top physics peter uwer humboldt-universität berlin

Top physicsPeter Uwer

Humboldt-UniversitätBerlin

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 2

Why are we interested in top-quarks ?

1) Top-quark = heaviest elementary particle discovered so far

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

Questions:

Important testground for theoretical developments

Many interesting phenomena/aspects

Interesting per se Required for precision



Precise measurements of its properties,search for possible deviations i.e. anomalous couplings

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

Important: precise predictions possible, only “two” input parameters: CKM matrix + top-quark mass

1) + 2)



3) Top-quark mass is an important input parameter of the SM

Fundamental parameter, should be known as precise as possible !

[Heinemeyer, Hollik, Stockinger,Weiglein, Zeune '12]


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

tt+Jet(s) production

tt cross section

b-quark distribution in decay

Top polarisation

Charge asymmetry

Measurement of the electric charge

Search for anomalous couplings, important background

Weak decay of a `free’ quark, bound on the top width and Vtb, search for anomalous couplings

Direct measurement of the CKM matrix element Vtb, top polarization, search for anomalous Wtb couplings

Measurement of the Yukawa coupling

Consistency checks with theo. predictions, new physics in the tt invariant mass spectrum

Test of the V-A structure in top decay

Measurement of the Z couplings

Sensitive to new physics tbH+

Sensitive to new physics

? new physics ?

Consistency Standard Model

See talks on Saturday:German Rodrigo and Aurelio Juste


Cross section for top-quarkpair production


Hadronic top-quark pair production

Partonic cross sections

~90% @ Tevatron, 10% @ LHC ~10% @ Tevatron, 90% @ LHC


Theory status: Total cross section

NLO QCD:

Beyond NLO QCD:

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

[Ahrens, Baernreuther, Beneke, Bonciani, Cacciari, Catani, Czakon, Ferroglia,

Kidonakis, Laenen, Mangano, Mitov, Moch, Nason, Neubert, Pecjak, Ridolfi, Schwinn,

Sterman, PU, Vogt, Yang…]

feasible

[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]

[Baernreuther, Czakon, Mitov ‘12]

NNLO approx

NNLO QCD for qqtt


Recent progress: qqtt @ NNLO/NNLL

Tevatron:

[Baernreuther, Czakon, Mitov arXiv:1204.5201]

ggtt @ NNLO is underway ~3%


LHC cross section measurements

[Ignacio Aracena, Moriond 2012]

Consistent picture

Most precise measurement: Lepton + jets 6.6% rel. uncertainty

(diff. channels / diff. experiments !)


Combination of measurements

ATLAS:

CMS:

All results consistent with SM

6.2 %

8 %


Aiming for precision: Beyond NNLO QCD

“Resonance structure” from would be bound state

~1 % shift of total cross section at LHC

[Kiyo,Kühn,Moch,Steinhauser,P.U. 08][Hagiwara, Sumino, Yokoya 08]

[Kühn, Scharf, P.U 06,07][Beenakker et al 94, Bernreuther, Fücker, Si 06’, 07]


Cross section measurements

Production mechanism seems well understood

Possible applications:

Gluon PDF / Gluon Luminosity Top-quark mass

Experimental goal seems feasible

Use cross section to constrain `parameters´

Severe constraint for new physics scenarios

Top-quark physics = precision physics


The top-quark mass


Top-quark mass measurements

[Stijn Blyweert, Moriond 2012]

Competitive with Tevatron


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 s,

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


Different mass definitions

Pole mass scheme

MS mass

Chose constants minimal to cancel 1/ poles in

Schemes defined in perturbation theory conversion possible

Common schemes:

Other schemes possible: 1S mass, PS mass,…


Conversion between schemes

Pole mass MS mass:

Important:

Difference can be numerically significant

[Chetyrkin,Steinhauser 99]

Example:

Difference is formally of higher order in coupling constant~10GeV

NLO predictions are required for meaningful measurements


Bad choices — Good choices

Scheme might be ill defined beyond perturbation theory

Example: Renormalon ambiguity in pole mass

Pole mass has intrinsic uncertainty of order QCD

“There is no pole in full QCD”!![Bigi, Shifman, Uraltsev, Vainshtein 94 Beneke, Braun,94 Smith, Willenbrock 97]


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 ?


