Searches for Anomalous Top Quark Production at the Tevatron

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Searches for Anomalous Top Quark Production at
the Tevatron

• Andrew Ivanov
• University of California, Davis
• for the CDF and D0 Collaborations

Top Quark Symposium April 08, 2005
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Why the top quark is so special

• Mass of order electroweak scale
• May play a special role in the dynamics of EWSB
• Serves as a powerful probe of physics beyond the
  SM

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Is the top quark ordinary or exotic ?

• Exotic in some way ?! New Physics might reveal
  itself through
• Oblique radiative corrections
• Exotic top quark decay modes (scalar bosons,
  FCNC)
• Exotic intermediate heavy states new gauge
  bosons or resonances
• Ordinary ? Becomes a major background to new
  physics
• Top sample might contain an admixture of exotic
  processes

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Is the top quark ordinary or exotic ?

• Exotic in some way ?! New Physics might reveal
  itself through
• Oblique radiative corrections
• Exotic top quark decay modes (scalar bosons,
  FCNC)
• Exotic intermediate heavy states new gauge
  bosons or resonances
• Ordinary ? Becomes a major background to new
  physics
• Top sample might contain an admixture of exotic
  processes

Focus of this talk
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Outline Searches for

• Resonance in mass of ttbar system
• Top quark pT
• Anomalous kinematics in top dilepton sample
• Anomalous kinematics in lepton jets sample
• Top Charge

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Why search for structure in the Mtt spectrum?

• Various exotic models predict the existence of
  particles decaying to tt
• Topcolor-Assisted Technicolor
• (Hill, Phys Lett. B345, 483 (1995) Hill
  and Parke Phys. Rev. D49, 4454 (1994))
• Extends technicolor models and attempts to
  explain EWSB by introducing a new strong
  interaction
• Predicts new massive bosons topgluons and a
  topcolor Z

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Run 1 Search for tt Resonances

• Each leptonjets event fit to ttbar hypothesis by
  constraining W and top masses
• Distribution smeared by
• ambiguity in correct jet-parton assignment
• detector resolution
• B-tagging reduces combinatorics
• Select configuration with best ?2 fit

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Run 1 Search for tt Resonances
Dominant background is from W jets
Likelihood fit is performed using X-gttt templates
with various masses
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No Evidence for tt Resonance in Run 1
Phys.Rev.Lett. 92, 221804 (2004)
Phys.Rev.Lett. 85, 2062 (2000)
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Run 1 Limits on Mass of the Topcolor Boson
Model independent search for a narrow resonance
X?tt exclude a narrow, leptophobic X boson with
a natural width of GX 0.012 MX MX lt 560
GeV/c2 and MX lt 480 GeV/c2
MX lt 780 GeV/c2 for GX 0.04 MX
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Run 2 Search continues

• MX 500 GeV/c2
• GX 0.012 MX
• ? 1pb

MX 700 GeV/c2
SM ttbar
W gt 3 jets
Monte Carlo

• Use Neural Nets trained with different X
  masses
• Use large statistics - lepton gt3 jets
  events sample (no b-tag information)

Fit Mtt distribution to combination of shapes
(lepton jets, use b-tag)
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Run 2 Search for Resonances Using Matrix Element
Reconstructed events
Generator Level
better reconstruction

• Employ matrix element (similar to the top
  mass measurement) information to weigh each
  parton configuration for direct Mtt
  reconstruction

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Why search for structure in top quark pT spectrum
?

• Many exotic models predict sizeable
  enhancements in the tt cross section at high
  transverse momenta pT gt 200 GeV/c
• (T.G. Rizzo , hep-ph/9902273 K. Lane,
  Phys. Rev. D52, 1546 (1995))
• e.g. Anomalous chromomagnetic moment

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Run 1 Top Quark pT Spectrum
Response functions

• Use an iterative procedure to unsmear the
  distribution of reconstructed top pT momentum
• Perform a likelihood fit to extract true top pT
• Use superposition of response functions in 4 bins
  of pT and background template

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Run 1 Top Quark pT Spectrum
Top Quark pT Spectrum
Upper limit R4(225ltPtlt425GeV/c)lt0.16 _at_ 95 C.L.
CDF Run 1
Phys.Rev.Lett. 87, 102001 (2001)
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Run 2 Search Top Quarks Kinematics

