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

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

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 – M W /M Z ratio 95 CDF and D0 announce discovery Some History - -

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

Vtb = 0.99 branching ratios: –t W b BR ~ 1 –t W s BR ~ –t W d BR ~ 5 x –t  c,u BR ~ –t Z c,u BR ~ 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/c 2

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

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

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:

Run I Results: Production Properties 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

Run I Results: Top Mass 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

Improved Method by DØ Use Probability density: Background probability –Main component W+jets (85% of background) –P bkg calculated from leading order matrix element from VECBOS –22 events remain: 12 signal, 10 background Dominant systematic is jet energy scale: 3.3 GeV/c 2 x : reconstructed 4-vectors LO Matrix element + phase space PDF’s Transfer function: parton values to measured quantities M t = ± 3.6 (stat) ± 3.9 (syst) GeV/c 2 = ± 5.3 GeV/c 2

New Run I Top Mass Result and implications on Higgs Mass New DØ combined mass: –M t = ± 5.1 GeV/c 2 New world average: –M t = ± 4.3 GeV/c 2 Global fit to electroweak data using this top mass –Method of LEPEWWG (hep-ex ) –Best-fit M H  113 GeV/c 2 –95% C.L. upper limit 237 GeV/c 2 Yellow region excluded: M H < GeV/c CL

Other Run I Results: Single Top and Branching Ratios 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): F 0 = 0.91  0.37  0.13 F 0 = 0.56  0.31  0.04  < 17 95% c.l  < 22 95% c.l  < % c.l  < % c.l

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

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 (stat) (sys)  0.4 (lumi) pb

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

Dilepton cross-section: ee, ,e  final 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: Combined result, (hep-ex/ , 1st Run II top paper): (stat) (sys)  0.4 (lumi) pb (stat) (sys)  0.5 (lumi) pb

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/  in all three channels ee: 156 pb -1 e  : 140 pb -1  : 143 pb -1 Dilepton cross-section: ee, ,e  final states (DØ)

(stat) (sys)  1.2 (lumi) pb Dilepton cross-section: ee, ,e  final states (DØ) (stat) (sys)  0.9 (lumi) pb (stat) (sys)  0.8 (lumi) pb Kinematic distributions below: Ht (left) and lepton Pt (right) (stat) (sys)  0.9 (lumi) pb

Lepton+jets cross-section using event topology DØ Signature: high-pT isolated lepton, missing ET and  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 -

 +jets 144 pb - 1 e+jets 141 pb -1 Lepton+jets cross-section using event topology DØ (stat) (sys)  0.6 (lumi) pb (stat) (sys)  0.4 (lumi) pb (stat) (sys)  0.4 (lumi) pb

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  (l+jets, SVX) = (stat) (sys) pb Double Tag Analysis Result: 5.4  2.2(stat)  1.1 (sys) pb

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  (l+jets, SVX) = (stat) (sys) pb

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  (l+jets, SVX) = (stat) (sys)  0.5 (lum) pb

Run II Top Cross Section Summaries

Perform kinematic fit: –find top mass that best fits event –loops over jet-parton assignments –Impose constraints: M t =M t, M(j,j) = M(l, ) = M W, with inputs: M W,  W,  t –loop over two solutions for p z of – 2-C fit performed Perform likelihood fit: –find top mass template that best fits data with background templates –background normalisation constrained b-jet W+W+ W-W- t t b-jet jet X l 5 vertices: 20 constraints 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: m t = (stat) ± 6.5 (sys) GeV

Top Mass: l + jets (DLM) Dynamic Likelihood Method: Likelihood defined as d  (M t ) 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.

Top Mass: l + jets (DLM) Result: m t = (stat) ± 6.2 (sys) GeV Systematic Uncertainties: Result:

Some other Run II Results: 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  ll)/  lj) (125 pb -1 )   < % c.l   < % c.l 175 ± 17 stat ± 8 sys GeV

Conclusions and Outlook We are now improving upon many Run I measurements but we are still at a very early stage of the Run II top physics programWe 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“Precision” top quark measurements in sight at Tevatron Future looks very bright: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/ ) Some measurement targets to aim for in Run II