experimental aspects of top quark physics lecture #3 regina demina university of rochester topical...
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Experimental aspects of top Experimental aspects of top quark physics quark physics
Lecture #3Lecture #3
Regina Demina
University of Rochester
Topical Seminar on Frontier of Particle Physics
Beijing, China08/15/05
08/15/05 Regina Demina, Lecture #3 2
OutlineOutline
• Top charge
• W-helicity in top decays
• ttbar resonances
• Single top
• Control questions
08/15/05 Regina Demina, Lecture #3 3
Top chargeTop 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
08/15/05 Regina Demina, Lecture #3 4
Top charge (the method)Top charge (the method)• Rely on charge conservation
– Qt=QW+Qb
– 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
Main question: is how to determine the charge of the b-quarkOr rather how to distinguish b-quark from anti-bJet-charge algorithmJet-charge algorithm
08/15/05 Regina Demina, Lecture #3 5
Jet charge algorithmJet charge algorithm• Total charge of the tracks within jet cone DR is
correlated with the charge of the initial quark (b)
• Track with pT>0.5 GeV/c and |ztrk-zPV|<0.1cm, charge qi• Jet charge is defined as
i
aTi
i
aTii
jetp
pqq
08/15/05 Regina Demina, Lecture #3 6
Jet charge algorithmJet charge algorithm• Optimize discriminating power:
• ab mean, Vb- variance
• Optimal values of a=0.6 and DR=0.5 were found using ttbar MC
bb
bb
VV
aaD
08/15/05 Regina Demina, Lecture #3 7
Jet charge calibration on dataJet charge calibration on data
• Use double-tagged muon+jet data – very clean bbar
• Reconstruct jet charge on probe jet
• Use correlations with muon’s charge for calibration
08/15/05 Regina Demina, Lecture #3 8
Jet chargeJet charge
• Performance on data must be corrected for– B-mixing (does not depend on pT
Rel)
– Sequential decays of b-hadrons (depends on pTRel)
– Contamination of ccbar (depends on pTRel)
No public result yet…
08/15/05 Regina Demina, Lecture #3 9
W helicity in top decaysW helicity in top decays
In SM F-=0.30, F0 =0.70, F+ =0
CDF RunI result:
F+<0.18(95%CL)
08/15/05 Regina Demina, Lecture #3 10
W helicityW helicityF+ <0.24 (90% CL) with tagF+ <0.24 (90% CL)
F0=0.89+0.30-0.34 (stat) _-0.17 (syst)F0>0.25 (95%CL)
LepPt: lep+jets + dileptonsF0=0.27+0.35-0.24F0 < 0.88 (95%cl)
CDF
CDF
08/15/05 Regina Demina, Lecture #3 11
ttbar resonances in ttbar resonances in l+jetsl+jets with b-tag with b-tag
• Check ttbar invariant mass for possible resonance production
DØ RunII Preliminary, 363pb-1
• Events are kinematically constrained – mT=175GeV/c2
– Leptonic and hadronic W masses
NNLOtt)=6.77±0.42
08/15/05 Regina Demina, Lecture #3 12
ttbar resonances in ttbar resonances in l+jetsl+jets with b-tag with b-tag
• Limit M(Z’)>680 GeV/c2 with =1.2%MZ’ at 95%CL
*R. Harris, C. Hill, S. Parke hep-ph/9911288
DØ RunII Preliminary, 363pb-1
*
Run I limit 560 GeV/c2
Run II limit 680 GeV/c2
08/15/05 Regina Demina, Lecture #3 13
Kinematics in l+jets sampleKinematics in l+jets sampleDØ RunII Preliminary, 363pb-1
08/15/05 Regina Demina, Lecture #3 14
Single (weak) top productionSingle (weak) top production
t-channel1.98+0.23-0.18pb
s-channel0.88+0.07-0.06pb(NLO, mt=175GeV,sqrt(s)=1.96TeV)
high pt leptonhigh missEttwo bjets(+1 light q)
s-channel is sensitive toextra particlesand dimensions
t-channel sensitive tonew top couplings(FCNC…)
productionproduction
decay
Vtb
08/15/05 Regina Demina, Lecture #3 15
Singletop search
s-channelt-channel
Just submitted PLB paper with most stringent limits to datePreliminary results for this summer, even better:
s< 5.0pb t< 4.4pb
hep-ex/0505063
The signal is kinematically between W+jets and ttbar
08/15/05 Regina Demina, Lecture #3 16
CMS is looking for new physics in particular the Higgs boson, supersymmetry and … One of the promising
reactions: Httbar
New Physics, New Particles?
We watch for interesting events:
~1 / 400,000 or ~ 100 per second
Higgs boson: h0 H0 A0 H±
Example of ttbar collision