Top quark properties and search for single top quark at the tevatron
Sponsored Links
This presentation is the property of its rightful owner.
1 / 34

Top Quark Properties and Search for Single Top Quark at the Tevatron PowerPoint PPT Presentation


  • 87 Views
  • Uploaded on
  • Presentation posted in: General

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)

Download Presentation

Top Quark Properties and Search for Single Top Quark at the Tevatron

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript


Top Quark Propertiesand Search for Single Top Quarkat 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)

    • 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 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

Top Width

Top Charge

W helicity

Top Spin

Top Mass

CP Violation

Anomalous Couplings

Production Kinematics

Production X-Section

Top Spin Polarization

Resonance Production

Y

|Vtb|

Rare/non SM decays

Branching Fractions


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%?

  • 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)

  • 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

hep-ex/0505091

Results consistent with the SM prediction


Exotic Decays of the top quark

  • Since R is about 1

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

  • Is X = W+ ?

    OR

  • could X =H+ ?.

    • as predicted by generic 2Higgs Doublet Models?


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 helicityTop Charge


W helicity in Top quark Decays

W-Left-Handed fractionF-

W+ Right-Handed fraction F+

W0Longitudinal fraction

F0

0

-1/2

+1

+1/2

+1/2

+1/2

W

W

b

t

t

t

b

b

W

-1/2

+1

+1/2

  • 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

V-A SUPPRESSED


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

Left-handed

Longitudinal

Right-handed


W Helicity

L=230 pb-1

hep-ex/0505031

  • 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

  • Is it the Standard Model top ?

    OR

  • 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

  • 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

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...

qB

qb

qB

qB

qb


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

    PRD 63, 014018 (2001)

    s-channel

    t-channel

    s(t-channel) (pb)

    SM

    3Dstheo

    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

    t-channel

    s-channel

    • 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

    d

    q

    t

    u

    W

    W

    q'

    b

    b

    t


    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

    (t-channel)

    Backgrounds

    • 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

    • 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

    • 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

    Likelihood

    Discriminant

    Decision Trees

    Cut-Based

    Neural Networks


    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

    Input Nodes: One for each variable xi

    full dataset

    electron

    muon

    =1 b-tag

    2 b-tags

    =1 b-tag

    2 b-tags

    Output Node: linear combination of hidden nodes

    constructnetworks

    Hidden Nodes: Sigmoid dependent on the input variables

    2d histograms, Wbb vs tt filter


    Result

    • 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

    e+m

    1 tag

    e+m

    1 tag

    e+m

    1 tag

    e+m

    1 tag


    Result

    • 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

    HT>212

    • Replace Neural Networks by Decision Trees

      • single tree, ~100 nodes

      • Remaining analysis steps identical

        • Same inputs

        • Same filter configuration

        • Binned likelihood analysis

    Fail

    Pass

    • Sensitivity comparable to Neural Network analysis

    Mt<352

    pt<31.6

    purity

    Expected/Observed limit:

    ss < 4.5 / 8.3 pb

    st < 6.4/ 8.1 pb


    4. Likelihood Discriminant Analysis

    • New Analysis based on 370pb-1 dataset

    • Different btagging algorithm and selection

    • Likelihood Discriminant:

      • Input Variables:


    Result

    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

    • 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

    using only muon channel data

    using only electron channel data


    Conclusion

    • 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.


  • Login