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Top Quark Physics

Top Quark Physics. Pierre Savard University of Toronto and TRIUMF APS Meeting Denver May 2004. Outline. History and Theoretical Overview Review of Experimental Results Electroweak constraints Run I Results Run II Results Outlook. Discovery of b quark in 1977

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Top Quark Physics

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  1. Top Quark Physics Pierre Savard University of Toronto and TRIUMF APS Meeting Denver May 2004

  2. Outline • History and Theoretical Overview • Review of Experimental Results • Electroweak constraints • Run I Results • Run II Results • Outlook

  3. Discovery of b quark in 1977 isospin analysis shows that b should have SU(2) partner Indirect evidence of top through loop contributions: BB Mixing Z bb rate MW/MZ ratio 95 CDF and D0 announce discovery Some History - -

  4. If we assume CKM matrix unitarity and measured mass ~ 175 GeV/c2 , then top properties are well understood within context of SM: Spin = ½ couplings: - +2/3e - color triplet - weak (T3)L - Yukawa coupling ~1 Width: G ~ 1.4 GeV lifetime t ~ 5 x 10-25 (no top hadrons!) Top Quark in the SM (I) Only quark decaying to W Mt at the ewk scale Large Mt-Mb difference implies large ewk loop corrections

  5. Vtb = 0.99 branching ratios: t W b BR ~ 1 t W s BR ~ 10-3 t W d BR ~ 5 x 10-5 t g c,u BR ~ 10-8 t Z c,u BR ~ 10-12 Fraction of longitudinal W bosons: Top Quark in the SM (II) • Experimental signatures depend on how W decays: ~70% of W bosons longitudinally polarised for Mt of 175 GeV/c2

  6. Theoretical models proposed to solve problems of SM often have top playing a leading role: In supersymmetric models, large top mass causes EWSB: In many dynamical symmetry breaking models, top interactions are modified: Examples: technicolour models like Top Flavour, Top Seesaw, Topcolour-assisted technicolour Need to test all aspects of top production and decay.Experimentally we still know very little about the top quark Top Quark Beyond the SM

  7. Booster p source Main Injector and Recycler Experimental Top Quark Physics Tevatron Collider is world’s only top quark production machine • Run 1, 100 pb-1: • collisions every ~ 3msec • beam energy 900 GeV • inst. Luminosity 1031 • Run 2: • collisions every ~400ns • beam energy 980 GeV • inst. Luminosity 1032 • CDF and D0 detectors underwent major upgrades for Run II

  8. First Experimental Results • Samples collected by identifying strong production of pairs of top quarks (have also looked for ewk production) • To help isolate signal, some analyses look for evidence of a B hadron decay: • Secondary Vertex Tagging (SVT or SVX) • Soft Lepton Tagging (SLT)bosons:

  9. Run I Results: Production Properties Test of QCD • Overall discrepancy could indicate non-SM production mechanisms • Inconsistencies between channels could indicate non-SM decay mechanisms • Run I results consistent with SM but with large statistical uncertainties

  10. Run I Results: Top Mass • Top mass important ewk parameter (due to t-b mass difference) • Uncertainty on top mass currently limiting factor in indirect determination of Higgs mass • Accurate measurement needed for self-consistency tests of SM • New Run I D0 l+jet result using matrix element technique

  11. Improved Method by DØ • Use Probability density: • Background probability • Main component W+jets (85% of background) • Pbkg calculated from leading order matrix element from VECBOS • 22 events remain: 12 signal, 10 background • Dominant systematic is jet energy scale: 3.3 GeV/c2 LO Matrix element + phase space Transfer function: parton values to measured quantities x :reconstructed 4-vectors PDF’s Mt = 180.1 ± 3.6 (stat) ± 3.9 (syst) GeV/c2 = 180.1 ± 5.3 GeV/c2

