1

Top Quark PhysicsSuyong ChoiKorea University

2Contents

• Top Quark in the Standard Model• Measurement of Production Cross Sections• Properties of the Top Quark• Summary and Outlook

TOP QUARK IN THE STANDARD MODEL

Properties of Top Quark

• Discrete quantum numbers• Spin• Weak isospin• Charge

• Mass

• Lifetime or Decay width

• Branching fraction

• Coupling – QCD, EW

QCD

• Top quark carries “color” and interacts with gluons• Production of top quark pairs is important probe of QCD

interactions

Electroweak La-grangian

• Fermion part

• Interaction with H, W, , Z

𝜓 𝑖=(𝜈𝑖ℓ𝑖−)𝑜𝑟 ( 𝑢𝑖𝑑𝑖 ′)

Interactions of Top Quark in SM

• Strong interaction:

• Coupling to neutral gauge boson:

• Coupling to charged gauge boson:

• Coupling to Higgs field (boson): • Coupling to Higgs field:

Top Quark Decays

• Decays to physical states

• Top decays almost 100% to W+b

Decays of Top Quark

Top Decay Width

• 1.3 GeV width for 172 GeV top quark

• s • Top quarks are produced and decay like free quarks with spin at

production information intact• Hadron formation time • Hadron formation is governed by light-quark dynamics• In contrast, B mesons decay isotropically

11Polarized Top Quark

• Top quark polarization is reflected in angular distribution of decay products

Particle

Charged lepton 1

Neutrino -0.31

B quark -0.41

TOP QUARK AND ELEC-TROWEAK PRECISION DATA

Top Quark Corrections to Electroweak Measurements

• Radiative corrections to W and Z propagator

• Quadratic sensitivity to fermion masses

𝐺𝐹

√2𝜌=𝑔

2+𝑔′ 2

8𝑀𝑍2 𝜌 ≈1+

3𝐺𝐹𝑚𝑡2

8𝜋 2√2

Top Quark and Electroweak Measurements

• W and Z masses

• Z mass was measured very precisely at LEP experiments

• could be inferred with knowledge of • Test of EW theory

𝑀 𝑍2 =

𝜋𝛼√2𝐺𝐹 𝜌 sin2𝜃 cos2𝜃

𝑀𝑊2 =

𝜋𝛼√2𝐺𝐹 sin 2𝜃

Prediction from LEP

• Top quark mass could bepredicted from precisionmeasurements

16Top Quark

CDF DØ

Top Mass Distributions from 1995 observation paper

CDF DØ

Run 2 results

17Success of the SM

Top Quark Mass from Electroweak Data

• LEP EW precision re-sults from

Connection with Higgs

• In conjunction with W and Z,we can gain information on Higgs mass

Δ 𝜌=− 38𝜋 cos2𝜃

ln𝑀𝐻

𝑀𝑊

20The Top Quark

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

• Boundary between metastability and stability

21LHC and Experi-ments

5 fb-1 @ 7 TeV20 fb-1 @ 8 TeV

22

Physics with Top Quarks

• Properties• Mass• Decay width• Spin• Coupling

• Cross section measure-ments• Production and decays

23

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

24

PRODUCTION

25 Pair Production

• Strongly produced

• Contribution of and changes as

Pair production diagams

26 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

27

Reconstructing tt-bar Events

• Lepton+Jets• 1 charged lepton• 4 hadronic jets (2 are b-quark jets)• Missing ET

• Problem• How to correctly assign jets to top or antitop • How to reconstruct neutrino momentum

Top Quark Recon-struction in L+Jets

• 1 unknown: neutrino • ,

• 3 constraints:

• Problem of combinatorics• 2 fold ambiguity – if 2 b-jets tagged• 6 fold ambiguity – if 1 b-jet tagged

𝑚 (ℓ𝜈𝑏1 )=𝑚(𝑏2 𝑗1 𝑗2)

