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Top-quark physics ─ Theoretical issues ─

Top-quark physics ─ Theoretical issues ─. Rencontre de Blois, 20.05.2014. Peter Uwer. GK1504. Outline. Motivation and introduction Issues: - Charge asymmetry - Top-quark mass 3. Conclusion. Introduction: Why are we interested in top-quarks.

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Top-quark physics ─ Theoretical issues ─

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  1. Top-quark physics─ Theoretical issues ─ Rencontre de Blois, 20.05.2014 Peter Uwer GK1504

  2. Outline • Motivation and introduction • Issues: - Charge asymmetry - Top-quark mass 3. Conclusion

  3. Introduction: Why are we interested in top-quarks 1) Top-quark = one building block of the Standard Model  Want to measure/know all properties as precise as possible Top-quark mass important input parameter 2) Top-quark physics as window to new physics could decay in new heavy particles ( resonances in tt) Very sensitive to EWSB, strong coupling to Higgs Important correction to Higgs mass Affects the running of the quadratic Higgs coupling (vac .stability) 1) and 2) are related through precision physics

  4. Introduction: New Physics searches  SM physics Vacuum stability Consistency of the SM [Degrassi, Di Vita, Elias-Miro,Spinosa,Giudici ’12, Alekhin, Djouadi, Moch ’12]  Precise theoretical predictions required

  5. Top-quark pair production ─ theory status Incl. cross section NLO Spin dependent cross section NLO Incl. cross section NNLO Combined NNLL and 1/b Full NNLL resummation Steps towards NNLL Analytic results NLO Bound state effects NLL resummation Th. uncertainty below 5% 2013 1989 1998 2004 2008 2010 [Czakon,Fiedler,Mitov 13] [Moch, PU 08] [Kiyo,Kuhn,Moch,Steinhauser,PU 08 Hagiwara, Sumion, Yokoya 08] [Bernreuther, Brandenburg, Si, PU ’04 [Dawson, Ellis, Nason ’89, Beenakker et al ’89,91] [Bonciani, Catani, Mangano,Nason ‘98, Kidonakis, Laenen, Moch, Vogt 01] [Ahrens, Beneke,Czakon, Ferroglia, Mitov, Schwinn…] [Czakon,Mitov 08] Impressive theoretical progress in last 25 years Similar for single top-quark production Many more results on differential distributions, add. jets, combination with parton shower, top decay… Further progress will require substantial effort from theory

  6. Current situation No smoking gun so far Most measurements in good agreement with predictions Precision physics with top-quarks has just started Future directions: more precise measurements more involved observables Issues (= something which could become a problem…): or may disappear… Understanding the top-quark mass Forward-backward charge asymmetry

  7. Charge asymmetry Remember: Furry’s theorem [Berends et al ’73, Kuhn, Rodrigo ‘98] + = 0 virt. corr. to tt Holds also for more complicated diagrams real corr. to tt t + + + = 0 = 0 = 0 t • Interference term does not contribute to total cross section • Asymmetric contribution if t-t phase space is un-integrated QCD:

  8. Forward-backward charge asymmetry (Tevatron) Qualitative picture: Assuming CP invariance: rapidity Definitions used by CDF/D0: Forward-backward charge asymmetry

  9. Charge asymmetry: Theory predictions 0.088 [Kühn, Rodrigo ´11] QCD+EW QCD ! QCD+EW Only LO Soft gluon resummation  Coherent picture of theoretical predictions, Theoretical uncertainties based on scale variations, possibly underestimates higher order effects (ratios!)

  10. Measurements from Tevatron [Bernreuther, Si, PRD86 (2012) 034026] [1] CDF, arXiv:1101.0034, [2] D0, arXiv:1107.4995, [7] CDF note 10807 At most 2.4 s deviation “Some tension” • O(100) theory papers • refined experimental studies (full data sample, lepton asymmetries)

  11. Recent results on the leptonic charge asymmetries CDF results: [CDF, 1404.3698] 9.1/fb D0 results: 5.4/fb [PRD84 (2011) 112005] 9.7/fb [D0, 1403.1294] Note: Lepton asymmetries depend on top polarization, ind. confirmation of Pt 0

  12. Recent results from CDF [CDF, Phys. Rev. D87 (2013) 092002] 9.4/fb

  13. Latest results from D0 D0, arXiv 1405.0421 Now in agreement with SM

  14. Charge asymmetry at the LHC top anti-top y No forward backward charge asymmetry at LHC due to P symmetric initial state 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

  15. Charge asymmetry at LHC [CMS PAS TOP-14-006]

  16. Summary on AFB and lessons to be learned The signal which could have been the first indication of new physics seems to have disappeared Charge asymmetry = just another subtle quantum effect Nothing particular to learn… …apart from understanding the quantum level !!! Important to probe theory at quantum level • We should measure these effects even if they look un-spectacular or out of reach as far as the SM predictions are concerned

  17. Top-quark mass: Recent results [arXiv:1403:4427] Best known quark mass, shouldn’t we be happy ?

  18. How do we measure a quark mass (in theory) ? Top-quark is not stable, even if it would be, confinement would prevent us from seeing free top-quarks What is the meaning of the top-quark mass ? Formal answer: Top-quark mass / Yukawa coupling just a parameter of the underlying theory (e.g SM) Value depends on renormalization scheme used to define the parameters in theor. predictions Measure mass in specific scheme through comparison/fit:

