# Presentation: 696932

## Top Quark Physics at the Tevatron

*Meenakshi Narain*
*Boston University*
***t****hinkshop***2**
**Fermilab, IL, Nov 10, 2000**

## Outline

- Brief History and Primer
- Results from Tevatron
- cross section
- mass
- Production and decay properties of the top
- Future Prospects

## The Top Quark

- building blocks of matter
- 1975 -  lepton Perl et al, PRL 35, 1489 (1975)
- 1977 - b quark Herb et al, PRL 39, 252 (1977)
- weak isospin = -0.504
- no flavor changing neutral currents
- need weak isospin partner  top quark
- 1995 - top quark CDF, PRL 74, 2626 (1995)
- 				      D0, PRL 74, 2632 (1995)

**e**

******e**

****

********

********

****

**u**

**d**

**s**

**t**

**c**

**b**

## The Top Quark

-         1999 Nobel Prize in Physics Poster

## The Top Quark

**1995**
**observation**
**Tevatron **

- History of :
- - direct and indirect mass measurements
- - lower limits from searches
- C. Quigg, Top-ology, Physics Today, May 1997

## Fermilab Tevatron

- central detector
- magnet
- silicon strips
- drift chamber
- calorimeter
- scintillator (central)
- gas (forward)
- muon detector

- central detector
- drift chambers
- transition radiation detector
- calorimeter
- U-liquid Ar
- uniform, hermetic
- muon spectrometer

## Top Quark Production

- Top-antitop quark pair production

- Single top quark production

- (Drell-Yan)

- (W-gluon fusion)

## Top Quark Decay

- Standard Model (with mt > MW+mb)
- expect t  Wb to dominate
- Decay channels

| t | W+ | l+ | qq | l+ | qq |
| --- | --- | --- | --- | --- | --- |
|  | b | b | b | b | b |
| t | W- | l- | l- | qq | qq |
|  | b | b | b | b | b |

## Event  Samples

- Event Requirements:
- High PT Leptons (leptonic W decay)
- Large Missing ET (neutrinos)
- 3 or more Jets with large ET.
- Jets from b-quarks
- Soft lepton tagged b-jets (CDF, DØ)
- Jet tagged in the SVX (CDF)
- Run I
- Integrated luminsoity:   120 pb-1
- Handful of tt events (100/ experiment)
- Run IIa : 2 fb-1
- Run IIb : 15 fb -1

| dilepton | 200 events |
| --- | --- |
| lepton+4jets | 1800 events |
| lepton+ 3jets/b-tag | 1400 events |
| lepton+ jets/2 b-tags | 450 events |

## Top quark production

***p***

***p***

- Production Cross Section
- Production Kinematics
- Resonance Production
- Spin Correlations
- Mass

## 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 models
- Measurement performed using various final states
- Dilepton channels
- ee, e,  and e final states
- Lepton + jets channels
- e+jets, +jets
- topological analysis
- b-tagging (using soft leptons from semileptonic decays of the b)
- All-jets channel
- Use topological variables
- exploit b-tagging using soft leptons
- Combine them using a Neural network technique

## Cross Section

- Dilepton channel
- ee, e,  + 2 jets + missing pT
- Lepton + jets channels
- e,  + 3 or 4 jets + missing pT
- All jets
- 5 or 6 jets, b-tag, neural networks

- 11 events in common

**Notes:**
all CDF: luminosity = 105.1 pb-1
CDF l+jets:
isolated lepton pT>20, |eta|<1.1 - pTmiss>20 - 3jets pT>15, |eta|<2.0 - remove lepton pairs - acceptance = 7.8+-1.1 %
SLT: lepton pT>2 inside jet pT>15 - efficiency b:7.3%, c:3.7%, fake:1.1% - 25 events, background 13.2+-1.2, xsec=9.2+-4.3 pb - backgrounds: fake tags
displaced vertex: 3 sigma = 450 micron - efficiency: b 23.1%, c: 4.0%, fake: 0.1% - 29 events, background 8.0+-1.0, xsec=5.1+-1.5 pb - backgrounds: fake tags, W+bb

CDF dilepton:
2 leptons, pT>20, |eta|<1.1, opposite sign, one isolated - not in Z mass region - pTmiss>25 - 2 jets, pT>10. |eta|<2 - backgrounds: WW, Z-tautau, fake leptons, DY, bb - 9 events, background 2.4+-0.5, xsec=8.4+4.5-3.5 pb

