top quark physics
DESCRIPTION
Top Quark Physics. Suyong Choi Korea University. Top Quark in the Standard Model Measurement of Production C ross S ections Properties of the Top Quark Summary and Outlook. Contents. Top quark in the Standard model. Discrete quantum numbers Spin Weak i sospin Charge Mass - PowerPoint PPT PresentationTRANSCRIPT
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Top Quark PhysicsSuyong ChoiKorea University
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2Contents
• Top Quark in the Standard Model• Measurement of Production Cross Sections• Properties of the Top Quark• Summary and Outlook
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TOP QUARK IN THE STANDARD MODEL
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Properties of Top Quark
• Discrete quantum numbers• Spin• Weak isospin• Charge
• Mass
• Lifetime or Decay width
• Branching fraction
• Coupling – QCD, EW
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QCD
• Top quark carries “color” and interacts with gluons• Production of top quark pairs is important probe of QCD
interactions
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Electroweak La-grangian
• Fermion part
• Interaction with H, W, , Z
𝜓 𝑖=(𝜈𝑖ℓ𝑖−)𝑜𝑟 ( 𝑢𝑖𝑑𝑖 ′)
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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:
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Top Quark Decays
• Decays to physical states
• Top decays almost 100% to W+b
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Decays of Top Quark
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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
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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
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TOP QUARK AND ELEC-TROWEAK PRECISION DATA
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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
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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
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Prediction from LEP
• Top quark mass could bepredicted from precisionmeasurements
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16Top Quark
CDF DØ
Top Mass Distributions from 1995 observation paper
CDF DØ
Run 2 results
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17Success of the SM
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Top Quark Mass from Electroweak Data
• LEP EW precision re-sults from
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Connection with Higgs
• In conjunction with W and Z,we can gain information on Higgs mass
Δ 𝜌=− 38𝜋 cos2𝜃
ln𝑀𝐻
𝑀𝑊
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20The 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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21LHC and Experi-ments
5 fb-1 @ 7 TeV20 fb-1 @ 8 TeV
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22
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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24
PRODUCTION
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25 Pair Production
• Strongly produced
• Contribution of and changes as
Pair production diagams
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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
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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
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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
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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
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TOP PRODUCTION CROSS SECTION
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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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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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35Measurement 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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37Mass 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
• Use lepton angular distribution in top rest frame
• W from top decays are either left-handed or longitudinal
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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
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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)
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44
Top Coupling with Vec-tor Bosons with and
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45 Production
• Major background to
• Number of b-tagged jets distribution
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SEARCHES WITH TOP QUARKS
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47
Search for Reso-nances Decaying into
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48Search for FCNC
Anomalous Single Top
𝑔𝑞→𝑡Search for
𝐵 (𝑡→𝑍𝑞)<0.0021@95% 𝐶𝐿
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49Search for
• Top-Higgs coupling almost 1• Consistent with backgrounds• Cross section limits at
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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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51
BACKUP
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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
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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
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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:
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Strong Coupling Constant
• is a function of and
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Top Quark Recon-struction in Dilepton
• Dilepton channel – 2 leptons + 2 b-jets + Missing ET
• Unknowns• Neutrino momentum components – 2 x 3 = 6
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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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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!
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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