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

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 2 Outline 1.Motivation and introduction 2.Issues: - Charge asymmetry - Top-quark mass 3. Conclusion

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 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 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) Top-quark mass important input parameter 1) and 2) are related through precision physics

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 4 Introduction: New Physics searches  SM physics [Degrassi, Di Vita, Elias-Miro,Spinosa,Giudici ’12, Alekhin, Djouadi, Moch ’12] Vacuum stability Consistency of the SM  Precise theoretical predictions required

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 5 Top-quark pair production ─ theory status Incl. cross section NLO Incl. cross section NNLO 1989 [Dawson, Ellis, Nason ’89, Beenakker et al ’89,91] [Bernreuther, Brandenburg, Si, PU ’04 [Czakon,Mitov 08] Spin dependent cross section NLO 2004 [Bonciani, Catani, Mangano,Nason ‘98, Kidonakis, Laenen, Moch, Vogt 01] NLL resummation Analytic results NLO Steps towards NNLL [Moch, PU 08] Bound state effects [Kiyo,Kuhn,Moch,Steinhauser,PU 08 Hagiwara, Sumion, Yokoya 08] Full NNLL resummation 2010 [Czakon,Fiedler,Mitov 13] 2013 Combined NNLL and 1/  [Ahrens, Beneke,Czakon, Ferroglia, Mitov, Schwinn…] 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… Th. uncertainty below 5% Further progress will require substantial effort from theory

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 6 Current situation No smoking gun so far Most measurements in good agreement with predictions Precision physics with top-quarks has just started  more precise measurements  more involved observables Future directions: Issues (= something which could become a problem…): Forward-backward charge asymmetry Understanding the top-quark mass or may disappear…

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 7 Charge asymmetry Remember: Furry’s theorem + Holds also for more complicated diagrams  Interference term does not contribute to total cross section  Asymmetric contribution if t-t phase space is un-integrated [Berends et al ’73, Kuhn, Rodrigo ‘98] + = t t virt. corr. to tt real corr. to tt QCD:QCD:

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 8 Forward-backward charge asymmetry (Tevatron) Qualitative picture: rapidity Definitions used by CDF/D0: Forward-backward charge asymmetry Assuming CP invariance:

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 9 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 gluon resummation QCD+EW QCD Only LO !

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 10 Measurements from Tevatron [1] CDF, arXiv: , [2] D0, arXiv: , [7] CDF note At most 2.4  deviation “Some tension” [Bernreuther, Si, PRD86 (2012) ]  O(100) theory papers  refined experimental studies (full data sample, lepton asymmetries)

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 11 Recent results on the leptonic charge asymmetries [CDF, ] CDF results: D0 results: [D0, ] [PRD84 (2011) ] 5.4/fb 9.7/fb 9.1/fb Note: Lepton asymmetries depend on top polarization, ind. confirmation of P t  0

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 12 Recent results from CDF [CDF, Phys. Rev. D87 (2013) ] 9.4/fb

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 13 Latest results from D0 D0, arXiv Now in agreement with SM

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 14 Charge asymmetry at the LHC 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 y top anti-top

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 15 Charge asymmetry at LHC [CMS PAS TOP ]

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 16 Summary on AFB and lessons to be learned Charge asymmetry = just another subtle quantum effect  We should measure these effects even if they look un- spectacular or out of reach as far as the SM predictions are concerned The signal which could have been the first indication of new physics seems to have disappeared Nothing particular to learn… …apart from understanding the quantum level !!! Important to probe theory at quantum level

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 17 Top-quark mass: Recent results Best known quark mass, shouldn’t we be happy ? [arXiv:1403:4427]

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 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:

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 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  Method should employ well defined mass scheme small perturbative and non-perturbative corrections

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 20 Different mass definitions  Pole mass scheme  MS mass Chose constants minimal to cancel 1/  poles in  1S mass  Potential subtracted mass Each scheme well defined in perturbation theory  conversion possible Position of would-be 1S boundstate in e+e-  tt [Hoang, Teubner 99] [Beneke 98]

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 21 Conversion between schemes Pole mass   MS mass: Example: 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