CDF hadronic:
5 jets, minimum separation, Sum pT>300 - either 1 SVX tag + topological cuts or 2 SVX tags - #s for 1 SVX tag: 187 events, bkg 151+-10
combined xsec = 7.6+3.5-2.7 pb

CDF combined xsec = 6.5+1.7-1.4 pb (pub: 7.6+1.8-1.5 pb)
change since pubs: b-tagging eff, luminosity (PRL 80 2773 1998)

## Cross Section

- D:  PRL _79_ 1203 (1997); CDF: PRL _80_ 2773 (1998)(+updates)

- Overall theoretical uncertainty is about 10% - 20%

## Cross Section

- Prospects for RunIIa (2 fb-1)
- Precision on top cross section ~8%
- Statistical Error : 4%
- Systematic errors assumed to scale with statistics
- errors from backgrounds:  decrease with increased statistics of control samples (2%)
- jet energy scale (2%)
- Radiation :
- Initial state (2%),
- Final state (1%)
- Limiting Factors ?
- error on geometric and kinematic acceptances  depend on differences between generators (Pythia, Herwig, Isajet) (4%)
-  luminosity error (4%)

## Top Quark Mass

- Fundamental parameter of Standard Model (SM)
- Affects predictions of SM via radiative corrections
- BB mixing
- W and Z mass
- measurements of MW, mt constrain MH
- Large mass of top quark
- Yukawa coupling  1
- may provide clues about electroweak symmetry breaking

## Top quark mass

- lepton+jets channel:
- 1 unknown (*p**z*)
- 3 constraints
*m*(*l***) = *m*(*qq*) = *m**W*
*m*(*l****b*) = *m*(*qqb*)
- 2-constraint kinematic fit
- up to 24-fold combinatoric ambiguity
- compare to MC to measure *m*t

## Complications

**Complications**

- Combinatorics:
- 4 possible j**l** pairings
- there are 12 possible assignments of the 4 jets to the 4 quarks (bbqq)
- only 6 if one of the jets is b-tagged
- only 2  for events with double b-tagged jets
- Gluon radiation
- can add extra jets

## Monte Carlo tests:
shaded plots show correct combinations
              (Herwig MC, mt = 175 GeV)

The width and shape of fitted mass distribution is due primarily to 
jet combinatorics
QCD radiation
Double b-tag helps... but, too few events in RunI

- Monte Carlo tests:
- shaded plots show correct combinations
-               (Herwig MC, mt = 175 GeV)
- The width and shape of fitted mass distribution is due primarily to
- jet combinatorics
- QCD radiation
- Double b-tag helps... but, too few events in RunI

**Complications (cont’d)**

##  In a sample of tt candidate events
For each candidate make a measurement of X = f(mt),    where X is a suitably selected estimator for the top mass, e.g. result of the kinematic fit  

This distribution contains                                     signal and background.

From MC determine shape of X as a  function of mt.
Determine shape of X for background (MC & data). 
Add these together and compare with data

- For each candidate make a measurement of X = f(mt),    where X is a suitably selected estimator for the top mass, e.g. result of the kinematic fit
- This distribution contains                                     signal and background.
- From MC determine shape of X as a  function of mt.
- Determine shape of X for background (MC & data).
- Add these together and compare with data

**The Basic procedure**

- likelihood  fit for mt.

## Top quark mass

- lepton+jets channel (DØ)

*m*t = 173.3±5.6±5.5 GeV

- dominated by
-  jet energy scale and
- gluon radiation

**Background-rich**

**Signal-rich**

## Top quark mass

- lepton+jets channel

- 173.3±5.6±5.5 GeV

- 176.1±5.1±5.3 GeV

- DØ

- CDF

## Top quark mass

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

*m*t = 186.0±10.0±5.7 GeV

## Top quark mass

- dilepton channel:
- 6 particles in the final state
- We measure 14 quantities:
- 2 charged leptons, 2jets, px()+px(), py()+py()
- 4 unknowns
- only *p**T* known
- 3 constraints
- underconstrained
- dynamical likelihood analysis