  12. New Run I Top Mass Result and implications on Higgs Mass • New DØ combined mass: • Mt = 179.0 ± 5.1 GeV/c2 • New world average: • Mt = 178.0 ± 4.3 GeV/c2 • Global fit to electroweak data using this top mass • Method of LEPEWWG (hep-ex 0312023) • Best-fit MH 113 GeV/c2 • 95% C.L. upper limit 237 GeV/c2 Yellow region excluded: MH < 114.4 GeV/c2 @95% CL

  13. Other Run I Results: Single Top and Branching Ratios Single top D0 cross section (s-channel) Single top D0 cross section (t-channel) Single top CDF cross section (s-channel) Single top CDF cross section (t-channel) Fraction of longitudinal W bosons (D0): Fraction of longitudinal W bosons (CDF): Branching ratios (CDF): s < 17 pb @ 95% c.l s < 22 pb @ 95% c.l s < 15.8 pb @ 95% c.l s < 15.4 pb @ 95% c.l F0 = 0.56 0.31  0.04 F0 = 0.91 0.37  0.13

  14. Run II Results • Integrated luminosity between 100 and 200 pb-1 • Focus on new cross section and mass results • Theoretical cross section: ~5.8-7.4 pb

  15. Dilepton cross-section: lepton+track (CDF) • Signature: 1 lepton + 1 isolated track, missing ET , 2 central jets • Higher acceptance reduced purity relative to Run 1, • Backgrounds: Z/ *  l+l-, WW, WZ, ZZ, W+jets Measured cross-section for different jet ET and track pT 19 events on 7.1 ± 1.2 background 11 e-track, 8 -track 7.0+2.7-2.3(stat) +1.5-1.3(sys) 0.4 (lumi) pb

  16. Lepton + track Kinematics • RunI: had seen hints of discrepancy in kinematic distribution: Ht:Scalar summed ET of jets, leptons, and missing ET Missing ET Leptons transverse momentum • With higher statistics in Run II, we observe good agreement with SM

  17. Dilepton cross-section: ee,mm,emfinal states (CDF) Different background composition; Lower acceptance, but higher S/B 13 events (1 ee, 3 , 9 e) , expect 10.6 SM with 2.4 ± 0.7 events. Result: 8.4+3.2-2.7(stat) +1.5-1.1(sys) 0.5 (lumi) pb Combined result, (hep-ex/0404036, 1st Run II top paper): 7.0+2.4-2.1(stat) +1.6-1.1(sys) 0.4 (lumi) pb

  18. Dilepton cross-section: ee,mm,emfinal states (DØ) • Physics background Z/*  l+l, W+W- estimated using MC • Instrumental background determined from data: • Due to fake missing ET in ee channel • Due to isolated fake e/m in all three channels ee: 156 pb-1 em: 140 pb-1 mm: 143 pb-1

  19. Dilepton cross-section: ee,mm,emfinal states (DØ) 13.1+5.9-4.7(stat) +2.2-1.7(sys) 0.9 (lumi) pb 19.1+13.0-9.6(stat) 3.7-2.6(sys) 1.2 (lumi) pb 11.7+19.7-14.1(stat) +7.9-5.0(sys) 0.8 (lumi) pb 14.3+5.1-4.3(stat) +1.9-2.0(sys) 0.9 (lumi) pb Kinematic distributions below: Ht (left) and lepton Pt (right)

  20. Lepton+jets cross-section using event topology DØ • Signature: high-pT isolated lepton, missing ET and  4 jets • Combine topological variables in event Likelihood. Choose variables with • Good signal-to-background discrimination • Small correlations • Low sensitivity to jet energy scale (e.g. sphericity, energy ratios) • Fit data to signal and background templates  extract tt fraction -

  21. Lepton+jets cross-section using event topology DØ 8.8+4.1-3.7(stat) +1.6-2.1(sys) 0.6 (lumi) pb 6.0+3.4-3.0(stat) +1.6-1.6(sys) 0.4 (lumi) pb 7.2+2.6-2.4(stat) +1.6-1.7(sys) 0.4 (lumi) pb e+jets 141 pb-1 m+jets 144 pb-1

