Top physics
Peter Uwer
Humboldt-Universität
Berlin
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
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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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 !
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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Important measurements
Consistency checks with theo. predictions,
new physics in the tt invariant mass spectrum
 Cross section for pair production
 Top quark mass measurement
Consistency Standard Model
 W-Polarisation in top decay
Test of the V-A structure in top decay
 ttH cross section
Measurement of the Yukawa coupling
 ttZ cross section
Measurement of the Z couplings
 Single top production
Direct measurement of the CKM matrix element Vtb,
top polarization, search for anomalous Wtb couplings
 Spin correlations
Weak decay of a `free’ quark, bound on
the top width and Vtb, search for anomalous couplings
 tt+Jet(s) production
Search for anomalous couplings, important background
 ttg cross section
Measurement of the electric charge
to new
physics tbH+
talks on
Saturday:
 b-quark distribution in decay See Sensitive
 Top polarisation
 Charge asymmetry
German
Rodrigo and Aurelio Juste
Sensitive to new physics
? new physics ?
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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Cross section for top-quark
pair production
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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Hadronic top-quark pair production
~90% @ Tevatron, 10% @ LHC
~10% @ Tevatron, 90% @ LHC
Partonic cross sections
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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Theory status: Total cross section
 NLO QCD:
[Dawson, Ellis, Nason ’89, Beenakker et al ’89,’91,Bernreuther,
Brandenburg, Si, PU ’04, Czakon,Mitov 08]
[Moch, PU 08, Cacciari, Frixone, Mangano, Nason Ridolfi 08, Kidonakis Vogt 08]
 Beyond NLO QCD:





[Ahrens, Baernreuther, Beneke, Bonciani,
Soft gluon resummation
Cacciari, Catani, Czakon, Ferroglia,
Kidonakis, Laenen, Mangano, Mitov, Moch,
Threshold corrections
Nason, Neubert, Pecjak, Ridolfi, Schwinn,
Sterman, PU, Vogt, Yang…]
Full scale NNLO (in)dependence
High energy behaviour
NNLO QCD for qqtt [Baernreuther, Czakon, Mitov ‘12]
feasible
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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Recent progress: qqtt @ NNLO/NNLL
[Baernreuther, Czakon, Mitov arXiv:1204.5201]
Tevatron:
ggtt @ NNLO is underway
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
~3%
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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
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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Combination of measurements
All results consistent with SM
ATLAS:
 6.2 %
CMS:
8%
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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Aiming for precision: Beyond NNLO QCD
[Hagiwara, Sumino, Yokoya 08]
[Kiyo,Kühn,Moch,Steinhauser,P.U. 08]
[Beenakker et al 94, Bernreuther, Fücker, Si 06’, 07]
[Kühn, Scharf, P.U 06,07]
“Resonance structure” from
would be bound state
~1 % shift of total cross section at LHC
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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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
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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The top-quark mass
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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Top-quark mass measurements
[Stijn Blyweert, Moriond 2012]
Competitive with Tevatron
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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Some basic facts about theory parameters
…and their determination.
Top-quarks don’t 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
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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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
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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Conversion between schemes
Example:
Pole mass   MS mass:
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
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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Bad choices — Good choices
 Scheme might be ill defined beyond perturbation theory
Example:
Renormalon ambiguity in pole mass
[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
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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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 ?
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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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 ?
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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Top quark mass from cross section
Mass scheme well defined, higher orders can be included
Drawback: Limited sensitivity to mt
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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Alternative observables ?
Compare b-quark mass measurement at LEP
using 3-jet rates [Bilenky, Fuster, Rodrigo, Santarmaria]
Use tt+1-jet events
For details, see Adrian Irles presentation
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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Spin correlations in top-quark
pair production
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
| IMFP 2012, Benasque, 24.05-03.06.2012 | page 24
Top-quark spin correlations
Parity invariance of QCD:
[Dharmaratna, Goldstein,’90,
Bernreuther, Brandenburg,PU. 95]
Top’s 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
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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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
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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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!
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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Spin correlations
[Parke, Mahlon ‘10]
Study (azimuthal) opening
angle distribution of
leptons in dilepton events
LHC:
gg dominates
Ansatz:
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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LHC measurement
[arXiv:1203.4081]
Observation of spin-correlations (5.1 s)
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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Constraining new physics
[Fujfer, Kamenik, Melic, arXiv1205.0264]
NLO corrections are known and found to be small
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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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
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
| IMFP 2012, Benasque, 24.05-03.06.2012 | page 31
Thank you
for your
attention !
Forward-Backward Charge Asymmetry in tt+1Jet
[Dittmaier, PU, Weinzierl PRL 98:262002, ’07]
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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Charge Asymmetry: Dependence on Pt(tt)
[Kühn, Top-quark workshop, Berlin 2012]
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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Non-perturbative corrections
Top-quark is a colour triplet
[Skands,Wicke ‘08]
 non-perturbative effects in the reconstruction of the top
momentum from colour singlet's
different modeling of nonperturbative physics / colour
reconnection
Non-perturbative
effects could result in uncertainty
of the order of 500 MeV
offset from generated mass
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
blue: pt-ordered PS
green: virtuality ordered PS
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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]
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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Charge asymmetry: Theory predictions
[Kühn, Rodrigo ´11]
QCD+EW
QCD
QCD+EW
Soft gluon
resummation
 Coherent picture of theoretical predictions,
Theoretical uncertainties based on scale variations,
possibly underestimates higher order effects (ratios!)
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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Tevatron results
[Bernreuther, Si ’12]
At most 2.4 s deviation
[1] CDF, arXiv:1101.0034, [2] D0, arXiv:1107.4995, [7] CDF note 10807
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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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
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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Charge asymmetry at LHC
top
anti-top
y
Effect expected to be small since qq makes only
a small fraction, more important for larger mtt
(Additional cuts may enhance asymmetry)
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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CMS results
[CMS-PAS-Top-11-030]
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
| IMFP 2012, Benasque, 24.05-03.06.2012 | page 41
ATLAS results
Inclusive:
[arXiv 1203.4211]
Theory (MC@NLO):
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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New physics scenarios
[arXiv 1203.4211]
inclusive
“Z´, W’ disfavoured, some tension”
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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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]
Peter Uwer (Humboldt-Uni. Berlin) | Top physics
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