## Dilepton events (cont’d)

- Two different strategies ...
- Find a kinematic variable with a strong mass dependence, OR, _calculate a weight as a function of m__t_
- Assume mt
- 18 degrees of freedom,
-  14 known quantities + 4 constraints
- t and t momenta can be determined
- Assign a weight
- using parton distribution functions and lepton pT’s (an extension of ideas proposed by Kondo and Dalitz & Goldstein)
- Characterizes how likely this event originates from a top quark of mass mt
- Weight curve is determined for each assumed value of mt  for a given event

- K. Kondo, J. Phys. Soc. Jpn. 57, 4126 (1988) and 60, 836 (1991).
-  R.H. Dalitz and G.R. Goldstein, Phys. Rev. D 51, 4763 (1995).

## Dilepton Sample

- Weight curves:
- Assume a top quark mass & reconstruct event.
- Calculate weight curve as a function of *m**t*.
- 			Fit as a function of mt

## Top quark mass

- dilepton channel

**168.4±12.3±3.6 GeV**

**167.4±10.3±4.8 GeV**

- DØ

- CDF

- 150

- 160

- 170

- 180

- 190

- 200

- m

- t

-  (GeV)

**168.4 ******** 12.8 GeV**

**173.3 ******** 7.8 GeV**

**172.1 ******** 7.1 GeV**

**167.4 ******** 11.4 GeV**

**176.1 ******** 7.4 GeV**

**186.0 ******** 11.5 GeV**

**176.1 ******** 6.6 GeV**

**174.3 ******** 5.1 GeV**

**D0 ll PRL 80, 2063 (1998)**

**D0 lj PRD 58, 52001 (1998)**

**D0 combined**

**CDF ll PRL 82, 271 (1999)**

**CDF lj PRL 80 2767 (1998)**

**CDF jj PRL 79, 1992 (1997)**

**CDF combined**

**Tevatron **FERMILAB-TM-2084

**Top quark mass**

**Notes:**
The current best CDF Top Mass results for each channel are: 
Lepton+jets: 176.1 +/- 5.1(stat.) +/- 5.3(syst.) GeV/c2 
Dilepton: 167.4 +/- 10.3(stat.) +/- 4.8(syst.) GeV/c2 
All-Hadronic: 186.0 +/- 10.0(stat.) +/- 5.7(syst.) GeV/c2 
COMBINED CDF Result: 176.1 +/- 6.6 GeV/c2, where the uncertainty consists of the statistical and systematic uncertainties added in quadrature. The relative contributions from the three channels are 65% for lepton+jets, 16% for all-hadronic and 19% for dilepton. D0's individual Mass Results are: 
Lepton+jets: 173.3 +/- 5.6(stat.) +/- 5.5(syst.) GeV/c2 
Dilepton: 168.4 +/- 12.3(stat.) +/- 3.6(syst.) GeV/c2 
All Hadronic: none available at this time 
COMBINED D0 Result: 172.1 +/- 7.1 GeV/c2 To get the Tevatron average, we combined the five channels (three from CDF and two from D0) separately. We did NOT just combine the experiment averages. To do that, CDF and D0 had to agree on which systematic uncertainties are correlated between the experiments. All the systematics that do not depend directly on Monte Carlo modelling were treated as uncorrelated. In other words, jet energy scale, backgrounds etc. are all uncorrelated between the two experiments. The Monte Carlo model systematics (initial state radiation, final state radiation, parton distributions, etc.) were defined to be 100% correlated since CDF and D0 rely on identical Monte Carlo models. This work was done with the full cooperation of D0. The D0 contacts were Mark Strovink and Boaz Klima. COMBINED TeVatron Result: 174.3 +/- 5.1 GeV/c2 

## cross section vs. mass

- Good agreement between observation and theory.
- These results and many kinematic distributions of top pairs are in good agreement with the Standard Model

## Higgs mass constraint

- SM predictions for *m**W*
- Degrassi *etal*, PL B418, 209 (1998)
- Degrassi, Gambino, Sirlin, PL B394, 188 (1997)

- mW =  80.398±0.041 GeV (CDF, DØ, LEP2)

## Precision measurement of top quark mass 
lepton+jets channel 

dilepton channel (smaller systematics) 
Improvements:
calibrate jet pT scale using data
Z+jet, +jet, Wjj, Zbb 
constrain gluon radiation effects in MC with data
Maybe use stringent cuts to reduce effects of hard gluon radiation...
double b-tag  reduce combinatorics

