Top quark properties and search for single top quark at the tevatron
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Top Quark Properties and Search for Single Top Quark at the Tevatron. Meenakshi Narain Boston University Presented at EPS 2005. Top Quark at the Tevatron. 8.2 fb -1. Design. 5.1 fb -1. 4.1 fb -1. We are here. Base. Top quark discovered a decade ago (in 1995)! Run I (1992-1996)

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Top quark properties and search for single top quark at the tevatron

Top Quark Propertiesand Search for Single Top Quarkat the Tevatron

Meenakshi Narain

Boston University

Presented at EPS 2005

Top quark at the tevatron

Top Quark at the Tevatron

8.2 fb-1


5.1 fb-1

4.1 fb-1

We are here


  • Top quark discovered a decade ago

    • (in 1995)!

  • Run I (1992-1996)

    • s = 1.8 TeV

    • Integrated luminosity

      • 120 pb-1

  • Run II (2001-present)

    • s = 1.96 TeV

    • 3 fold increase performance since June’03

    • Integrated luminosity by June ‘05:

      • Delivered >1fb-1

      • On tape ~800pb-1

      • Analyzed up to ~350 pb-1

World’s only top factory!

Top quark physics

Top Quark Physics

  • Top is very massive

    • It probes physics at much higher energy scale than the other fermions.

  • Top decays before hadronizing

    • momentum and spin information is passed to its decay products.

    • No hadron spectroscopy.

  • Top mass constrains the Higgs mass

    • Mtop, enters as a parameter in the

      calculation of radiative corrections to other

      Standard Model observables

    • it is also related, along with the mass of

      the W boson, to the that of the Higgsboson.

Mtop (world average)= 172.7  2.9 GeV

top ~ 10-24 sec

The top properties tour

The Top Properties Tour

Top Width

Top Charge

W helicity

Top Spin

Top Mass

CP Violation

Anomalous Couplings

Production Kinematics

Production X-Section

Top Spin Polarization

Resonance Production



Rare/non SM decays

Branching Fractions

Top quark decay properties

Top Quark Decay Properties

  • Does top quark decay 100% of the times to Wb?

    • B(t Wb)

  • Search for exotic decay modes of the top quark

    • t H+b

  • Properties of the W-t-b vertex

    • W Helicity

    • Top quark Charge

Is b t wb 100

Is B(t Wb) ~ 100%?

  • Within the SM, assuming unitarity of the CKM matrix, B(tWb)~1.

  • An observation of a B(tWb) significantly different than unity would be a clear indication of new physics:

    • non-SM top decay, non-SM background to top decay, fourth fermion generation,..

Measurement of b t wb b t wq

Measurement of B(tWb)/B(tWq)

  • B(tWb) can be accessed directly in single top production.

    Top decays give access to B(tWb)/B(tWq):

  • R can be measured by comparing the number of ttbar candidates with 0, 1 and 2 jets tagged.

    • In the 0-tag bin, a discriminant variable exploiting the differences in event kinematics between ttbar and background is used.

In the SM

Lepton+jets (~230 pb-1)

Lepton+jets and dilepton (~160 pb-1)

DØ Run II Preliminary


Results consistent with the SM prediction

Exotic decays of the top quark

Exotic Decays of the top quark

  • Since R is about 1

    • Top quark decays to a b-quark  t X+b

  • Is X = W+ ?


  • could X =H+ ?.

    • as predicted by generic 2Higgs Doublet Models?

Search for t h b

Search for tH+b

  • If MH±<mt-mb

    • then t H+b competes with t W+b

    • results in B(tWb)<1.

  • H decays are different than W decays

    • affect (tt) measurements in different channels (dileptons, lepton+jets, lepton+tau).

  • Perform simultaneous fit

    • to the observation in all channels and

    • determine model-dependent exclusion region in (tan, MH±).

Are the other properties of the top quark as expected w t b vertex w helicity top charge

Are the other Properties of the Top Quark as Expected?W-t-b Vertex: W helicityTop Charge

W helicity in top quark decays

W helicity in Top quark Decays

W-Left-Handed fractionF-

W+ Right-Handed fraction F+

W0Longitudinal fraction




















  • Large top quark mass:

    • Are there new interactions at energy scales near EWSB?

