Experimental aspects of top quark physics Lecture

1
Experimental aspects of top quark physics
Lecture 3

• Regina Demina
• University of Rochester
• Topical Seminar on Frontier of Particle Physics
• Beijing, China
• 08/15/05

2
Outline

• Top charge
• W-helicity in top decays
• ttbar resonances
• Single top
• Control questions

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Top charge

• Electric charge is as fundamental quantum number
  as mass
• SM leaves little options for the predicted value
  of top-quark charge (2/3e)
• Some beyond SM models suggest that the discovered
  top quark has actually a charge of -4/3e, while
  true top quark resides at 270GeV and thus have
  eluded discovery
• Chang,Chang,Ma PhysRevD59,091503(1999)
• From the experimental point of view this
  measurement is rather challenging, so we could
  not resist

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Top charge (the method)

• Rely on charge conservation
• QtQWQb
• Q(top) - Q(antitop)
• Q (leptonic W)Q (lepton)
• Q (hadronic W) - Q (leptonic W)
• Use double-tagged sample
• Very good S/B
• 2 b-jets/event
• fewer permutations
• B-jet to W is assigned using kinematic fitting
  procedure (HITFIT), which utilizes constraints
  due to
• top mass, W-mass
• total energy-momentum conservation (zero pT of
  ttbar system)
• Use only the best solution from HITFIT

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Jet charge algorithm

• Total charge of the tracks within jet cone DR is
  correlated with the charge of the initial quark
  (b)
• Track with pTgt0.5 GeV/c and ztrk-zPVlt0.1cm,
  charge qi
• Jet charge is defined as

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Jet charge algorithm

• Optimize discriminating power
• ab mean, Vb- variance
• Optimal values of a0.6 and DR0.5 were found
  using ttbar MC

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Jet charge calibration on data

• Use double-tagged muonjet data very clean bbar
• Reconstruct jet charge on probe jet
• Use correlations with muons charge for
  calibration

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Jet charge

• Performance on data must be corrected for
• B-mixing (does not depend on pTRel)
• Sequential decays of b-hadrons (depends on pTRel)
• Contamination of ccbar (depends on pTRel)

No public result yet
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W helicity in top decays
In SM F-0.30, F0 0.70, F 0
CDF RunI result Flt0.18(95CL)
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W helicity
F lt0.24 (90 CL) with tag F lt0.24 (90 CL)
LepPt lepjets dileptons F00.270.35-0.24 F
0 lt 0.88 (95cl)
F00.890.30-0.34 (stat) _-0.17 (syst) F0gt0.25
(95CL)
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ttbar resonances in ljets with b-tag

• Check ttbar invariant mass for possible resonance
  production

DØ RunII Preliminary, 363pb-1
sNNLO(tt)6.770.42

• Events are kinematically constrained
• mT175GeV/c2
• Leptonic and hadronic W masses

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ttbar resonances in ljets with b-tag

• Limit M(Z)gt680 GeV/c2 with G1.2MZ at 95CL

DØ RunII Preliminary, 363pb-1

R. Harris, C. Hill, S. Parke hep-ph/9911288
Run I limit 560 GeV/c2
Run II limit 680 GeV/c2
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Kinematics in ljets sample
DØ RunII Preliminary, 363pb-1
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Single (weak) top production
production
production
Vtb
s-channel 0.880.07-0.06pb (NLO,
mt175GeV,sqrt(s)1.96TeV)
t-channel 1.980.23-0.18pb
t-channel sensitive to new top couplings(FCNC)
s-channel is sensitive to extra particles and
dimensions
decay
high pt lepton high missEt two bjets (1 light q)
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Singletop search
s-channel
t-channel
The signal is kinematically between Wjets and
ttbar
hep-ex/0505063
Just submitted PLB paper with most stringent
limits to date Preliminary results for this
summer, even better sslt 5.0pb stlt 4.4pb
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New Physics, New Particles?
Higgs boson h0 H0 A0 H
CMS is looking for new physics in particular the
Higgs boson, supersymmetry and One of the
promising reactions Httbar
We watch for interesting events 1 /
400,000 or 100 per second
Example of ttbar collision