Top Quark Physics Meenakshi Narain Boston University Frontiers of Matter XIth Rencontres de Blois June 27 - July 3, 1999

1
Top Quark PhysicsMeenakshi NarainBoston
UniversityFrontiers of MatterXIth
Rencontres de BloisJune 27 - July 3, 1999
2
Outline

• Introduction
• Results from Tevatron
• Future Prospects at the Tevatron and the LHC
• Topics of discussion
• Top quark mass
• Top quark production cross sections
• Production dynamics
• Single Top quarks
• t-W-b vertex
• Top quark width
• Polarization
• Branching ratios, rare decays
• Is the Top quark special?

3
The Top Quark

• Discovery of the b quark and ??lepton
• ? existence of the top quark
• Properties of the b quark required the existence
  of the top quark
• weak isospin of b quark is measured to be -0.504
• b quark needs a weak isospin partner ? top quark
• Absence of flavor changing neutral currents in
  B-decays.
• Searches for the top quark
• ee- collisions
• 1979 - 1984 DESY (PETRA) Mtop gt 23.3
  GeV
• 1987 - 1990 KEK (TRISTAN) Mtop gt 30.2 GeV
• 1989 - 1990 SLC LEP (CERN) Mtop gt 45.8 GeV
• pp collisions SM fits (LEP SLC)
• CERN
• 1990 Mtop gt 69 GeV
• Tevatron
• 1991 gt 77 GeV
• 1992 gt 91 GeV
• 1993
• 1994 gt 131 GeV

1995 observation
4
Top Production

• Accelerator sites
• Tevatron pp collisions 0.9 ? 0.9 TeV (Run I)
• 1 ? 1
  TeV (Run II)
• LHC 7 ? 7 TeV pp collisions
• Top pair production
• s(pp tt X) ? 4.7 - 6.2 pb (Fermilab - Run
  I)
• ( Berger et al. , Bonciani et al., Laenen et
  al. , Nason et al.)
• ? 550 tt events produced (100 pb-1)
• Run II, cross sections increase by 40 as the COM
  rises from 1.8 to 2 TeV.
• ? 15,000 tt events produced (2 fb-1)
• s(pp tt X) ? 830 pb (LHC)
• ? 8 million tt pairs produced / low lumi. year
• Single Top production
• Fermilab (Run I)
• s(pp Wg ? t X) 1.7? 0.2 pb (Stelzer et
  al.)
• s(pp W ? t X) 0.72?0.04 pb (Smith et
  al.)
• LHC
• s(pp Wg ? t X) ? 250 pb
• s(pp W ? t X) ? 70 pb

5
Top Decays

• Assuming SM
• t Wb W ln or qq
• 3 generations (mt 175 GeV)
• Vtb ? 0.9991 ? 0.0002
• ? ?(t ?bW) ? 1.55 GeV
• Corresponds to a lifetime
• ?(t) ? 0.4 ? 10-24 s
• time scale for confinement 1/?QCD? few ? 10- 24
  s
• top quark decays before it can be hadronized
• tt Channels

6
Cross Section

• Measurement of ?(tt) is of interest
• test of QCD predictions
• Any discrepancy indicates possible new physics
• production via a high mass intermediate state
• Non Wb decay modes
• Measurement combines various final states
• Dilepton (ee, em, mm 2 jets missing ET)
• Lepton jets (e, m 3 or 4 jets missing ET).
• All jets (5 or 6 jets, b-tags, NN) and e/??

7
Cross Section

• Summary of top pair production measurements

D? PRL 79 1203 (1997), hep-ex/9704015 CDF PRL
80 2773 (1998)
8
Top Quark Mass

• Many Standard Model (SM) predictions depend on
  the value of the top quark mass through loop
  corrections to tree level processes e.g.
• BB mixing
• radiative corrections to W and Z mass ? (Mtop)2
• W and Z masses are
  sensitive to the
  Higgs mass ?ln(mH)
• The large mass of the top quark may well provide
  clues to the nature of electroweak symmetry
  breaking

