top quark physics: an overview young scientists’ workshop, ringberg castle, july 21 st 2006 andrea...
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Top Quark Physics:An Overview
Young Scientists’ Workshop, Ringberg castle, July 21st 2006Andrea Bangert
2
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.
3
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.
4
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)
6
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%
7
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
8
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 %.
9
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]
10
Tevatron Results: Top Cross SectionCDF
theoretical cross section: theory = 6.78 ± 1.2 pb [Bonciani 1998, Kidonakis 2003, Cacciari 2004]
11
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.
12
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.
13
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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Reconstructed Top Mass• Variation of parameter D in the KT jet reconstruction algorithm affects mt. [Nabil Ghodbane, ATLAS MPI Group]
17
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.
18
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.