Total uncertainty  2-3 GeV  (per experiment)
W mass uncertainty  40 MeV 
 constrain Higgs mass to 80%

- lepton+jets channel
- dilepton channel (smaller systematics)
- Improvements:
- calibrate jet pT scale using data
- Z+jet, +jet, Wjj, Zbb
- constrain gluon radiation effects in MC with data
- Maybe use stringent cuts to reduce effects of hard gluon radiation...
- double b-tag  reduce combinatorics
- Total uncertainty  2-3 GeV  (per experiment)
- W mass uncertainty  40 MeV
-  **constrain Higgs mass to 80%**

**Mass: Future Prospects**

|  | Run I (DØ) | Run IIa (2 fb-1) |
| --- | --- | --- |
| statistics | 5.6 GeV | 1.3 GeV |
| jet pT scale | 4.0 GeV | 2.2 GeV |
| MC generator | 3.1 GeV | 0.7 GeV |
| MC model | 1.6 GeV | 0.4 GeV |
| fit procedure | 1.3 GeV | 0.3 GeV |
| Total syst | 5.5 GeV | 2.3 GeV |
| Total | 7.8 GeV | 2.7 GeV |

- 0.5 GeV

- 1.0 GeV

- 1.6 GeV

_**w/STT**_

**Notes:**
for mW = 40 MeV and mt = 2.5 GeV constrain mH to 80% precision

## Single Top Production

- Electroweak process:
- SM cross sections
- (ppWgt+X) =  1.70.2 pb
- (Stelzer et al.)
- (ppW*t+X) = 0.720.04 pb
- (Smith et al.)
- direct access to W-tb vertex
- measure top quark width and |Vtb|
- (qq  tb)   (t  W+b)  |Vtb|2
- Measure of CKM element without any assumptions on number  of generations
- probe of anomalous couplings
- large production rates
- anomalous angular distributions

**Notes:**
(ppWgt+X) = 2.40.12 pb (Stelzer et al.)
(ppW*t+X) = 0.880.05 pb (Smith et al.)
(cross sections at s = 2 TeV)

## Single Top Production

- Event topology
- W decay products ( lepton+neutrino)  &
- s-channel  (W*)
- Two high PT, central b-jets
- t-channel (Wg)
- One high PT central b-jet (from top)
- One soft, central b-jet
- One high PT, forward light quark jet
- Backgrounds:
- Top pair production, W+jets,  Multijets
- Ability to extract signal depends on
- b-tagging efficiency (especially in the  forward direction)
- fake lepton and fake b-quark jet reconstruction rates
- Important to separately isolate the  processes.
- different systematic errors for Vtb
- different sensitivities to new physics
- measure W and top helicities
- sensitivity to V+A, anomalous couplings, CP violation etc

**Notes:**
We present preliminary results of a search for electroweak  single top quark production in 106 pb^-1 of CDF Run 1 data. We select events with a lepton, missing energy, and 1, 2, or 3 jets,  of which at least one jet is b-tagged.  We apply a cut on reconstructed mass of 140 < M_{l nu b} < 210 GeV/c^2 and fit the Ht distribution  of the remaining events.  We set an upper limit on the single top  production cross section of 13.5 pb,  roughly five and a half times larger than the Standard Model prediction of 2.43 pb.   alternative analysis:
We set an upper limit on the combined cross section of single top quark production via the s-channel W* and W-gluon fusion processes. Using the Run 1 SECVTX b-tagged W+2jets sample, we estimate a single top production cross section to be 5.2 +4.9/ -3.4(stat) +/-1.4(syst) pb and extracted a 95% C.L. upper limit of 18.6 pb on the single top production cross section. 

