Highlights from Top Quark Physics
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Highlights from Top Quark PhysicsSuyong ChoiKorea University
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2Contents
• Introduction• Measurement of Production Cross Sections• Properties of the Top Quark• Summary and Outlook
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3Top Quark
CDF DØ
Top Mass Distributions from 1995 observation paper
CDF DØ
Run 2 results
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4Success of the SM
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5The Top Quark
• Top quark is special• Most massive• Interaction only within 3rd generation• top-Higgs coupling ~ 1
• Boundary between metastability and stability
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6LHC and Experi-ments
5 fb-1 @ 7 TeV20 fb-1 @ 8 TeV
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Physics with Top Quarks
• Properties• Mass• Decay width• Spin• Coupling
• Cross section measure-ments• Production and decays
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Cross Sec-tions at Teva-tron and LHC
• Higher cross sectionand higher luminosity at LHC• Top quark factory• Rare processes with top
quarks• New physics with top quarks
• Tevatron and LHC are complementary
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PRODUCTION
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10 Pair Production
• Strongly produced
• Contribution of and changes as
Pair production diagams
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11 channels
Multijet
e+jets
mu+jets
Dilepton:ee, e,
tau+X
• per lepton flavor• Multijet – Highest statistics, but large backgrounds and combinatorics• Lepton+jets – Highest statistics and usually yields best measurement • Dilepton – Smaller statistics but clean, less combinatoric, solving for 2 neutrino
momenta not trivial
Lepton+jets
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Pair Production Cross Section
• Experimental error comparable to theory error• QCD explains well the inclusive pair production
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Lepton Forward-Backward Asymmetry
• Lepton asymmetry reflects• Asymmetry in production• Polarization of : vs
• SM predicts small asymmetry in production and no polar-ization
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14Lepton AFB in
Channel LuminosityCDF Lepton+jets 9.4 fb-1
D0 Lepton+jets 9.7 fb-1
D0 Dilepton 9.7 fb-1
SM prediction @ NLO:
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Single Top Produc-tion
• Electroweak production• Cross section of the same
order as pair production
• Sensitive probe of withoutthe assumption of 3 generationof quarks
W associated
s and t channel
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Single Top Production t-channel
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Observation of Wt Single Top Production
Signal Region Control Region
𝜎 (𝑝𝑝→𝑊𝑡 )=23.4−5.4+5.5 𝑝𝑏 significance
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18Measurement of
• From single top quark production cross section, we can measure directly without assuming 3 generation of quarks
• Current best direct measurement:
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PROPERTIES
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20Mass of Top Quark• Tevatron: GeV – 0.5% accuracy
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Mass Difference of and
• CPT violated if • and distinguished by electric charged of lepton
Δ𝑚𝑡=−272±196 (𝑠𝑡𝑎𝑡 )±122(𝑠𝑦𝑠𝑡)
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Decay Width of Top Quark
• In SM, top quark width at NLO is
• 1.29 GeV/c2
• Lifetime of
• Decay width reflected in reconstructed mass distribution
• CDF measures
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Electric Charge of Top Quark
• B-jet charge calculatedfrom tracks associatedwith b-jet
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W Polarization from Top
• W from top decays are either left-handed or longitudinal
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25Spin Correlation
• and the spins of top quarks are correlated• Due to , spin state of top at pro-
duction reflected in decay prod-ucts
• Lepton is the most sensitive probe of top spin polarization
• Tevatron and LHC has different contributions of and
• ATLAS observed spin correla-tions at 5.1 s.d.
𝑓 𝑆𝑀=1.30±0.14 (𝑠𝑡𝑎𝑡 )−0.22+0.27 (𝑠𝑦𝑠𝑡)
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Top Coupling with Vec-tor Bosons with and
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27 Production
• Major background to
• Number of b-tagged jets distribution
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Summary and Out-look
• Approaching 20 years of rich physics program at hadron colliders with top quark events
• Top quark production and properties consistent with SM
• Many measurements systematics limited. What can you do with millions of top quark events?
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29Introduction
• When was discovered in 1977, it was considered as a bound state of quarks. Hence extra quark was thought to exist.
• It took a long time until top quark was discovered in 1995 by CDF and D-Zero experiments using Fermilab Tevatron accelerators
• Being the most massive quark, it may hold the key.
• With the luminosity and energy reach of the LHC at CERN, top quarks can be studied with unprecedented precision.• 1.96 TeV → 8 TeV
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Strong Coupling Constant
• is a function of and
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SEARCHES WITH TOP QUARKS
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Search for Reso-nances Decaying into
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33Search for FCNC
Anomalous Single Top
𝑔𝑞→𝑡Search for
𝐵 (𝑡→𝑍𝑞)<0.0021@95% 𝐶𝐿
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34Search for
• Top-Higgs coupling almost 1• Consistent with backgrounds• Cross section limits at