Top physics results Greg Heath University of Bristol
Top physics results Greg Heath University of Bristol UK HEP forum, Cosener's, 7 th September 2011
Motivation • Large cross-section – About 20× Tevatron at 7 Te. V • Opportunity to study “bare” quark – Decays before hadronisation • Handles on New Physics – Higgs mass uncertainties – New phenomena coupling to top – Top as background to discovery channels • Exercise full detector and reconstruction in complex multi-parton events – Leptons, jets, MET, b-tagging, tau-id UK HEP forum, Cosener's, 7 th September 2011 2
Top production and decay • Top pair production from initial gg pair dominates • Top pair decay modes – – – All hadronic (6 jets) – large BR, large QCD background Lepton+jets (4 jets) – smaller BR (per lepton), smaller background Dileptonic (2 jets) – smallest BR, cleanest • Also consider single top production Theory predictions UK HEP forum, Cosener's, 7 th September 2011 3
Top pair cross-section UK HEP forum, Cosener's, 7 th September 2011 4
2011 dilepton analyses • ATLAS • • Robust cut and count analyses B-tagging improves efficiency Systematic errors dominate CMS UK HEP forum, Cosener's, 7 th September 2011 5
Lepton+jets analyses with b-tags • b-tagging probability as input to likelihood fits to signal+background • Combined fit to jet multiplicity bins CMS lepton+jets with b-tags ATLAS single lepton with b-tags m+jets e+jets m+jets UK HEP forum, Cosener's, 7 th September 2011 e+jets 6
Lepton+jets untagged (ATLAS) 2011 data (0. 7 fb-1) Likelihood fit to event kinematics UK HEP forum, Cosener's, 7 th September 2011 7
All-hadronic measurement (CMS) • Kinematic fit to events with b-tagging • Fit mass distribution to estimate t-tbar contribution UK HEP forum, Cosener's, 7 th September 2011 8
Dilepton measurements with ts CMS – cut-based selection including b-tagging ATLAS – m+jets background minimised using BDT UK HEP forum, Cosener's, 7 th September 2011 9
Cross-section summary Individual combinations (l+jets and dilepton) ATLAS CMS UK HEP forum, Cosener's, 7 th September 2011 10
Other cross-section measurements • Single top – t-channel – s-channel and t. W • Resonance searches • Charge asymmetry UK HEP forum, Cosener's, 7 th September 2011 11
Single top production • Three EW diagrams contribute to production of single top quarks at hadron colliders • t-channel production has the largest predicted cross-section • Main backgrounds t-tbar and W+jets UK HEP forum, Cosener's, 7 th September 2011 12
t-channel cross-section • t-channel (dominant) – – • 2 jets Leptonic decay of W – – • Central b-tagged jet Forward light jet Isolated lepton Large MT(ln) Variety of approaches – Backgrounds are large 3 jets Boosted decision tree (BDT) Cut-based UK HEP forum, Cosener's, 7 th September 2011 13
t-channel cross-section ATLAS 2011 cut-based: CMS 2010 combined: (dominated by BDT) UK HEP forum, Cosener's, 7 th September 2011 14
Wt and s-channel single top searches ATLAS 1. 2 s 95% CL signal exp 16± 6 total exp 285± 17 observed 296 UK HEP forum, Cosener's, 7 th September 2011 15
Searches for t-tbar resonance ATLAS l+jets Reject extra jets from radiation ATLAS dilepton CMS l+jets Kinematic fit and b-tagging UK HEP forum, Cosener's, 7 th September 2011 16
Charge asymmetry • Small effect predicted in SM – Top quark h distribution narrower than antiquark • But dominant gg production is symmetric – Measured quantity: – Theory predicts small positive asymmetry • Lepton+jets selection • Fit to identify top and anti-top • Unfold measured asymmetry CMS result: ATLAS result: Theory: AC=0. 006 Measured: AC=-0. 024± 0. 016± 0. 023 Theory: AC=0. 013± 0. 001 Measured: AC=-0. 016± 0. 030+0. 010 -0. 019 UK HEP forum, Cosener's, 7 th September 2011 17
Top properties UK HEP forum, Cosener's, 7 th September 2011 18
mt in lepton+jets (ATLAS 2011) • Template method – Pick the “best” jet combination hypothesis per event – Kinematic fit with m. W constraint – Simultaneous fit to mt and Jet Scale Factor UK HEP forum, Cosener's, 7 th September 2011 19