9
Mass leptonjets

• leptonjets channel ttlnb qqb

CDF
DØ
173.35.65.5 GeV
175.94.85.3 GeV
10
Mass All Hadronic

• all hadronic channel ttqqb qqb (CDF)
• large background
• 3-constraint kinematic fit

mt 186.010.05.7 GeV
11
Mass Dileptons

• dilepton channel
• ttlnb lnb
• dynamical likelihood analysis

DØ
CDF
168.412.33.6 GeV
167.410.34.8 GeV
12
Top quark mass
Combining CDF and D? mass measurements FERMILAB-
TM-2084
13
Results from Run I

• These results and many kinematic distributions of
  top pairs (e.g. pT(top tt), ?top, ?tt,, m(tt)
  etc) are in good agreement with the Standard
  Model
• Single top production (CDF)
• ? (W-gluon) lt 15.4 pb at 95 CL
• ? (W) lt 15.8 pb at 95 CL
• B(t ? Wb) and Vtb (CDF)
• Vtb 0.99 ? 0.29 or Vtb ? 0.76 (95 CL)
• W boson helicity in top decays
• Top quark Spin Correlations (D?)
• Rare Decays

Good agreement between observation and theory.
14
Future Prospects

• Top quark is heavy!
• mass is 40x larger than the next most massive
  b-quark.
• Is this just an accident OR does it point to
  some deeper truth about the nature of electroweak
  symmetry breaking ?
• Detailed investigation of other properties
• spin correlations
• provides information on spin and existence of
  anomalous couplings of the top quark
• W-t-b coupling
• Single top production
• Electroweak coupling of the top quark
• rare decays
• probe for physics beyond SM
• production dynamics
• additional interactions?
• ? Is it standard top?

15
Future Prospects ...

• Current sample size is very small
• ? 120 events provide measurements of the
• top quark mass ? 3?
• pair production cross section ? 30?
• searches for resonances, rare decays are underway
  but cannot provide a significant result due to
  small statistics
• Further study of properties of the top quark and
  SM predictions need large samples
• ?Tevatron is the ONLY site for top quark
  production for the next 5-7 years
• until LHC The Top Factory operates!
• RunII of the Tevatron scheduled to begin in mid
  2000 will provide at least 2 fb-1 of data
• x20 more data
• upgraded detectors...
• better charged particle tracking and momentum
  measurement
• displaced vertex reconstruction (offline and
  trigger)
• better triggering on low pT muons and electrons
• helps with identifying b-quark jets

16
tt Event Yields

• tt production cross section increases by 40
  for ?s change from 1.8 to 2 TeV .
• For mt175 GeV
• Expected event yields for 2 fb-1
• channel events SB
• dilepton 200 51
• lepton³4jets 1,800
• lepton ³3jets/b-tag 1,400 31
• lepton ³4jets/dbl b-tags 450 121
• (Note b-tag ? displaced vertex and semileptonic
  tags)

s(pp tt X) ? 7.5 pb (Berger and
Contoponagos)
17
Top Quark Mass

• Precision measurement of top quark mass possible
  in both leptonjets and dilepton channels.
• Systematic effects will probably be comparable or
  better(?) in dilepton channel (currently
  dominated by statistical error)
• e.g. uncertainties (ljets channel) (in GeV)
• Run I Run II
• statistics 5.6 1.3
• jet pT scale 4.0 2.2
• MC generator 3.1 0.7 (limit?)
• MC model 1.6 0.4
• fit procedure 1.3 0.3
• Total syst 5.5 2.3
  
• Total 7.8 2.7
  
• Run II expectations
• calibrate jet pT scale using data
• Zjet, gjet, W?jj, Z?bb
• double b-tag ? reduce combinatorics
• constrain MC model using data
• Total uncertainty ? 2-3 GeV (per experiment)

18
Constraining M(Higgs)

• mt and mH affect the SM prediction for mW via
  radiative corrections
• measure mW and mt ? constrain mH
• for dmW 40 MeV and dmt 2.5 GeV constrain mH
  to 80 precision