G. Watts:
Run 2
100-200 events per experiment
Increased data set size allows for better selection cuts
Better Signal-to-background ratio
Measurements
Partial Widths (tWX)
Vtb (12% error)
10% error)
Top Quark Polarization

## Single Top Production

-  < 13.5 pb
- at 95% CL

- CDF preliminary

**Notes:**
We present preliminary results of a search for electroweak  single top quark production in 106 pb^-1 of CDF Run 1 data. We select events with a lepton, missing energy, and 1, 2, or 3 jets,  of which at least one jet is b-tagged.  We apply a cut on reconstructed mass of 140 < M_{l nu b} < 210 GeV/c^2 and fit the Ht distribution  of the remaining events.  We set an upper limit on the single top  production cross section of 13.5 pb,  roughly five and a half times larger than the Standard Model prediction of 2.43 pb.   alternative analysis:
We set an upper limit on the combined cross section of single top quark production via the s-channel W* and W-gluon fusion processes. Using the Run 1 SECVTX b-tagged W+2jets sample, we estimate a single top production cross section to be 5.2 +4.9/ -3.4(stat) +/-1.4(syst) pb and extracted a 95% C.L. upper limit of 18.6 pb on the single top production cross section. 

G. Watts:
Run 2
100-200 events per experiment
Increased data set size allows for better selection cuts
Better Signal-to-background ratio
Measurements
Partial Widths (tWX)
Vtb (12% error)
10% error)
Top Quark Polarization

## Single Top Production

- s channel:  < 39 pb  at 95% CL
- t channel:  < 58 pb  at 95% CL

- DØ preliminary

- Search  using 92 pb-1 data from Run I for
- electroweak production of single top quarks
- Use various requirements to obtain the best significance of S/B

-  Not enough sensitivity during Run I to obtain a signal

**Notes:**
We present preliminary results of a search for electroweak  single top quark production in 106 pb^-1 of CDF Run 1 data. We select events with a lepton, missing energy, and 1, 2, or 3 jets,  of which at least one jet is b-tagged.  We apply a cut on reconstructed mass of 140 < M_{l nu b} < 210 GeV/c^2 and fit the Ht distribution  of the remaining events.  We set an upper limit on the single top  production cross section of 13.5 pb,  roughly five and a half times larger than the Standard Model prediction of 2.43 pb.   alternative analysis:
We set an upper limit on the combined cross section of single top quark production via the s-channel W* and W-gluon fusion processes. Using the Run 1 SECVTX b-tagged W+2jets sample, we estimate a single top production cross section to be 5.2 +4.9/ -3.4(stat) +/-1.4(syst) pb and extracted a 95% C.L. upper limit of 18.6 pb on the single top production cross section. 

G. Watts:
Run 2
100-200 events per experiment
Increased data set size allows for better selection cuts
Better Signal-to-background ratio
Measurements
Partial Widths (tWX)
Vtb (12% error)
10% error)
Top Quark Polarization

## Single Top Production

- Improved techniques
- Efforts underway to improve analysis techniques to increase eff and purity.
- use Neural Networks to discriminate between signal and background
- Different backgrounds have very different kinematic properties.
- Train networks (20) for each background and signal type combination separately
- Preliminary study:
- 16% efficiency for signal
- 0.7% background survives
- Application to D0 RunI data soon.
- Run II:
- Using 2 fb-1, expect to measure
- (qq  tb) to  20%
-  (t  W+b) to  25%
- Vtb to 12%
- Note single top will be a background for Higgs searches and any new physics

**Notes:**
We present preliminary results of a search for electroweak  single top quark production in 106 pb^-1 of CDF Run 1 data. We select events with a lepton, missing energy, and 1, 2, or 3 jets,  of which at least one jet is b-tagged.  We apply a cut on reconstructed mass of 140 < M_{l nu b} < 210 GeV/c^2 and fit the Ht distribution  of the remaining events.  We set an upper limit on the single top  production cross section of 13.5 pb,  roughly five and a half times larger than the Standard Model prediction of 2.43 pb.   alternative analysis:
We set an upper limit on the combined cross section of single top quark production via the s-channel W* and W-gluon fusion processes. Using the Run 1 SECVTX b-tagged W+2jets sample, we estimate a single top production cross section to be 5.2 +4.9/ -3.4(stat) +/-1.4(syst) pb and extracted a 95% C.L. upper limit of 18.6 pb on the single top production cross section. 

