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