Top physics

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Top physics
Peter Uwer
Humboldt-Universität Berlin
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Why are we interested in top-quarks ?
1) Top-quark heaviest elementary particle
discovered so far
Questions

• Is the top-quark point-like ?
• Why is the top-quark so heavy ?
• How is the mass generated ?

Important testground for theoretical developments
Many interesting phenomena/aspects
Interesting per se
Required for precision
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Why are we interested in top-quarks ?
2) Top-quarks - a sensitive tool to explore the
electroweak symmetry breaking
? Top-quark plays special role in many extensions
of the Standard Model, ideal tool to search for
new physics
1) 2)
Precise measurements of its properties, search
for possible deviations i.e. anomalous couplings
Important precise predictions possible, only
two input parameters CKM matrix top-quark
mass
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Why are we interested in top-quarks ?
3) Top-quark mass is an important input parameter
of the SM
Heinemeyer, Hollik, Stockinger, Weiglein, Zeune
'12
Fundamental parameter, should be known as
precise as possible !
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Important measurements

• Cross section for pair production
• Top quark mass measurement
• W-Polarisation in top decay
• ttH cross section
• ttZ cross section
• Single top production
• Spin correlations
• ttJet(s) production
• ttg cross section
• b-quark distribution in decay
• Top polarisation
• Charge asymmetry

Consistency checks with theo. predictions, new
physics in the tt invariant mass spectrum
Consistency Standard Model
Test of the V-A structure in top decay
Measurement of the Yukawa coupling
Measurement of the Z couplings
Direct measurement of the CKM matrix element Vtb,
top polarization, search for anomalous Wtb
couplings
Weak decay of a free quark, bound on the top
width and Vtb, search for anomalous couplings
Search for anomalous couplings, important
background
Measurement of the electric charge
See talks on Saturday German Rodrigo and Aurelio
Juste
Sensitive to new physics t?bH
Sensitive to new physics
? new physics ?
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Cross section for top-quark pair production
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Hadronic top-quark pair production
90 _at_ Tevatron, 10 _at_ LHC
10 _at_ Tevatron, 90 _at_ LHC
Partonic cross sections
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Theory status Total cross section
Dawson, Ellis, Nason 89, Beenakker et al
89,91,Bernreuther, Brandenburg, Si, PU 04,
Czakon,Mitov 08

• NLO QCD

Moch, PU 08, Cacciari, Frixone, Mangano, Nason
Ridolfi 08, Kidonakis Vogt 08

• Beyond NLO QCD

Ahrens, Baernreuther, Beneke, Bonciani,
Cacciari, Catani, Czakon, Ferroglia, Kidonakis,
Laenen, Mangano, Mitov, Moch, Nason, Neubert,
Pecjak, Ridolfi, Schwinn, Sterman, PU, Vogt,
Yang

• Soft gluon resummation
• Threshold corrections
• Full scale NNLO (in)dependence
• High energy behaviour

NNLOapprox

• NNLO QCD for qq?tt

Baernreuther, Czakon, Mitov 12
feasible
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Recent progress qq?tt _at_ NNLO/NNLL
Baernreuther, Czakon, Mitov arXiv1204.5201
Tevatron
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gg?tt _at_ NNLO is underway
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LHC cross section measurements
Ignacio Aracena, Moriond 2012
Consistent picture
(diff. channels / diff. experiments !)
Most precise measurement Lepton jets ? 6.6
rel. uncertainty
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Combination of measurements
All results consistent with SM
? 6.2
ATLAS
? 8
CMS
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Aiming for precision Beyond NNLO QCD
Beenakker et al 94, Bernreuther, Fücker, Si 06,
07
Hagiwara, Sumino, Yokoya 08
Kühn, Scharf, P.U 06,07
Kiyo,Kühn,Moch,Steinhauser,P.U. 08
Resonance structure from would be bound state
1 shift of total cross section at LHC
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Cross section measurements

• Production mechanism seems well understood

• Experimental goal

seems feasible

• Severe constraint for new physics scenarios

Top-quark physics precision physics
Possible applications
Use cross section to constrain parameters

• Gluon PDF / Gluon Luminosity
• Top-quark mass

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The top-quark mass
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Top-quark mass measurements
Stijn Blyweert, Moriond 2012
Competitive with Tevatron
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Some basic facts about theory parameters
and their determination.
Top-quarks dont appear as asymptotic states (no
free quarks due to confinement)
Top-quark mass is just a parameter like as,
only defined in a specific theory/model i.e. SM

• renormalisation scheme dependent,
• only indirect determination possible through
  comparison (fit) theory ? ? experiment

Parameter determination relies on theory, scheme
dependence encoded in theor. predictions
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Different mass definitions
Common schemes

• Pole mass scheme

• MS mass

Chose constants minimal to cancel 1/e poles in
Other schemes possible 1S mass, PS mass,
Schemes defined in perturbation theory ?
conversion possible
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Conversion between schemes
Pole mass ? ? MS mass
Example
Important

• Difference can be numerically significant

Chetyrkin,Steinhauser 99
10GeV

• Difference is formally of higher order in
  coupling constant

NLO predictions are required for meaningful
measurements
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Bad choices Good choices

• Scheme might be ill defined beyond perturbation
  theory

Renormalon ambiguity in pole mass
Example
Bigi, Shifman, Uraltsev, Vainshtein 94 Beneke,
Braun,94 Smith, Willenbrock 97
!
There is no pole in full QCD
Pole mass has intrinsic uncertainty of order LQCD
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Template method kinematic reconstruction
Present measurements

• Distribution invariant masse of top quark decay
  products
• Rely mostly on parton shower predictions
• No NLO so far available (?)