  • helicity of the W boson:

    • examines the nature of the tbW vertex

    • provides a stringent test of Standard Model

V-A coupling

gWtb  |Vtb| (V-A)

F0= 0.7 F-= 0.3 F+=0


W helicity

W helicity

  • In the Standard Model (with mb=0):

  • The PT of the lepton has information about the helicity of the W boson:

    • longitudinal: leptons are emitted perpendicular to the W (harder lepton PT)

    • left-handed: leptons are emitted opposite to W boson (softer lepton PT)

SM: F-= 0.3 F0= 0.7 F+=0




W helicity1

W Helicity

L=230 pb-1


  • Likelihood analysis of PT spectrum

  • Consider dilepton channels

  • Fix F0=0.7, measure F+ (F-=1-F0-F+)

  • Binned likelihood and estimate F+ using Bayesian method

    Run I best (DØ 125 pb-1): F0=0.560.31

  • Likelihood analysis of cos *

  • Consider lepton+jets channels

  • Fix F0=0.7, measure F+ (F-=1-F0-F+)

  • Two analysis: topological and b-tag

    Run I best (CDF 109 pb-1): F+<0.18 @ 95% CL

Lepton+jets (1 b-tag)

Results consistent with the SM prediction: F0=0.7, F+=0

Measurement of top quark charge

Measurement of top quark Charge

  • Is it the Standard Model top ?


  • An exotic doublet of quarks (Q1, Q4)

    • with charges (-1/3,-4/3) and M ~ 175 GeV/c2

    • while M(top) ~ 274 GeV/c2

      • W.-F. Chang et al.,hep-ph/9810531

  • q = -4/3 is consistent with EW data,

    • new b-couplings improve the EW fit

      • (E. Ma et al. , hep-ph/9909537)

Top quark charge measurement

Top Quark Charge Measurement

  • Goal: discriminate between

    • |Qtop| = 2e/3 and |Q”top”| = 4e/3

  • Top quark charge is given by the sum of the charge of its decay products

  • Determine:

    • Charge of W (lepton)

    • Charge of b-jet Qjet = qi pTia/  pTia

      • (here, a=0.6)

    • Associate b-jets to correct W (charged lepton)

  • The charge of the quark is correlated with the charge of the highest pT hadron during hadronization

Top quark charge

Top Quark Charge

qB = b hadr. side

qb = b lept. side

qb = b lept. side

qB = b hadr. side

  • We need an observable and an expectation for the ”2/3” and ”4/3” scenarios

  • Consider only lepton+jets double-tagged events

    • Two top quarks in the event  measure the charge ”twice”

  • The exotic scenario is obtained by permuting the charge of the tagged jets

  • qb and qB are taken from the data derived jet charge templates

  • Results coming soon...






Top quark production properties

Top Quark Production Properties

  • Since top decay properties look quite consistent with SM predictions….

  • What about its production?

    • Could it be a “t-prime”?

      • Search for t’t’ production (t’ Wq)

  • Could the ttbar pair originate from the decay of a resonance?

    • Model independent search for narrowresonance Xtt used to exclude a leptophobic X boson:

  • What about single top production?

  • Run I search of X with G=1.2%M:

    MX>560 GeV @ 95% CL (DØ) and MX>480 GeV @ 95% CL (CDF)

    Search for single top quark

    Search for Single Top Quark

    Search for single top

    Search for Single Top

    PRD 63, 014018 (2001)



    s(t-channel) (pb)



    sNLO = 0.88pb ± 8%

    sNLO = 1.98pb ± 11%

    s(s-channel) (pb)

    hep-hp/207055 (Harris, Laenen, Phaf, Sullivan, Weinzierl)

    • Electroweak Production of top quark:

      • Measure production cross sections

      • Direct measurement of |Vtb| (s  |Vtb|2)

      • Top spin physics (~100% polarized top quark)

      • s- and t-channels sensitive to different New Physics

      • Irreducible background to associated Higgs production

      • Exotic Models (FCNC, Top Flavor, 4th Gen)

    Single top status

    Single Top Status



    • Cross sections:s-channel t-channel s+t

      • NLO calculation: 0.88pb (±8%) 1.98pb (±11%)

      • Run I 95% CL limits, DØ: < 17pb < 22pbCDF: < 18pb < 13pb < 14pb

      • Run II CDF 95% CL limits: < 14pb < 10pb < 18pb

    • Other Standard Model production mode (Wt) negligible











    Signature backgrounds

    Signature & Backgrounds

    Signal for s and t channel mostly similar

    • Lepton + Missing ET + Jets

    • t-channel extra b tends to be forward

    • Similar to top pair production, but with less jets

    Harder Signal To Find



    • W/Z + jets Production

    • Fake Leptons

    • Top Pair Production

    • WW, WZ, Ztt, etc.