  22. Lepton+jets cross-section using SVX tag CDF • Analysis requirements at least 1 displaced vertex tag (SVX) • Event b-tagging efficiency ~ 55%, fake tag rate (QCD jets) ~0.5% • Main backgrounds: W + heavy flavour, W + fake tag, QCD • Count events with 3 or more jets and Ht > 200 GeV 162 pb-1 s (l+jets, SVX) = 5.6+1.2-1.0(stat) +1.0-0.7(sys) pb Double Tag Analysis Result: 5.4  2.2(stat) 1.1 (sys) pb

  23. All jets cross-section using SVX Tags (CDF) • Final state: 6 jets, 2 b-quark jets (top needle in a haystack of QCD) • Use dedicated trigger (4 jets > 15 GeV and sumEt >125 GeV) • S/B of 1/2500 increased to 1/24 with sumEt> 320 GeV and topo. cuts: aplanarity, centrality • Require 6 to 8 jets, and SVX tags • Dominant systematic uncertainty due to jet energy scale s (l+jets, SVX) = 7.8+2.5-1.0(stat) +4.7-2.3(sys) pb

  24. All jets cross-section using NN and SVX Tag (DØ) • Final state: 6 jets, 2 b-quark jets • Derive SVX tag rate function in the same multijet events. Apply to untag sample to predict background shape • Three NNs combine various kinematic variables: Ht, sphericity, aplanarity, centrality etc. • 220 observed with expected background of 186 5  12 s (l+jets, SVX) = 7.7+3.4-3.3(stat) +4.7-3.8(sys)  0.5 (lum) pb

  25. Run II Top Cross Section Summaries

  26. l W+ b-jet n 5 vertices: 20 constraints X t t jet W- jet b-jet b-jet Top mass: l + jets (template) • Perform kinematic fit: • find top mass that best fits event • loops over jet-parton assignments • Impose constraints: Mt=Mt , M(j,j) = M(l,) = MW,with inputs: MW, GW, Gt • loop over two solutions for pz of n • 2-C fit performed • Perform likelihood fit: • find top mass template that best fits data with background templates • background normalisation constrained

  27. Top mass: l + jets (template) • Choose events with 4 jets, 1 vertex tag • 28 events in 162 pb-1 with estimated bckg of 7.0 ± 0.8 • Syst. uncertainty dominated by jet-energy scale. Result: mt = 174.9 +7.1-7.7 (stat) ± 6.5 (sys) GeV

  28. Top Mass: l + jets (DLM) Dynamic Likelihood Method: Likelihood defined as ds(Mt) per unit phase volume of final partons times the transfer function (jets to partons): See original paper by K.Kondo J.Phys. Soc. 57, 4126 (1988) use 162 data sample: 22 events with 4.2 ± 0.8 background predicted.

  29. Top Mass: l + jets (DLM) Result: Systematic Uncertainties: Result: mt = 177.8 +4.5-5.0 (stat) ± 6.2 (sys) GeV

  30. Some other Run II Results: • Single top cross section (t-channel 162 pb-1) • Single top cross section (channels combined, 162 pb-1) • top mass in dilepton channel (126 pb-1) • cross section ratio s(ll)/s(lj) (125 pb-1) s < 8.5 pb @ 95% c.l s < 13.7 pb @ 95% c.l 175 ± 17stat ± 8sys GeV 1.45+ 0.83- 0.55 0.27+ 0.35- 0.21

  31. We are now improving upon many Run I measurements but we are still at a very early stage of the Run II top physics program “Precision” top quark measurements in sight at Tevatron Future looks very bright: Top factory (LHC) will turn on in a few years. Fantastic top physics to be done with ATLAS and CMS (e.g. see hep-ph/0003033) Conclusions and Outlook Some measurement targets to aim for in Run II

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