Main issues

• Corrections due to color reconnection / non
  perturbative physics (? momentum reconstruction
  of color triplet)
• Precise mass definition ?

How important ?
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Impact on current measurements
Different channels and different experiments give
consistent results
Large effects unlikely
Possible improvements of current measurements
Template method

• Study additional distributions / observables
• Compare with NLO templates

Matrix element method

• Matrix element method at NLO

Alternative measurements ?
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Top quark mass from cross section
Mass scheme well defined, higher orders can be
included
Drawback Limited sensitivity to mt
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Alternative observables ?
First measurement of the Running b-quark mass at
high scale
Compare b-quark mass measurement at LEP using
3-jet rates
Bilenky, Fuster, Rodrigo, Santarmaria
Use tt1-jet events
For details, see Adrian Irles presentation
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Spin correlations in top-quark pair production
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Top-quark spin correlations
Dharmaratna, Goldstein,90, Bernreuther,
Brandenburg,PU. 95
Parity invariance of QCD
Tops produced in qq?tt and gg? tt are
essentially unpolarized
But
Spins of top quark and antiquark are correlated
Bernreuther,Brandenburg 93, Mahlon, Parke 96,
Stelzer,Willenbrock 96, Bernreuther,
Brandenburg, Si, P.U. 04
Quantum mechanics
close to threshold
? Spins are parallel (qq) or anti-parallel (gg)
close to threshold
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Why are spin correlations interesting ?

• You also measured the charge asymmetry.

• LHC can improve a lot compared to Tevatron

• Sensitive test of production and decay, may put
  severe constrains on new physics scenarios

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Spin correlations How to measure it
Basic ingredients

• Top quark decays before hadronization
• Parity violating decay t ? Wb

f
Polarisation can be studied through the angular
distribution of the decay products!
?
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Spin correlations
Parke, Mahlon 10
Study (azimuthal) opening angle distribution of
leptons in dilepton events
LHC
gg dominates
Ansatz
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LHC measurement
arXiv1203.4081
Observation of spin-correlations (5.1 s)
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Constraining new physics
Fujfer, Kamenik, Melic, arXiv1205.0264
NLO corrections are known and found to be small
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Summary

• Tremendous progress in the recent past
• Top-quark physics is now precision physics
• Already after one year LHC is competitive or
  even better than Tevatron
• Ideal laboratory to search for new physics

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Thank you for yourattention !
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Forward-Backward Charge Asymmetry in tt1Jet
Dittmaier, PU, Weinzierl PRL 98262002, 07
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Charge Asymmetry Dependence on Pt(tt)
Kühn, Top-quark workshop, Berlin 2012
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Non-perturbative corrections
Skands,Wicke 08
Top-quark is a colour triplet
? non-perturbative effects in the reconstruction
of the top momentum from colour singlet's
different modeling of non-perturbative physics /
colour reconnection
Non-perturbative effects could result in
uncertainty of the order of 500 MeV
blue pt-ordered PS green virtuality ordered PS
offset from generated mass
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Top-quark charge asymmetry
Berends, Gaemers, Gastmans 73, Berends, Kleiss,
Jadach, Was 83
Compare
Kühn
Similar effect
Charge asymmetry SM
-
Kühn, Rodrigo 98,07,12, Almeida, Sterman,
Vogelsang 08, Bernreuther, Si 10, Hollik, Pagani
11 Ahrens, Ferroglia,Neubert,Pecjak, Yang 11
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Charge asymmetry Theory predictions
Kühn, Rodrigo 11
QCDEW
QCD
QCDEW
Soft gluon resummation
? Coherent picture of theoretical
predictions, Theoretical uncertainties based on
scale variations, possibly underestimates higher
order effects (ratios!)
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Tevatron results
Bernreuther, Si 12
At most 2.4 s deviation
1 CDF, arXiv1101.0034, 2 D0,
arXiv1107.4995, 7 CDF note 10807
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Charge asymmetry at LHC

• No forward-backward asymmetry since pp is P
  symmetric

However

• t tend to follow initial q, while tb tend to
  follow initial qb
• initial state is not symmetric with respect to
  q,qb
• q tend to be more energetic

should be broader w.r.t
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Charge asymmetry at LHC
Effect expected to be small since qq makes only a
small fraction, more important for larger mtt
(Additional cuts may enhance asymmetry)
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CMS results
CMS-PAS-Top-11-030
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ATLAS results
arXiv 1203.4211
Inclusive
Theory (MC_at_NLO)
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New physics scenarios
arXiv 1203.4211
inclusive
Z, W disfavoured, some tension
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Final remarks on asymmetry

• Discrepancy has reduced with new CDF measurement
• Theory is only LO, in ttj where also NLO is
  known, large higher-order corrections observed
• Charge asymmetry very sensitive to Pt(tt)
• LHC uncertainties are still large

No conclusive picture yet
Future
Improve current measurements
Look into observables which can be measured at
LHC and Tevatron
Aguilar Saavedra, Juste 12