  19. Requirements for a good observable • Observable should show good sensitivity to m • Observable must be theoretically calculable, at least predictions at NLO accuracy required • Theory uncertainties should be small small perturbative and non-perturbative corrections • Method should employ well defined mass scheme

  20. Different mass definitions Pole mass scheme MS mass Chose constants minimal to cancel 1/e poles in 1S mass [Hoang, Teubner 99] Position of would-be 1S boundstate in e+e-  tt Potential subtracted mass [Beneke 98] Each scheme well defined in perturbation theory  conversion possible

  21. Conversion between schemes Example: Pole mass   MS mass: Difference is formally of higher order in coupling constant Which scheme shall we use?  Scheme should be well defined, should lead to small perturbative corrections

  22. Intrinsic uncertainty of the pole mass Expect non-perturbative corrections since full S-matrix has no pole Qualitatively: Quantitative understanding: Renormalon ambiguity in pole mass [Bigi, Shifman, Uraltsev, Vainshtein 94 Beneke, Braun,94 Smith, Willenbrock 97] Pole mass has intrinsic uncertainty of orderLQCD (recently confirmed by lattice studies)

  23. A related issue: color reconnection [Mangano, Top workshop, July 2012, CERN] • To avoid non perturbative effects, observable should not strongly rely on pt

  24. What is currently done in experiment [arXiv 1403.4427] ATLAS-CDF-CMS-D0 combination: “The systematic uncertainty related to the specific MC choice is found to be marginal with respect to the possible intrinsic difference between the top-quark mass implemented in any MC and the pole mass definition” Related uncertainty

  25. Do we really care ? [CMS-PAS-FTR-13-017] See also Jorgen’s talk  Yes, aiming for a precision of or even below 500 MeV

  26. Alternatives: MS mass from cross section [Langenfeld, Moch, PU 09] Mass scheme well defined, higher orders can be included Tevatron, D0 only exp. uncertainties Drawback: Limited sensitivity to mt

  27. Alternative Methods Invariant mass of J/Y + lepton in top decay / MlB Top-quark mass from jet rates “Endpoint method” e+e- theshold scan

  28. Towards a “global fit”… Idea: Include top-quark cross section in PDF analysis and fit mt together with as and PDF’s in particular gluon distribution Correlations with as and PDF’s are automatically taken into account Result: [Alekhin,Blümlein,Moch ’13]

  29. Summary Precise theoretical predictions for top-quark physics available So far no significant deviations from SM predictions found Precision physics has just started! Very precise measurements for top-quark mass available Further improvements require to put more theory in

  30. Comparison pole mass versus MS mass Perturbative expansion using different mass schemes: [Dowling,Moch 13] LO, NLO, NNLO

  31. Top mass in leptonic final states with J/Y [A. Kharchilava, CMS-Note 1999-065, Phys. Lett. B476 (2000) 73, Corcella, Mangano, Seymour ’00, Chierici, Dierlamm, CMS-Note 2006-058] Advantages: Experimentally very clean Independent from production Good sensitivity Disadvantages: Small branching fraction  Relies on Monte Carlo modeling Which mass do we measure ?

  32. Top mass in leptonic final states with J/Y Recent progress: [Biswas, Melnikov, Schulze ‘10] Slope difference of 0.01 compared to MC results  3 GeV shift NLO corrections are important linear fit Further studies required

  33. Top-quark mass from jet-rates (ttj) [S. Alioli, P.Fernandez, J.Fuster, A. Irles, S. Moch, PU, M. Vos, to appear] Use tt+1-jet events • Large event rates (~30 % of inclusive tt events) • NLO corrections available • NLO+shower available [Dittmaier, PU, Weinzierl ´07,´08, Melnikov, Schulze ’10, Melnikov, Scharf, Schulze ´12] [Alioli, Moch, PU ´11, Kardos, Papadopoulos, Trocsanyi ‘11] Similar to b-quark mass measurement at LEP using 3-jet rates [Bilenky, Fuster, Rodrigo, Santarmaria ‘95] Less sensitive to color reconnection Mass parameter fixed through NLO calculation MS mass in principle possible

  34. Top-quark mass from jet rates [S. Alioli, P.Fernandez, J.Fuster, A. Irles, S. Moch, PU, M. Vos, to appear] To enhance mass sensitivity study: with i.e. m0 = 170 GeV

  35. Mass dependence [S. Alioli, P.Fernandez, J.Fuster, A. Irles, S. Moch, PU, M. Vos, to appear] Crossing due to normalization threshold high energy

  36. Sensitivity [S. Alioli, P.Fernandez, J.Fuster, A. Irles, S. Moch, PU, M. Vos, to appear] 25.5 17 8.5

  37. New physics scenarios [JHEP02(2014)107]  No evident explanation in terms of new physics [Delaunay, Gedalia, Hochberg, Perez, Soreq `11] EFT approach New physics further constraint if additional observables are included

  38. Uncertainty estimates I Non-perturbative effects at the LHC [Skands,Wicke ‘08] Simulate top mass measurement using different models/tunes for non-perturbative physics / colour reconnection different offset for different tunes! Non-perturbative effects result in uncertainty of the order of 500 MeV blue: pt-ordered PS green: virtuality ordered PS offset from generated mass

  39. Uncertainty estimates II Suppose top-quark form T-mesons and would not decay: HQET: do not depend on mt and are calculable in HQET Estimate from B-physics / QCD sum rules: Identifying would lead to a systematic (calculable) shift of 500 MeV

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