    Much worse than for pair production because of lower jet multiplicity

    Anything with a lepton + jets + ET signature

    Discriminating variables

    Discriminating Variables

    • Object kinematics

      • Jet pT for different jets

        • Tagged, untagged,...

    • Event kinematics

      • H (total energy)

      • HT (transverse energy)

      • M (invariant mass)

      • MT (transverse mass)

      • Summing over various objects in the event

    • Angular variables

      • Jet-jet separation

      • Jet pseudorapidity (t-channel)

      • Top quark spin

    Separating signal from backgrounds

    Separating Signal from Backgrounds

    • Four analysis methods

    • Three (Cut, NN, DT) use the same structure:

      • Optimize separately for s-channel and t-channel

        • Optimize separately for electron and muon channel (same variables)

      • Focus on dominant backgrounds: W+jets, tt

        • W+jets – train on tb-Wbb and tqb-Wbb

        • tt – train on tb – tt  l + jets and tqb – tt  l + jets

      • Based on same set of discriminating variables

      • 8 separate sets of cuts/networks/trees



    Decision Trees


    Neural Networks

    1 cut based analysis

    1. Cut-Based Analysis

    • Cuts on sensitive variables to isolate single top

      • Separate optimizations for s-channel and t-channel

      • Loose cuts on energy-related variables:pT (jet1tagged) H(alljets – jet1tagged)H(alljets – jet1best)HT (alljets)M(toptagged)M(alljets)M(alljets – jet1tagged)ŝ

    Factor 2 improvement!

    2 neural network analysis

    2. Neural Network Analysis

    Input Nodes: One for each variable xi

    full dataset



    =1 b-tag

    2 b-tags

    =1 b-tag

    2 b-tags

    Output Node: linear combination of hidden nodes


    Hidden Nodes: Sigmoid dependent on the input variables

    2d histograms, Wbb vs tt filter



    • No evidence for single top signal

      • Set 95% CL upper cross section limit

      • Using Bayesian approach

      • Combine all analysis channels (e, m, =1 tag, 2 tags)

      • Take systematics andcorrelations into account

    Systematic uncertainty:

    Expected limit: set Nobs to background yield

    Expected/Observed limit:

    ss < 9.8 / 10.6 pb

    st < 12.4 / 11.3 pb

    Neural network output

    Neural Network Output


    1 tag


    1 tag


    1 tag


    1 tag



    • No evidence for single top signal

      • Set 95% CL upper cross section limit

      • Using Bayesian approach and binned likelihood

        • Built from 2-d histogram of Wbb NN vs tt NN

        • Including bin-by-bin systematics and correlations

    Expected/Observed limit:

    ss < 4.5 / 6.4 pb

    st < 5.8 / 5.0 pb

    3 decision tree analysis

    3. Decision Tree Analysis


    • Replace Neural Networks by Decision Trees

      • single tree, ~100 nodes

      • Remaining analysis steps identical

        • Same inputs

        • Same filter configuration

        • Binned likelihood analysis



    • Sensitivity comparable to Neural Network analysis




    Expected/Observed limit:

    ss < 4.5 / 8.3 pb

    st < 6.4/ 8.1 pb

    4 likelihood discriminant analysis

    4. Likelihood Discriminant Analysis

    • New Analysis based on 370pb-1 dataset

    • Different btagging algorithm and selection

    • Likelihood Discriminant:

      • Input Variables:



    Expected/Observed limit:

    ss < 3.3 / 5.0 pb

    st < 4.3/ 4.4 pb

    Best Limit !!!!

    Comparison of LH with NN analysis

    Single top summary

    Single Top Summary

    • Cross sections:s-channel t-channel s+t

      • NLO calculation: 0.88pb (±8%) 1.98pb (±11%)

      • Run I 95% CL limits, DØ: < 17pb < 22pbCDF: < 18pb < 13pb < 14pb

      • Run II CDF 95% CL limits: < 14pb < 10pb < 18pb

      • RunII DØ 95% Cl Limits:

        (230 pb-1)

        Cut Based < 10.6pb < 11.3pb

        Decision Tree < 8.3pb < 8.1pb

        Neural Network* < 6.4pb < 5.0pb

        (370 pb-1) (new analysis)

        Likelihood Discriminant< 5.0pb < 4.4pb

    * = Accepted for publication, hep-ex/0505063

    Sensitivity to non sm single top

    Sensitivity to non-SM Single Top

    using only muon channel data

    using only electron channel data



    • Measurements of various top quark properties are underway and will improve with larger data sets

    • The single top cross section limits and sensitivity of the analyses are getting to a level where we can expect to observe single top quark production soon!.

      Stay Tuned.

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