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 Required for precision

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

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 | IMFP 2012, Benasque, 24.05-03.06.2012 | page 3

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 | IMFP 2012, Benasque, 24.05-03.06.2012 | page 4

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 g 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 V tb , 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 ? new physics ? +

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 5

Cross section for top-quark pair production

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 6

Hadronic top-quark pair production

~90% @ Tevatron, 10% @ LHC Partonic cross sections ~10% @ Tevatron, 90% @ LHC

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 7

Theory status: Total cross section

 NLO Q C D: [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 Q C D :      Soft gluon resummation Threshold corrections Full scale NNLO (in)dependence [Ahrens, Baernreuther, Beneke, Bonciani, Cacciari, Catani, Czakon, Ferroglia, Kidonakis, Laenen, Mangano, Mitov, Moch, Nason, Neubert, Pecjak, Ridolfi, Schwinn, Sterman, PU, Vogt, Yang…] High energy behaviour NNLO Q C D for qq  tt [Baernreuther, Czakon, Mitov ‘12] feasible

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 8

Recent progress: qq



tt @ NNLO/NNLL

[Baernreuther, Czakon, Mitov arXiv:1204.5201] Tevatron: gg  tt @ NNLO is underway ~3%

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 9

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 | IMFP 2012, Benasque, 24.05-03.06.2012 | page 10

Combination of measurements

All results consistent with SM ATLAS: CMS:   6.2 % 8 %

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 11

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 | IMFP 2012, Benasque, 24.05-03.06.2012 | page 12

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 | IMFP 2012, Benasque, 24.05-03.06.2012 | page 13

The top-quark mass

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 14

Top-quark mass measurements

[Stijn Blyweert, Moriond 2012] Competitive with Tevatron

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 15

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 a 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

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 16

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 | IMFP 2012, Benasque, 24.05-03.06.2012 | page 17

Example:

Conversion between schemes

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 | IMFP 2012, Benasque, 24.05-03.06.2012 | page 18

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

Q C

D”

!

L

Q C D

Pole mass has intrinsic uncertainty of order

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 19

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 | IMFP 2012, Benasque, 24.05-03.06.2012 | page 20

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 | IMFP 2012, Benasque, 24.05-03.06.2012 | page 21

Top quark mass from cross section

Mass scheme well defined, higher orders can be included Drawback: Limited sensitivity to m t

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 22

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 | IMFP 2012, Benasque, 24.05-03.06.2012 | page 23

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 Q C D : [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 | IMFP 2012, Benasque, 24.05-03.06.2012 | page 25

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 | IMFP 2012, Benasque, 24.05-03.06.2012 | page 26

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 | IMFP 2012, Benasque, 24.05-03.06.2012 | page 27

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 | IMFP 2012, Benasque, 24.05-03.06.2012 | page 28

LHC measurement

[arXiv:1203.4081] Observation of spin-correlations (5.1

s)

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 29

Constraining new physics

[Fujfer, Kamenik, Melic, arXiv1205.0264] NLO corrections are known and found to be small

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 30

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 | IMFP 2012, Benasque, 24.05-03.06.2012 | page 33

Charge Asymmetry: Dependence on P

t

(tt)

[Kühn, Top-quark workshop, Berlin 2012]

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 34

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 non perturbative physics / colour reconnection Non-perturbative effects could result in uncertainty of the order of 500 MeV offset from generated mass blue: pt-ordered PS green: virtuality ordered PS

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 35

Compare

Top-quark charge asymmetry

[Berends, Gaemers, Gastmans ´73, Berends, Kleiss, Jadach, Was ´83] – + + – + – + ─ + – [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 | IMFP 2012, Benasque, 24.05-03.06.2012 | page 36

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 | IMFP 2012, Benasque, 24.05-03.06.2012 | page 37

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 | IMFP 2012, Benasque, 24.05-03.06.2012 | page 38

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 | IMFP 2012, Benasque, 24.05-03.06.2012 | page 39

Charge asymmetry at LHC

top anti-top

y

Effect expected to be small since qq makes only a small fraction, more important for larger

m tt

(Additional cuts may enhance asymmetry)

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 40

CMS results

[CMS-PAS-Top-11-030]

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 41

Inclusive: Theory (MC@NLO):

ATLAS results

[arXiv 1203.4211]

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 42

New physics scenarios

inclusive [arXiv 1203.4211] “Z´, W’ disfavoured, some tension”

Peter Uwer (Humboldt-Uni. Berlin) | Top physics | IMFP 2012, Benasque, 24.05-03.06.2012 | page 43

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 P t (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 | IMFP 2012, Benasque, 24.05-03.06.2012 | page 44