mt in lepton+jets (CMS 2010) • Ideogram method – Kinematic fit and per-event likelihood vs mt – Uses all jet combination hypotheses – JES as systematic correction • Additional CMS measurement in dilepton events: UK HEP forum, Cosener's, 7 th September 2011 20
Measurement of pole mass ATLAS • Direct measurements yield MC mass parameter mt • Theoretical crosssection calculations depend on the pole mass – Mass uncertainty 4× smaller than cross-section uncertainty – Pole mass looks smaller than MC mass UK HEP forum, Cosener's, 7 th September 2011 21
t-tbar mass difference • Ideogram method modified CMS – Lepton+jets – Use lepton to distinguish t and tbar – Fit mass of hadronic top only – 2011 data (1. 09 fb− 1) UK HEP forum, Cosener's, 7 th September 2011 22
Spin correlation ATLAS • Top quark decays before hadronisation – Parton level Data vs MC Spin correlations at production propagated to decay products • Charged lepton from W decay carries full spin information • Measure distribution of Df between leptons in dilepton events • Fit to a mixture of SM and uncorrelated templates • Results consistent with SM expectations – SM fraction FSM=1. 06± 0. 21 UK HEP forum, Cosener's, 7 th September 2011 23
W helicity measurement ATLAS • Test of V−A structure of top quark decay • Angular distribution of lepton and b quark in W rest frame • Fit to LH, RH and longitudinally polarised W • Results consistent with SM expectations – Longitudinal fraction F 0=0. 75± 0. 08 • Improved limits on anomalous couplings UK HEP forum, Cosener's, 7 th September 2011 24
Summary and outlook • LHC is a top factory – Initial measurements in l+jets and dilepton channels now systematics dominated – Cross-section precision (7%) approaching that of theory prediction – Single top production confirmed • Top quark properties consistent with SM (so far) • Top is an active field in searches for New Physics – Many studies just beginning UK HEP forum, Cosener's, 7 th September 2011 25
List of sources UK HEP forum, Cosener's, 7 th September 2011 26
Top pair cross section measurements • L+jets: – – ATLAS 2010 analysis: 3, 4, >4 -jet subsamples (ATLAS-CONF-2011 -035) CMS 2010 analysis similar (ar. Xiv: 1108. 3773) B-tagging gives best results Untagged analysis on 2011 data (ATLAS-CONF-2011 -121) • Dilepton: – ATLAS 2011 0. 70 fb-1 (ATLAS-CONF-2011 -100) – CMS 2011 1. 14 fb-1 (CMS PAS TOP-11 -005) • Simple cut-based, no b-tagging • Combinations – – ATLAS combine the above two results (ATLAS-CONF-2011 -108) CMS combine the two 2010 measurements (ar. Xiv: 1108. 3773) • All-hadronic and tau dilepton – – CMS all-hadronic measurement on 2011 data (CMS PAS TOP-11 -007) ATLAS all-hadronic search on 2010 data (ATLAS-CONF-2011 -066) ATLAS mu+tau measurement (ATLAS-CONF-2011 -119) CMS mu+tau measurement (CMS PAS TOP-11 -006) UK HEP forum, Cosener's, 7 th September 2011 27
Production measurements; single top • Spin correlation (ATLAS-CONF 2011 -117) • Charge asymmetry – (ATLAS-CONF-2011 -106) – (CMS PAS TOP-11 -014) • Single top – – t-channel measurement (ATLAS-CONF-2011 -101) t-channel measurement (ar. Xi. V: 1106. 3052) s-channel search (ATLAS-CONF-2011 -118) Search for Wt (ATLAS-CONF-2011 -104) UK HEP forum, Cosener's, 7 th September 2011 28
Top mass and properties, exotica • Top mass – – Template method (ATLAS-CONF-2011 -120) Mass from cross-section (ATLAS-CONF-2011 -054) Ideogram method (CMS PAS TOP-10 -009) Mass in dileptons (included in ar. Xi. V: 1105. 5661) • Mass difference (CMS PAS TOP-11 -019) • W polarisation (ATLAS-CONF-2011 -122) • Resonance search – – – Dilepton channel (ATLAS-CONF-2011 -123) Lepton+jets channel (ATLAS-CONF-2011 -087) CMS search (CMS PAS TOP-10 -007) • Exotic searches – – Search for FCNC (ATLAS-CONF-2011 -061) Search for anomalous MET (ATLAS-CONF-2011 -036) UK HEP forum, Cosener's, 7 th September 2011 29
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