SM predictions for mW Degrassi etal, PL B418,
209 (1998) Degrassi, Gambino, Sirlin, PL B394,
188 (1997)
19
Single Top Production

• s(pp Wg ? t X) 2.4? 0.12 pb
• (Stelzer et al.)
• s(pp W ? t X) 0.88?0.05 pb
• (Smith et al.)
• (cross sections at ?s 2 TeV)
• Provides direct access to t-W-b vertex
• Top quark width G(tX) and Vtb
• Partial width from single top cross section
• s (qq tb) µ G (t Wb)
• µ Vtb2
• Probe of anomalous couplings
• large production rates
• anomalous angular distributions

20
Single Top Production

• Events with one lepton?2jets (1 b-jet)
• Expect ? 150 events with SB 110 (2fb-1)
• Challenging measurement...
• More optimization needed e.g. HT, M(l?b)

21
Production Dynamics

• Resonances in tt production?
• Dynamical models of EWSB e.g. Top condensate,
  multiscale technicolor models imply
• color octet resonances ? tt
• with masses of several hundred GeV.
• Technicolor gg ??T? (tt, gg)
• Topcolor qq ? V8? (tt, bb)
• Look for peak in tt invariant mass (e.g.
  topcolor Z)
• Limits from RunI 500 GeV
• Limits from RunII could be 1TeV

expect 17 from tt 70 from Z 700 ? mtt ?900
Run I D?
22
tt Spin Correlation

• Significant asymmetry exists in same vs.
  opposite-spin top quark pairs ? 70 tt opposite
  helicity
• Spin correlation ? Angular correlation in q vs
  q- space. Any non-zero measurement
• Confirms top quark spin 1/2
• Proof that top-quark lifetime is shorter than
  hadronization time scale
• ?t ?1.5 GeV ?QCD ? 0.2 GeV
• Lower bound on ?t and Vtb
• Vtd2Vts2Vtb2gt(0.03)2 (assume ?3
  generations)
• Probe presence of non-standard interactions
• Angular correlation of leptons in dilepton
  events
• A 3? measurement is possible (RunII)

Correlation factor k contains all info.
23
t-W-b Vertex

• Top quarks decay before they hadronize
• polarization of W
• non-standard top couplings may result in
  different W polarization
• Charged lepton pT
• angular distribution
• â
• Longitudinal W vs.
• Left Handed Ws
• dB(tbWlong) 5

24
New Physics?

• Rare decays - SM and beyond
• Within Standard Model
• t Wb g/g
• t Wb Z Near threshold
• t Wb H0 Might be beyond
  threshold
• t W s/d Measure CKM matrix
  element
• Beyond SM Run II
• t c/u g/g (FCNC) lt 1.4 / 0.3
• t c/u Z (FCNC) lt 2
• t c/u H0 (FCNC)
• t H b (SUSY) lt 11
• SM predicts branching fractions of FCNC decays ?
  10-10
• Observation of these decays would signal new
  physics

25
Prospects at the LHC

• LHC will be a TOP FACTORY
• Enhance the detailed studies of top quark
  properties after Run II of the Tevatron.
• Probe of new physics and maybe lead to
  uderstanding of the origin of EWSB.
• Top quark events will be a calibration tool for
  LHC calorimeters
• Need to understand top production for background
  studies of new physics
• Expect (from 1 yr of low luminosity running - 10
  fb-1)
• 8 million tt events produced
• 1.4 M single leptonjets events with one b-tag
• 400 k dilepton events
• 3 million single top events produced
• 10k events after analysis selection
• Physics highlights
• Top quark mass sensitivity
• Measurement of the ttH Yukawa coupling
• Single top cross section and Vtb measurement
• Rare decays

26
Top Quark Mass

• Lepton jets events reconstruct 40k events / 10
  fb-1
• Source ?mt (GeV)
• statistical 0.06
• light quark jet scale 0.3
• b-jet scale 0.7
• b fragmentation 0.3
• initial state radiation 0.6
• final state radiation 0.4
• multiple interactions 1.0
• background 0.2
• Total 1.5 GeV
• event-by-event use m(W) to calibrate light jets
• backgrounds dominated by wrong combinations
• improve systematics by using selected sub
  samples? e.g. top events with pT(top) ? 250 GeV
• reconstruct 4.3k events per 10 fb-1 .
• tt descendants well separated in detector
• large reduction in combinatorial background
• higher pT for descendants
• reduces non-tt bkg, smaller jet corrections, less
  sensitivity to gluon radiation?

