Top Quark Physics:An Overview

Young Scientists’ Workshop, Ringberg castle, July 21st 2006Andrea Bangert

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Why Study the Top Quark?

• Top is a fundamental particle of the SM: as such its parameters should be measured.

• Top may play a role in the mechanism of electroweak symmetry breaking.

• More precise constraints on mt help to constrain mH.

• Top decay provides a unique opportunity to study the decay of a bare quark.

• Top will provide a large background for searches for physics beyond the SM.

• Top provides a useful calibration tool for the LHC detectors.

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A Brief History of Top

• 1977: meson was discovered (b + anti-b).

• T3(b) = -1/2 → T3(t) = +1/2• 1993: Constraints from

electroweak precision data gave

mt = 164 ± 27 GeV [Mele]• 1995: The top quark was

observed directly at the Tevatron.

• 2006:

mt = 172.5 ± 2.3 GeV• 2008: The LHC will be a

top factory, producing 8 million pairs/year at luminosity L~1033cm-2s-1.

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Electroweak Precision Measurements

EM, GF, mZ → sin2θW 1 - (mW / mZ)2

mW2 ~ 1 / GF sin2 θW (1-▲r)

▲r contains all one-loop corrections to mW.

(▲r)top ~ GFmt2 / tan2 θW

(▲r)Higgs ~ GF mW2 ln (mH

2 / mZ2)

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Electroweak Precision Measurements (II)

LEP: mw = 80.383 ± 0.035 GeVTeVatron: mt=172.5 ± 2.3 GeV

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Top Pair Production• Top-antitop pair production proceeds via the

strong interaction.

Tevatron ~ 6.8 pb, LHC ~ 800 pb

• gluon fusion: Tevatron 10%, LHC 90%

• quark-antiquark annihilation: Tevatron 90%, LHC 10%

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W

Single Top Production• Top is produced singly via the weak charged-current

interaction.

W• t-channel, W-gluon fusion

• Wt production

• W* process, s-channel

Tevatron ~ 2 pbLHC ~ 250 pb

Tevatron ~ 0 pbLHC ~ 60 pb

Tevatron ~ 1 pbLHC ~ 10 pb

W

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Decay Topologies

•Top decays via t→Wb.•Final states in decay of top-antitop pair are:

Dileptonic:Both W bosons decay via W→lν,Γ ~ 4.9 %.

Lepton + jets: 1 W decays via W→lν, 1 W decays to produce hadron jets. Γ ~ 50.7 %. l = e, μ Γ ~ 29.9 %

Hadronic: Both W bosons decay to produce hadron jets. Γ ~ 44.4 %.

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TeVatron Results: Top Mass

• mt = 172.5 ± 2.3 GeV

• Good agreement of indirect determination with direct measurement: mt = 172.6 + 13.2 – 10.2 GeV

[LEP 1 and SLD]

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Tevatron Results: Top Cross SectionCDF

theoretical cross section: theory = 6.78 ± 1.2 pb [Bonciani 1998, Kidonakis 2003, Cacciari 2004]

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Monte Carlo Analysis in preparation for LHC

• Analyses to be performed include mt and t.

• Lepton + jets channel, l = e,μ; Γ ~ 29.9%.

• Selection criteria: • At least 1 isolated lepton,

pT > 20 GeV, |η| < 2.5• Missing ET > 20 GeV • At least 4 jets with

pT > 40 GeV, |η| < 2.5 • b-tagging is not required.

• Reconstruction: • Choose the three jets which

maximize the total pT.

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Monte Carlo Analysis: Top Mass

•Reconstruction (continued):

•Choose the two jets which have invariant mass closest to mW.

•Apply mjj = mW ± 10 GeV as selection criteria.

•Jets are reconstructed using the Cone or KT algorithms.

•Top mass distribution (Gaussian) and combinatorial background (Chebyshev) are depicted.

•Only signal events and combinatoric background are depicted. •Plot by Nabil Ghodbane.

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Monte Carlo Analysis: t

• Top pair events from all channels were generated using MC@NLO / HERWIG.

• 1500 events / mt.• No detector simulation

or event reconstruction was performed.

• t and mt will both be measured: this analysis will be used as consistency check.

Summary

• Top is worth studying because: • it is a fundamental particle of the SM,• it may play a role in electroweak symmetry breaking, • It helps to constrain mH, • It will provide a background for future studies, • It offers a useful tool for calibrating the detector.

• Top-antitop pair production proceeds via the strong interaction.

• Single top production proceeds via the weak interaction. • Top decays via the weak interaction, t Wb. • mt = 172.5 ± 2.3 GeV [CDF and D0 combined result]

• t= 7.3 ± 0.9 pb [CDF combined result] • mt and t will be among first measurements made at

LHC.

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Backup SlidesBackup Slides

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Reconstructed Top Mass• Variation of parameter D in the KT jet reconstruction algorithm affects mt. [Nabil Ghodbane, ATLAS MPI Group]

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Charge of the Top Quark (I)

• The SM top quark has charge Qt = + 2/3.• The particle of mass mt = 172.5 GeV which was

first observed in 1995 at the TeVatron is believed to be the SM top quark, i.e. the weak isospin partner of the b quark with Qt = + 2/3.

• The charge of the particle observed in 1995 at the TeVatron has never been directly measured.

• It is possible that the observed particle is instead an exotic quark

Q with charge QQ = - 4/3 which decays via Q→W-b.

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Charge of the Top Quark (II)

• Experiments at the Tevatron will be able to exclude to 95% CL the possibility that QQ = - 4/3.

• Experiments at the LHC will be able to measure the charge of the top quark to an accuracy of 10%.

• One possibility for directly measuring the top charge at the LHC is by considering gg → ttg events,

~ Qt2.

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W Helicity in Top Decays

• The W boson in top quark decay must be left-handed or longitudinal; it cannot be right-handed.

• Weak vertex factor for t → Wb: - igWVtb ½ (1 - 5)• mb ~ 0 → b quark is left-handed