAS : two main approaches • SV based : search for a secondary vertex inside the jet: • signed decay length significance : S(Lxy) = Lxy/s(Lxy). • IP based : count tracks with large impact parameter significance (IPS): Impact parameter significance : S(IP) = d0/(d0) • (complimentary) : look for soft leptons inside the jet • JetFitter: takes into account track & vertex info, energy fraction of charged tracks, S(Lxy) in a neural net • MV1: combination of all above methods (default) • b-tagging performance : characterized by b-tagging efficiency (probability to identify a b-jet as such) and rejection of non-b-jets. A typical working point at 70% efficiency using the MV1 tagger has a rejection factor of about 140. P. Skubic - OU , ATLAS

More Related