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 22 Intrinsic uncertainty of the pole mass Renormalon ambiguity in pole mass Pole mass has intrinsic uncertainty of order  QCD [Bigi, Shifman, Uraltsev, Vainshtein 94 Beneke, Braun,94 Smith, Willenbrock 97] Qualitatively: Expect non-perturbative corrections since full S-matrix has no pole Quantitative understanding: (recently confirmed by lattice studies)

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 23 A related issue: color reconnection [Mangano, Top workshop, July 2012, CERN]  To avoid non perturbative effects, observable should not strongly rely on p t

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 24 What is currently done in experiment ATLAS-CDF-CMS-D0 combination: [arXiv ] “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

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 25 Do we really care ? [CMS-PAS-FTR ] See also Jorgen’s talk  Yes, aiming for a precision of or even below 500 MeV

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 26 Alternatives: MS mass from cross section [Langenfeld, Moch, PU 09] Tevatron, D0 Drawback: Limited sensitivity to m t Mass scheme well defined, higher orders can be included Mass scheme well defined, higher orders can be included only exp. uncertainties

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 27 Alternative Methods 1.Invariant mass of J/  + lepton in top decay / M lB 2.Top-quark mass from jet rates 3.“Endpoint method” 4.e+e- theshold scan

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 28 Towards a “global fit”… Include top-quark cross section in PDF analysis and fit m t together with as and PDF’s in particular gluon distribution Idea: Correlations with as and PDF’s are automatically taken into account Result: [Alekhin,Blümlein,Moch ’13]

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 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

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 30 Comparison pole mass versus MS mass [Dowling,Moch 13] LO, NLO, NNLO Perturbative expansion using different mass schemes:

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 31 Top mass in leptonic final states with J/  [A. Kharchilava, CMS-Note , Phys. Lett. B476 (2000) 73, Corcella, Mangano, Seymour ’00, Chierici, Dierlamm, CMS-Note ] Advantages:  Experimentally very clean  Independent from production  Good sensitivity Disadvantages:  Small branching fraction   Relies on Monte Carlo modeling  Which mass do we measure ?

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 32 Top mass in leptonic final states with J/  Recent progress: [Biswas, Melnikov, Schulze ‘10] linear fit  Slope difference of 0.01 compared to MC results  3 GeV shift NLO corrections are important  Further studies required

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 33 Top-quark mass from jet-rates (ttj) Similar to b-quark mass measurement at LEP using 3-jet rates [Bilenky, Fuster, Rodrigo, Santarmaria ‘95] Use tt+1-jet events [S. Alioli, P.Fernandez, J.Fuster, A. Irles, S. Moch, PU, M. Vos, to appear]  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]  Less sensitive to color reconnection  Mass parameter fixed through NLO calculation  MS mass in principle possible

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 34 Top-quark mass from jet rates To enhance mass sensitivity study: with i.e. m 0 = 170 GeV [S. Alioli, P.Fernandez, J.Fuster, A. Irles, S. Moch, PU, M. Vos, to appear]

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 35 Mass dependence high energy threshold Crossing due to normalization [S. Alioli, P.Fernandez, J.Fuster, A. Irles, S. Moch, PU, M. Vos, to appear]

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 36 Sensitivity [S. Alioli, P.Fernandez, J.Fuster, A. Irles, S. Moch, PU, M. Vos, to appear]

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 37 New physics scenarios [JHEP02(2014)107]  No evident explanation in terms of new physics New physics further constraint if additional observables are included [Delaunay, Gedalia, Hochberg, Perez, Soreq `11] EFT approach

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 38 Uncertainty estimates I Non-perturbative effects at the LHC Simulate top mass measurement using different models/tunes for non-perturbative physics / colour reconnection [Skands,Wicke ‘08] Non-perturbative effects result in uncertainty of the order of 500 MeV blue: pt-ordered PS green: virtuality ordered PS offset from generated mass different offset for different tunes!

Peter Uwer (HU Berlin) | Top-quark physics – Theoretical issues ─ | Blois, | page 39 Uncertainty estimates II Suppose top-quark form T-mesons and would not decay: HQET: do not depend on m t and are calculable in HQET Estimate from B-physics / QCD sum rules: Identifying would lead to a systematic (calculable) shift of 500 MeV