Top Quark Physics

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
• Run I Results
• Run II Results
• cross section measurements
• mass measurements
• other results
• Conclusions and Outlook

3
Some History

• 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 DØ announced discovery

-
-
4
Top Quark in the SM (I)

• If we assume CKM matrix unitarity and
  measured mass 180 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!)

Mt at the ewk scale Only quark decaying to
W Large Mt-Mb difference implies large ewk loop
corrections
5
Top Quark in the SM (II)
-

• Experimental tt signatures depend on how W
  decays

• 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
  

70 of W bosons longitudinally polarised for Mt
of 175 GeV/c2
6
Top Quark Beyond the SM

• 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

7
Experimental Top Quark Physics
Tevatron Collider is worlds 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 DØ detectors underwent major upgrades for
  Run II

8
Run I 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)

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 DØ ljet result using matrix element
  technique

11
Improved Method by DØ

• Use Probability density
• Background probability
• Main component Wjets (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
PDFs
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 lt 114.4 GeV/c2 _at_95 CL
13
Other Run I Results Single Top and Branching
Ratios

• Single top DØ cross section (s-channel)
• Single top DØ cross section (t-channel)
• Single top CDF cross section (s-channel)
• Single top CDF cross section (t-channel)
• Fraction of longitudinal W bosons (DØ)
• Fraction of longitudinal W bosons (CDF)
• Branching ratios (CDF)

s lt 17 pb _at_ 95 c.l
s lt 22 pb _at_ 95 c.l
s lt 15.8 pb _at_ 95 c.l
s lt 15.4 pb _at_ 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 7 pb

15
Dilepton cross-section leptontrack (CDF)

• Signature 1 lepton 1 isolated track, missing
  ET , 2 central jets
• Higher acceptance reduced purity relative to Run
  1,
• Backgrounds Z/ ? ? ll-, WW, WZ, ZZ, Wjets
  (fake leptons)

Measured cross-section for different jet ET and
track pT
19 events on 7.1 1.2 background 11 e-track, 8
?-track
7.02.7-2.3(stat) 1.5-1.3(sys) ? 0.4 (lumi) pb
16
Lepton track Kinematics
RunI had seen hints of discrepancy in kinematic
distribution
HtScalar 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,em 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
8.43.2-2.7(stat) 1.5-1.1(sys) ? 0.5 (lumi) pb
Combined result, (hep-ex/0404036, 1st Run II top
paper)
7.02.4-2.1(stat) 1.6-1.1(sys) ? 0.4 (lumi) pb
18
Dilepton cross-section ee,mm,em final states (DØ)

• Physics background Z/? ? ll, WW- estimated
  using MC
• Instrumental backgrounds determined from data
• fake missing ET in ee channel
• 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,em final states (DØ)
13.15.9-4.7(stat) 2.2-1.7(sys) ? 0.9 (lumi) pb
19.113.0-9.6(stat) 3.7-2.6(sys) ? 1.2 (lumi) pb
11.719.7-14.1(stat) 7.9-5.0(sys) ? 0.8 (lumi)
pb
14.35.1-4.3(stat) 1.9-2.0(sys) ? 0.9 (lumi) pb
Kinematic distributions below Ht (left) and
lepton Pt (right)
20
Leptonjets 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
Leptonjets cross-section using event topology DØ
8.84.1-3.7(stat) 1.6-2.1(sys) ? 0.6 (lumi) pb
6.03.4-3.0(stat) 1.6-1.6(sys) ? 0.4 (lumi) pb
7.22.6-2.4(stat) 1.6-1.7(sys) ? 0.4 (lumi) pb
ejets 141 pb-1
mjets 144 pb-1
22
Leptonjets 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 gt 200 GeV

162 pb-1
s (ljets, SVX) 5.61.2-1.0(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 gt 15 GeV and sumEt
  gt125 GeV)
• S/B of 1/2500 increased to 1/24 with sumEtgt 320
  GeV and topo. cuts aplanarity, centrality
• Require 6 to 8 jets, and SVX tags
• Dominant systematic uncertainty due to jet energy
  scale

s (all jets, SVX) 7.82.5-1.0(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
• Require 1 SVT tag rate and use untagged 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 (all jets, SVX) 7.73.4-3.3(stat)
4.7-3.8(sys) ? 0.5 (lum) pb
25
Run II Top Cross Section Summaries
26
Top mass l jets template (CDF)

• Perform kinematic fit
• find top mass that best fits event
• loops over jet-parton assignments
• Impose constraints MtMt , 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/pb 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 (CDF)

• 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)
• Longitudinal W fraction (162-193 pb-1)

s lt 8.5 pb _at_ 95 c.l
s lt 13.7 pb _at_ 95 c.l
175 17stat 8sys GeV
1.45 0.83 - 0.55
0.27 0.35 - 0.21
31
Conclusions and Outlook (I)

• New Run I measurement from D0 shifts world
  average top mass to 178 GeV/c2
• Run II measurements up to date are consistent
  with SM expectations for Mt 180 GeV/c2
• 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

32
Conclusions and Outlook (II)

• 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)

Some measurement targets to aim for in Run II