27
Top Quark Yukawa Coupling

• In the SM, fermions acquire mass via Yukawa
  couplings to Higgs field. (free parameters in the
  SM)
• for the top quark
• Large value of m(top) has generated proposals for
  alternate mechanisms (e.g. topcolor)
• A direct measurement of yt is of extreme
  interest!
• Measure yt via associated Higgs production
  (ttH)
• for m(H) ? 130 GeV, H?bb is the dominat decay
• look for events with W(?l?)W(?jj)4b-jets
• for m(H)100 GeV and data size of 100 fb-1
• N(signal) 61 events, N(bkg) 150 events
• ? yt (stat.) 12
• with 300 fb-1
• ? yt (stat.) 5
• m(H) 100 GeV
• ? yt (stat.) 10

28
Single Top Events

• Single top cross section gt 300pb
• contributions from W-gluon, W, Wt processes
• Large backgrounds from tt, Wbb processes
• ability ti extract signal depends on
• b-tagging efficiency (especially in the forward
  direction)
• fake lepton and fake b-jet reconstruction rates
• Important to separately isolate the 3 processes.
• different systematic errors for Vtb
• different sensitivities to new physics
• heavy W ?enhancement in s-channel W
• FCNC gu ?t ?enhancement in Wg fusion
• measure W and top helicities
• sensitivity to VA, anomalous couplings, CP
  violation etc
• Expected samples and sensitivities for 30 fb-1
• Process Nsig Nbkg S/B
  S/?B
• Wg fusion 27k 8.5k 3.1
  286
• Wt 6.8k 30k 0.22
  39
• s-channel 1.1k 2.4k 0.46
  23

29
Conclusions

• Much work accomplished at Tevatron (Run I) since
  the discovery of top quark in 1995. e.g.
• measurement the top quark cross section
• measurement the top quark mass
• initial studies of tt event properties
• Detailed study of top quark properties will be
  feasible during Run II, scheduled to begin in mid
  2000 (expect ? 2 fb-1 and upgraded detectors)
• The LHC top factory will significantly improve
  the precision of the measurements as well as open
  possibilities of new measurements e.g. Yukawa
  coupling, rare decays, CP violation etc.
• Impact of top quark studies
• tt cross section
• test of QCD, new interactions?
• top quark mass
• provides constraints on Higgs mass
• tt resonances
• Wtb vertex
• spin correlations
• observation of single top production
• test of electroweak coupling of the top

30
Top Production

• At the Tevatron (900 ? 900 GeV pp collisions)
  and LHC (7 ? 7 TeV pp collisions) top quark is
  produced in various ways
• Top pair production
• Single Top production

Run I
Run II
LHC

90
80
10
10
20
90
Drell Yan
W-gluon fusion
31
tt Spin Correlation

• Significant asymmetry exists in same-spin vs.
  opposite-spin top quark pairs
• expect 70 tt opposite helicity
• Polarization state is transmitted to the angular
  distribution of decay products.
• Non-zero measurement
• Confirms top quark spin 1/2
• Þ set lower limit on top quark width
• Þ probe presence of non-standard
  interactions
• Use charged lepton, lightest quark angular
  distributions in leptonjet events
• difficult to identify the down quark jet.
• OR, angular correlation of leptons in dilepton
  events.
• Possible to measure ? 3 ? effect in 2fb-1

32
b-tagging in Top Events

• Displaced vertex tagging
• 50 - 60 efficiency for b-jets in tt event
• backgrounds
• Þ 2 for Wjets
• Soft lepton tagging
• tag b-jet via its semileptonic decay b m/eX
• Soft e-tag Þ E/p helps identify soft-e inside
  b-jet
• Soft m-tag Þ extended to forward region,