Impact on current measurements

Different channels and different experimentsgive consistent results

Large effects unlikely

Study additional distributions / observables Compare with NLO templates

Possible improvements of current measurements:

Template method:

Matrix element method at NLO

Matrix element method

Alternative measurements ?


Top quark mass from cross section

Drawback: Limited sensitivity to mt

Mass scheme well defined, higher orders can be included


Alternative observables ?

Compare b-quark mass measurement at LEPusing 3-jet rates [Bilenky, Fuster, Rodrigo, Santarmaria]

For details, see Adrian Irles presentation

Use tt+1-jet events

First m

easurement of th

e

Running b-quark mass a

t

high scale


Spin correlations in top-quarkpair production


Top-quark spin correlations

Quantum mechanics:

close tothreshold:

Spins are parallel (qq) or anti-parallel (gg) close to threshold

Parity invariance of QCD:

But: Spins of top quark and antiquark are correlated[Bernreuther,Brandenburg 93, Mahlon, Parke 96, Stelzer,Willenbrock 96, Bernreuther, Brandenburg, Si, P.U. 04]

[Dharmaratna, Goldstein,’90, Bernreuther, Brandenburg,PU. 95]

Top’s produced in qqtt and gg tt are essentially unpolarized


Why are spin correlations interesting ?

You also measured the charge asymmetry….

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

LHC can improve a lot compared to Tevatron


Spin correlations: How to measure it

Basic ingredients: Top quark decays before hadronization Parity violating decay t Wb

Polarisation can be studied through the angular distribution of the decay

products!

f


Spin correlations

Study (azimuthal) opening angle distribution of

leptons in dilepton events

gg dominates

[Parke, Mahlon ‘10]

LHC:

Ansatz:


LHC measurement

Observation of spin-correlations (5.1

[arXiv:1203.4081]


Constraining new physics

[Fujfer, Kamenik, Melic, arXiv1205.0264]

NLO corrections are known and found to be small


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

Thank you for your

attention !


Forward-Backward Charge Asymmetry in tt+1Jet

[Dittmaier, PU, Weinzierl PRL 98:262002, ’07]


Charge Asymmetry: Dependence on Pt(tt)

[Kühn, Top-quark workshop, Berlin 2012]


Non-perturbative corrections

Top-quark is a colour triplet non-perturbative effects in the reconstruction of the top momentum from colour singlet's

[Skands,Wicke ‘08]

blue: pt-ordered PSgreen: virtuality ordered PSoffset from generated mass

different modeling of non-perturbative physics / colour

reconnection

Non-perturbativeeffects could result in uncertainty

of the order of 500 MeV


Top-quark charge asymmetry

Compare

+

+–

+ –

+–

+–

+–

+–

-

+–

+–+ –+

–

+–

+–

──

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]

[Berends, Gaemers, Gastmans ´73, Berends, Kleiss, Jadach, Was ´83]

[Kühn]


Charge asymmetry: Theory predictions

[Kühn, Rodrigo ´11]

Coherent picture of theoretical predictions,Theoretical uncertainties based on scale variations,

possibly underestimates higher order effects (ratios!)

Soft gluonresummation

QCD+EW

QCD+EW

QCD


Tevatron results

[Bernreuther, Si ’12]

[1] CDF, arXiv:1101.0034, [2] D0, arXiv:1107.4995, [7] CDF note 10807

At most 2.4 deviation


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


Charge asymmetry at LHC

y

top

anti-top

Effect expected to be small since qq makes onlya small fraction, more important for larger mtt

(Additional cuts may enhance asymmetry)


CMS results

[CMS-PAS-Top-11-030]


ATLAS results

Inclusive:

Theory (MC@NLO):

[arXiv 1203.4211]


New physics scenarios

[arXiv 1203.4211]

inclusive

“Z´, W’ disfavoured, some tension”


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:

Look into observables which can be measured at LHC and Tevatron [Aguilar Saavedra, Juste ‘12]

Improve current measurements

top physics peter uwer humboldt-universität berlin

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