Searches for New Physics in the CDF Top Sample

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Searches for New Physics in the CDF Top Sample

• Robin D. Erbacher
• Fermi National Accelerator Laboratory
• (for the CDF Collaboration)

SUSY 2004 -- Tsukuba, Japan Saturday June 19,
2004
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Elucidating the Top Quark in Run 2

• Top pairs s(tt) 7 pb
• Top production rate
• Mass of top
• W helicity in top events
• QCD tests
• New physics in X? tt
• Anomalous couplings,
• new particles
• Single top s(tb) 3 pb
• Vtb
• QCD tests
• New physics?

Vtb
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Top Physics at the Tevatron
t?Wb

• Main usable top event topologies
• tt ? lnlnbb di-lepton 5 em
• tt ? lnqqbb leptonjets 30 em
• tt ? qqqqbb all hadronic 45

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ttbar Production Cross Section

• Measuring the top cross section precisely is
  important for exploring the possibilities for new
  physics
• Test of QCD Higher cross section could be a
  sign of non-Standard Model top production
  mechanisms
• Resonant state X? tt OR Anomalous couplings in
  QCD?
• It could also mean new physics is entering into
  the top sample!

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Cross Section Summary
We have many different approaches to
measuring the top cross section, allowing us to
carefully cross check the results-- and look for
anomalies.
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Run 1 Anomalies in the Top Di-lepton
Sample?
Four di-lepton candidates with very high MET.
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Run 2 Di-lepton Kinematics
Distributed as expected? More data to come
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Analysis of SM Agreement Probability Using
Kinematics
PT(lepton)
a 1.6
Topological weighting parameter (TW)
Probability(a) of CDF outcome with SM hypothesis
using four kinematic variables MET, Tw, PT,
Df(MET,l) 1.6
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Run 1 Excess in the b-tagged Lepton Jets
Sample?

• Observed excess of
• b-tags in the 2 jet bin
• Too many SVX double tags (more than one b-tagged
  jet/event)
• Too many multiple tags (more than one b-tag/jet)
• A lot of speculation,
• but nothing solid.

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Run 2 Lepton Jets Events with at Least One
b-tag
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Run 2 LeptonJets Events with Two b-tags
Very Clean Sample Number of Jets for
Double-tagged Events
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LeptonJets Cross SectionsUsing Event Kinematics
W ? 3 Jets Sample Composition WJets
75 QCD fakes 6 ttbar
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• 3 component likelihood fit
• ttbar shape from MC
• Wjets shape from MC
• QCD shape from data
• QCD from non-iso leptons

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LeptonJets Cross SectionUsing Many Kinematics
Neural Network
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Search for High ET Events
HT distribution for W4p, ttbar, and t' where
M(t')225 GeV We can set limits on new physics
processes in top sample
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Can a t' Exist?

• Z width measurement rules out a fourth generation
  with a light neutrino m(n4)ltm(Z)/2
• Even if m(n4)gtm(Z)/2, precision electroweak data
  restrict masses and couplings of new quarks and
  leptons (PDG)
• He/Polonsky/Su (hep-ph/0102144) a generic 4th
  chiral generation is consistent with EWK data
  can accommodate a heavy Higgs (500 GeV) without
  any other new physics (similarly with 2HDM)
• N2 SUSY requires three more mirror generations
  the SUSY breaking mechanism can induce
  couplings of the mirror quarks with the known
  ones
• Other models (eg Beautiful Mirrors hep-ph/
  0109097) include possibilities of a new heavy
  up-type quark decaying to Wb

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Does t' decay to Wq ?

• The most natural decay would be t' ? W b'.
  This is suppressed if M(t') lt M(b') M(W)
• There is reason to believe (from precision ewk
  data) that the mass splitting between a t' and a
  b' quark should be relatively small (PDG). So
• If M(t') lt M(b') M(W), then.
• t' ? W b (Cabibbo suppressed)
• t' ? Ws, Wd (doubly/triply Cabibbo suppressed)
• t' ? l n b' (virtual W)

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HT Plot with t' Signal, M(t') 225 GeV
Plot for fit result with t signal included at
95 CL limit s(ttbar) ?6.12 pb in this fit

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Result Limits (pb) Versus M(t')
Constraints vary with assumed top mass, but not
by much.
Expected 1s sensitivity
Mtop sconstraint
170 7.8 ? 1.0 pb
175 6.7 ? 0.9 pb
180 5.75 ? 0.7 pb
Taken from hep-ph/0303085
Limits on s?BR(t'?Wq)2
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Projected Limits with Higher Luminosity
This assumes no improvement in systematic errors

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W Helicity in Top Events
SM Only longitudinal and left-handed Ws can be
produced in the top rest frame. Use lepton pT
spectra to determine the fraction F0 of
longitudinally polarized Ws. F0 0.7 in the
Standard Model Result F0 0.27 0.35
0.21 Or F0 lt 0.88 _at_ 95 CL
The combined dilepton and leptonjets b-tagged
events plotted against the best fit.
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W Helicity in Top Dileptons
The dilepton events plotted against the best
fit. Here we see a slight 2s effect.
Result F0 -0.54 0.12 0.47 Or F0 lt
0.52 _at_ 95 CL
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Summary

• Other analyses hunting for hints of new physics
  in the
• top quark datasets
• W helicity from Run 1 about to be published (fit
  to VA fraction directly) Another Run 2
  analysis cosq
• Tests of top quark branching ratios and Standard
  Model consistency
• Ongoing search for single top in lepton jets
  events
• Tests of consistency of kinematics with the SM in
  the top quark sample
• Search for top decaying to charged Higgs

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Conclusions
As we increase our datasets at the Tevatron in
Run 2, CDF will have much to say about the
possibility of new physics in our top quark
samples. Stay tuned CDF top quark publications
are beginning to roll
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Searches for Single Top
t-Channel Search s(t-chan) lt 8.5 pb _at_ 95 C.L.
Combined Channel Search s(st) lt 13.7 pb _at_ 95
C.L.
We are still looking for single top!
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New Top Mass World Average

• 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 mas
• 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
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Current Top Mass Results
LeptonJets Mass using SecVTX b-tagging
LJ Dynamic Likelihood Method (Kondo et al)
mt 174.9 7.1-7.7 (stat) 6.5 (sys) GeV
mt 177.8 4.5-5.0 (stat) 6.2 (sys) GeV