Top physics

1. Top physics Peter Uwer Humboldt-Universität Berlin

2. 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

3. 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

4. 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 !

5. 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 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 ?

6. Cross section for top-quark pair production

7. Hadronic top-quark pair production ~90% @ Tevatron, 10% @ LHC ~10% @ Tevatron, 90% @ LHC Partonic cross sections

8. 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

9. Recent progress: qqtt @ NNLO/NNLL [Baernreuther, Czakon, Mitov arXiv:1204.5201] Tevatron: ~3% ggtt @ NNLO is underway

10. LHC cross section measurements [Ignacio Aracena, Moriond 2012] Consistent picture (diff. channels / diff. experiments !) Most precise measurement: Lepton + jets  6.6% rel. uncertainty

11. Combination of measurements All results consistent with SM  6.2 % ATLAS:  8 % CMS:

12. 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

13. 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

14. The top-quark mass

15. Top-quark mass measurements [Stijn Blyweert, Moriond 2012] Competitive with Tevatron

16. 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

17. 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

18. 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

19. 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 orderLQCD

20. 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 ?

21. 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 ?

22. Top quark mass from cross section Mass scheme well defined, higher orders can be included Drawback: Limited sensitivity to mt

23. 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 tt+1-jet events For details, see Adrian Irles presentation

24. Spin correlations in top-quark pair production

25. Top-quark spin correlations [Dharmaratna, Goldstein,’90, Bernreuther, Brandenburg,PU. 95] Parity invariance ofQCD: Top’s produced in qqtt andgg tt are essentiallyunpolarized 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

26. 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

27. 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! 

28. Spin correlations [Parke, Mahlon ‘10] Study (azimuthal) opening angle distribution of leptons in dilepton events LHC: gg dominates Ansatz:

29. LHC measurement [arXiv:1203.4081] Observation of spin-correlations (5.1 s)

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

31. 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

32. Thank you for yourattention !

33. Forward-Backward Charge Asymmetry in tt+1Jet [Dittmaier, PU, Weinzierl PRL 98:262002, ’07]

34. Charge Asymmetry: Dependence on Pt(tt) [Kühn, Top-quark workshop, Berlin 2012]

35. 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

36. 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]

37. 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!)

38. 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

39. 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

40. 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)

41. CMS results [CMS-PAS-Top-11-030]

42. ATLAS results [arXiv 1203.4211] Inclusive: Theory (MC@NLO):

43. New physics scenarios [arXiv 1203.4211] inclusive “Z´, W’ disfavoured, some tension”

44. 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]