G. Watts:
Run 2
100-200 events per experiment
Increased data set size allows for better selection cuts
Better Signal-to-background ratio
Measurements
Partial Widths (tWX)
Vtb (12% error)
10% error)
Top Quark Polarization

## Spin Correlations

- Standard Model predicts:
- 90% of top quark pairs produced at s=1.8 TeV come from qq annihilation via spin-1 gluon
-  source of spin correlation
- width  t = (t bW)  1.55 GeV
- lifetime   4  10-25 s
- QCD time scale 1/QCD  few  10-24 s
- Top quark decays before losing the spin information at production
- spin can be reconstructed, as decay products carry spin information.
- Motivation
- Experimental proof that top-quark lifetime is shorter than spin-decorrelation time scale
- Lower bound on t and |Vtb|
- Direct probing of the properties of quark, free of QCD long-distance effects
- Probe for non-standard interactions:
- Both production and decay side

## Spin Configurations

- Optimal spin quantization basis:
- Off-diagonal
- Determined once the velocity and scattering angle is known
- Only like-spin combinations are produced in this optimized basis
- G. Mahlon and S. Parke,  PLB 411, 173 (1997)

## Differential decay rate of top quark with spin fully aligned:

To find the direction of spin:
measure angle between the off-diagonal basis and the lepton flight direction in  rest frame of the top -, +
Spin correlation  correlation in + vs - space

- To find the direction of spin:
- measure angle between the off-diagonal basis and the lepton flight direction in  rest frame of the top -, +
- Spin correlation  correlation in + vs - space

**Decay of Polarized ****Top Quark**

- correlation parameter
- SM value  0.9

## Angular Correlations

- No spin
- correlation
- =0

- Off-diagonal
- basis
- =0.87

- Helicity basis
- =0.40

- Beamline basis
- =0.68

## Spin Correlations

- 6 dilepton events
- binned 2D likelihood fit

- > -0.25 @ 68% CL

- DØ

- Fermilab-Pub-00/046-E

## Prospects for Run II

- Based on ensemble tests of 150 events
- likelihood
- One can distinguish =0 from =1 at greater than 2

- RMS=0.42

- RMS=0.43

- PDE

## Top Quark pT

- CDF preliminary

- Another tool for investigating anomalous tt production mechanisms

**Notes:**
We have investigated the kinematic properties of top quark pair production by analyzing the Run I lepton plus jets data sample. We select 61 candidate events and reconstruct these events by using a kinematic fitter that constrains the reconstructed top quark mass to its measured value. We measure and place confidence limits on the top quark pT distribution by using a likelihood technique that is designed to correct for experimental bias introduced during event reconstruction. The observed distribution is in agreement with the Standard Model prediction. We use these data to place limits on the production of high pT top quarks suggested in some models of anomalous top quark pair production. 

G. Watts:
Look for deviations in the top quark production variable pT, 
predicted by Extended Technicolor 
R4 is where most difference is expected
Use standard Lepton + Jets sample, Hadronic Top only (the two are correlated)
Unfold reconstruction smearing, Bin in true top pT
Excellent agreement with the SM

## Top-Antitop Resonances

- A general purpose tool for search for heavy objects decaying to top pairs
- Dynamical models of EWSB
- color octet resonances  tt
- mass  several hundred GeV.
- Technicolor   gg T (tt, gg)
- Topcolor      qq  V8 (tt, bb)
-  peak  in tt invariant mass

- DØ

**Notes:**
Some Technicolor theories predict existence of heavy objects that decay to tt pairs.
top gluons and a Z’ in topcolor assisted Technicolor.

## Top-Antitop Resonances

- CDF

- Use W+  4jet events
- No evidence for a deviation from expectation (KS prob 20%)
- Use tt invariant mass spectrum to set limits on  narrow Z’ resonances in topcolor models
- During RunIIa, can probe Z’  resonances up to 1 TeV.

**Notes:**
In this analysis, we examine the Run 1 t-tbar invariant mass spectrum (Mttbar) in search of non-standard model t-tbar resonance states. After establishing the primary event selection criteria, we perform a detailed combinatorics study to determine an appropriate method for measuring Mttbar. After using Monte Carlo samples to optimize a chi^2 cut which maximizes discovery potential, we apply our cuts to the Run 1 data (106 pb-1). A shape-fitting likelihood analysis is then performed to establish production cross-section limits at the 95% confidence level for X to ttbar. These limits are then compared to topcolor models of Hill et al., and exclude the existence of topcolor Z' within a range of masses in these models. 

G. Watts:
Perform binned likelihood fit using Mtt templates.
Z’ tt
SM tt
QCD W+Jets
In the region less than 650 GeV/c2
Existence of a narrow width topcolor Z’ which maximizes the predicted Z’ production cross-section is excluded.