• Variables of interest
• hadronic/leptonic top pT,
• ttbar system pT and??,
• ?????? of the two top quarks,
• Additional jets are source of combinatorial
  background

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Run 1 Anomalies in the Top Dilepton Sample?
1 ev. 1 ev. 7 ev.
Flavor asymmetry an excess of em events
An excess of events with large missing
transverse ET and lepton pT
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A Subset of Run 1 Dilepton Events
hep-ph/9609313
Barnett and Hall, Phys. Rev. Lett. 77 3506
(1996)
tt MC SUSY MC
Data
Stnd Model
D0
SUSY ???
CDF
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SM Kinematic Test with Run 2 Dilepton Events

• Stay model-independent
• Choose a-priori potentially sensitive to new
  physics kinematic variables
• Perform Kolmogorov-Smirnov consistency test
  between data and Monte Carlo expectation
• Search for and isolate a subset with the most
  non-SM features (SUSY events?!)

Sensitivity given 13 events observed (kinematics
only)
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Kinematics in Run 2 Top Dilepton Sample
Missing ET
Leading lepton pT
?
Df (leading lepton, met)
topness ttbar decay goodness-of-fit
more top-like
Overall agreement of 1.0-4.5 mainly due to an
excess at low lepton pT
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Run 2 Dilepton events
Phys.Rev.Lett. 93, 142001 (2004)
FERMILAB-PUB-04-396-E ( Subm. to Phys.Rev.Lett.)
1 ev. 3 ev. 9 ev.
topness
Low pT-lepton events are accompanied with b-jets
- likely being from tt
Flavor asymmetry remains?!.. More data to come
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New Particles Decaying to Wq?

• Can be a fourth generation up-type quark
• He/Polonsky/Su (hep-ph/0102144)
• a generic 4th chiral generation is
  consistent with EWK data accommodates a heavy
  Higgs (500 GeV) without any other new physics
• Beautiful Mirrors model
• Wagner et al (hep-ph/ 0109097)
• predicts a new heavy up-type quark decaying
  to Wb naturally accomodates the LEP b
  forward-backward asymmetry results
• From the precision EWK data the mass splitting
  between a t' and a b' quark is relatively small.
  Therefore if Mt lt Mb MW t' -gt l ? b'
  (virtual W)

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Run 2 Search for t-gtWq

• Lepton jets channel
• Use the variable HT sum of transverse momenta
  of all objects in the event
• Sensitive to any new high-pT physics
• Perform a likelihood fit and set a limit on
  non-SM processes

Data does not prefer t contribution

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Projected Limits Higher Luminosity / New NN
Analysis
HT likelihood sensitivity study

• Use Neural Nets trained with different t
  masses
• Perform likelihood fit with the NN output
  variable

W gt 3 jets
ttbar
t-gtWb

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Top Charge

• Is it the Standard Model top ?
• W.-F. Chang et al.,hep-ph/9810531 proposes an
  exotic doublet of quarks (Q1, Q4) with charges
  (-1/3,-4/3) and M 175 GeV/c2
• while Mtop 274 GeV/c2
• q -4/3 is consistent with EW data, new
  b-couplings improve the EW fit (E. Ma et al. ,
  hep-ph/9909537)

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Top Quark Charge Measurement

• Determine
• charge of W (lepton)
• pairing between W and b
• flavor of b-jet
• Use both dilepton and lepton jets samples
• Variables to use for flavor b-tagging
• sign of max pT track in b-jet cone
• jet charge - sum of charge for tracks inside
  of the cone
• sign of min pT track
• sign of fragmentation track
• Expect with 500 pb-1 to rule out q -4/3

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Top Quark Charge Measurement
Sensitivity Study Limit depends on lepton-
b-jet mismatching (fraction of mismatches)

• Jet Charge Algorithm Qjet ??qi pTia/ ?
  pTia

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Near Future is Very Exciting ...

• Current Run 2 top analyses in progress utilize
  datasets of integrated luminosity 340 pb-1
• Results with this amount of data are expected
  to be reported at the Summer conferences
• A lot of work was performed on reducing
  systematic uncertainties in the past months
• Expect better limits and more precise
  measurements very soon
• Many new analyses testing various top
  properties are in preparation

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More Distant Future is Even More Exciting

• More data is on tape
• Peak Luminosity beats new records every day
• Improved trigger system guarantees a higher
  purity data
• Stay Tuned