𝑚 (ℓ𝜈 )=𝑀𝑊

𝑚 ( 𝑗1 𝑗 2 )=𝑀𝑊

Top Quark Recon-struction in L+Jets

• Have to consider • experimental uncertainties on measurements• finite widths of W and top

• Numerically minimize event-by-event

30

TOP PRODUCTION CROSS SECTION

31

Pair Production Cross Section

• Experimental error comparable to theory error• QCD explains well the inclusive pair production

32

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

33

Single Top Production t-channel

34

Observation of Wt Single Top Production

Signal Region Control Region

𝜎 (𝑝𝑝→𝑊𝑡 )=23.4−5.4+5.5 𝑝𝑏 significance

35Measurement of

• From single top quark production cross section, we can measure directly without assuming 3 generation of quarks

• Current best direct measurement:

36

PROPERTIES

37Mass of Top Quark• Tevatron: GeV – 0.5% accuracy

38

Mass Difference of and

• CPT violated if • and distinguished by electric charged of lepton

Δ𝑚𝑡=−272±196 (𝑠𝑡𝑎𝑡 )±122(𝑠𝑦𝑠𝑡)

39

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

40

Electric Charge of Top Quark

• B-jet charge calculatedfrom tracks associatedwith b-jet

41

W Polarization from Top

• Use lepton angular distribution in top rest frame

• W from top decays are either left-handed or longitudinal

42

Spin Correlation in Production

• On average, spin of top and antitop are unpolarized, but event-by-event, their spins are correlated• Most prominent in initial state: aligned top spin• For gg mostly anti-aligned spins• Results depend on spin quantization axis chosen

𝑞 𝑞

𝑡

𝑡

Produced at Rest

𝑞 𝑞

𝑡

𝑡

Relativistic top

43Spin 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)

44

Top Coupling with Vec-tor Bosons with and

45 Production

• Major background to

• Number of b-tagged jets distribution

46

SEARCHES WITH TOP QUARKS

47

Search for Reso-nances Decaying into

48Search for FCNC

Anomalous Single Top

𝑔𝑞→𝑡Search for

𝐵 (𝑡→𝑍𝑞)<0.0021@95% 𝐶𝐿

49Search for

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

50

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?

51

BACKUP

52Introduction

• 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

The Top Quark

• 6th quark discovered• Partner to bottom quark predicted after discovery of • “gauge anomaly” of SM must be absent• Electric charge inferred from using quarkonium model• Weak isospin inferred from forward backward asymmetry

• 1995 by D-Zero and CDF experiments @ Fermilab

54Lepton 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:

55

Strong Coupling Constant

• is a function of and

Top Quark Recon-struction in Dilepton

• Dilepton channel – 2 leptons + 2 b-jets + Missing ET

• Unknowns• Neutrino momentum components – 2 x 3 = 6

57

Lepton Forward-Backward Asymmetry

• Lepton asymmetry reflects• Asymmetry in production• Polarization of : vs

• SM predicts small asymmetry in production and no polar-ization

Top Quark Recon-struction in Dilepton

• Constraints

𝑚 (ℓ1𝜈1𝑏1 )=𝑚 (ℓ2𝜈2𝑏2)

𝑚 (ℓ1𝜈1 )=𝑀𝑊

𝑝𝜈1 , 𝑥+𝑝𝜈 2, 𝑥=𝑀𝐸𝑇 𝑥

𝑝𝜈1 ,𝑦+𝑝𝜈2 , 𝑦=𝑀𝐸𝑇 𝑦

2-fold ambiguity inLepton-jet assignment

𝑚 (ℓ2𝜈2 )=𝑀𝑊

System is Underconstrained!

Top Quark Recon-struction in Dilepton

• If top quark mass is assumed, then the number of constraints = number of un-knowns• But, still, up to 4 solutions are possible – intersection of 2 ellipses in 2-D space * 2 fold

ambiguity

• Numerically maximize likelihood event-by-event

• Assume momentum and mass spectra of system