Highlights from Top Quark Physics

1

Highlights from Top Quark PhysicsSuyong ChoiKorea University

2Contents

• Introduction• Measurement of Production Cross Sections• Properties of the Top Quark• Summary and Outlook

3Top Quark

CDF DØ

Top Mass Distributions from 1995 observation paper

CDF DØ

Run 2 results

4Success of the SM

5The Top Quark

• Top quark is special• Most massive• Interaction only within 3rd generation• top-Higgs coupling ~ 1

• Boundary between metastability and stability

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

10 Pair Production

• Strongly produced

• Contribution of and changes as

Pair production diagams

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

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

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

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

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

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?

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

33Search for FCNC

Anomalous Single Top

𝑔𝑞→𝑡Search for

𝐵 (𝑡→𝑍𝑞)<0.0021@95% 𝐶𝐿

34Search for

• Top-Higgs coupling almost 1• Consistent with backgrounds• Cross section limits at