## Top quark decay

***p***

***p***

***W****** ***

***l***

******

- W helicity

- Decay modes
- Branching ratios
- CKM matrix element |Vtb|
- Rare decays
- Non-SM decays

## W Helicity in Top Decays

- Top quarks decay before they hadronize
- polarization of W :
- non-standard top couplings may result in different W polarization
**Charged lepton p****T**** &**
**angular distribution**
**              **
**Longitudinal W vs.**
**Left Handed W’s**
- tbWlong)  5%

## W Helicity in Top Decays

- SM top (spin 1⁄2, V-A coupling)
- top quark decays to longitudinal (hW=0) or left-handed (hW=-1) W bosons
- Lepton pT distribution in *t**bl*** distinguishes the two helicity states.
- hW= 0: hard pT
- hW=-1: soft pT
- Check for V+A component
- F+1 determined by repeating fit with F0 constrained to SM value

## W Helicity in Top Decays

- F0=0.91 ± 0.37 (stat) ± 0.12 (sys)
- F+1=0.11 ± 0.15 (stat) ± 0.06 (sys)
- CDF, PRL 84, 216 (2000)

-  Use dilepton and lepton+jets tt samples:

**Notes:**
We fit the lepton Pt spectrum in ttbar events to the hypothesis that the W boson's have helicities of either -1 or 0 in the top rest frame. The fit extracts the fraction of W's which are in the 0 helicity state. The fit functions were derived from GGWIG, a modified verstion of HERWIG which allows the user to set the W helicity in top decays by hand. The background is constrained by a gaussian function whose width is given by the uncertainty on the size of the background. Here we fit the lepton Pt spectrum in ttbar events to the hypothesis that 75% of the W boson's in top decay have a helicity of 0 and that the rest have a helicity of either +1 or -1. The fit extracts the fraction of W's in the +1 helicity state. Note that the maximum possible physical value of the fraction of W's in the +1 helicity state (F+) is 0.25 because we constrain the fraction of W's in the helicity 0 state be 0.75. The standard model predicts that F+ = 0. => F+=0.11+-0.15+-0.06
G. Watts: Run 2:  ~ 6%

## |Vtb|

- |Vtb| expected to be close to 1 (, assuming 3 generations
- If 4th generation existsno constraints
- Any departure of |Vtb| from 1  indication of non standard physics
- Extract from the measurement of R
- Measure R using b-tagging in tt decays
- Count events with zero, single and double tags in in l+jets and dilepton events.
- CDF (RunI)
- R = 0.94+0.31-0.24
- IVtb| = 0.97+0.16-.12  or IVtb|  > 0.75 at 95% C.L.
- Assuming 3 generations
- IVtb| > 0.046 at 95% CL
- Without the 3 generation hypothesis
- Run II projections
- Vtb    2% 	 (2 fb-1)
- benefits from improvements in b-tagging efficiency and reduced systematic errors

- R  =

## New Physics?

- Rare decays - SM and beyond
-      Within Standard Model
*t** Wb + g*/
*t** Wb + Z            *Near threshold* *
*t** Wb + H**0**          *Might be beyond threshold
*t** W + s/d           *Measure CKM matrix element
- 	Beyond SM			Run II
*t** c/u + g*/*     (FCNC) **	**  **< 1.4% / 0.3%*
*t** c/u + Z       (FCNC)* 	  < 2%
*t** c/u + H**0**     (FCNC)*
*t** H**+** + b         (SUSY)**	** < 11%*
- Rare decays (CDF)
- BR(*t**Zq*) < 33% @ 95% CL
- BR(*t****q*) < 3.2% @ 95% CL
- Search for tH+b (DØ)
-            SM predictions for  FCNC decays  10-10
- Observation of these decays would signal new physics

## 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 dominant decay
- look for events with W(l)W(jj)+4b-jets
- A recent feasibility study finds it may be possible to carry out this measurement at the Tevatron with large data samples in RunII
- (Goldstein et al., hep-ph/0006311)

*y*

*vev*

*m**t*

## Conclusion

**Thanks to**
- CDF and DØ collaborations

- Tevatron Run I
- top pair production cross section agrees with expectations
- mt = 174.3±5.1 GeV
- is the top quark “standard”?
- Tevatron Run II
- precision studies of top quark properties