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Top-quark physics ─ Theory Status ─

Top-quark physics ─ Theory Status ─. ICHEP 2014, Valencia, 7.7.2014. Peter Uwer. GK1504. Content. Tremendous progress in the theoretical description of top-quark physics Possible, due to many synergies in recent developments Per cent level accuracy for inclusive quantities

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Top-quark physics ─ Theory Status ─

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  1. Top-quark physics─ Theory Status ─ ICHEP 2014, Valencia, 7.7.2014 Peter Uwer GK1504

  2. Content Tremendous progress in the theoretical description of top-quark physics Possible, due to many synergies in recent developments Per cent level accuracy for inclusive quantities (10%) level accuracy for exclusive processes even for “large” multiplicities Many recent results publicly available for the analysis in form of Computer code  Top-quark physics ideal laboratory for precise tests of the Standard Model and New Physics searches Heaviest elementary fermion discovered so far  strong coupling to the Higgs, very sensitive to electro weak symmetry breaking

  3. Steps towards precision NNLO predictions for top-quark pair and single top-quark production Beyond fixed order 1/b, ln(b) LL,NLL,NNLL Fixed-order Perturbative Corrections (NLO,NNLO) Cannot cover everything since time is to short Focus on new directions / developments illustrated through specific examples Apologies to all those, who's work is not mentioned Towards distributions in NNLO tt+2Jets Matched and merged @ NLO Precise theoretical predictions Predictions for observable final state Top-decay… Matching with Parton-shower à la MC@NLO and Powheg INPUT mt, as, Vtq, PDFs, Unified description of tt and tW production Top-quark mass measurements Combined fit mt,as,gPDF

  4. Inclusive top-quark pair production in NNLO Top-quark pair production in Born approximation: Recent progress: NNLO / NNLL QCD predictions [Czakon,Mitov,Fiedler ‘12, ‘13] (Distributions: work in progress) Very active field, see also related work: [Bonciani, Ferroglia,Gehrmann,Manteuffel,Studerus ’11 ,Abelof, Gehrmann-de Ridder, ‘12] Bonciani, Ferroglia,Gehrmann,Manteuffel,Studerus ’13] Abelof,Gehrmann-de Ridder, Maierhöfer,Pozzorini ’14,…

  5. Inclusive top-quark pair production in NNLO [Czakon,Mitov,Fiedler ‘12, ‘13] Residual scale (in)dependence [Hathor 2.1, Top++ 2.0] restricted scale variation to avoid large ratios NLO  NNLO shifts cross section by 13pb (5.4%) Significant reduction of scale dependence

  6. Inclusive top-quark pair production in NNLO [Czakon,Mitov,Fiedler ‘12, ‘13] Residual scale (in)dependence [Hathor 2.1, Top++ 2.0] restricted scale variation to avoid large ratios NNLO  NNLL shifts cross section by 6.7pb (2.7%) NNLL resummation further stabilizes residual scale dep.

  7. Inclusive top-quark pair production in NNLO Scale (in)dependence using the MS mass m(m) Mass is just another parameter, can be renormalized/defined using different schemes Use mass renormalized in minimal subtraction (“running mass”, MS mass)

  8. Inclusive top-quark pair production in NNLO [Czakon,Mitov,Fiedler ‘12, ‘13 +Hathor 2.1] Scale (in)dependence using the MS mass Scale variation: 0.5 1 2 Scale (in)dependence further improved! For differential distributions using the MS mass in NLO see [Dowling,Moch ‘13]

  9. Steps towards distributions in NNLO accuracy [Guzzi, Lipka, Moch ‘14] Construct ansatz for one-particle inclusive distributions based on soft gluon resummation +…

  10. Steps towards distributions in NNLO accuracy [Guzzi, Lipka, Moch ‘14] (normalized!) Scale dependence is reduced Agreement with data is improved How well is inclusive cross section reproduced ? Impact of damping factors ?

  11. Top-quark pair production in association with jets Higher order corrections typically large for high multiplicities  NLO required Average momentum scale decreases with increasing jet mult. Soft and coll. emission important  parton shower Different phase space regions require different theoretical treatment

  12. Matching and Merging [Frederix, Frixione 12 Höche,Krauss,Schönherr,Siegert 13, Gehrmann,Höche,Krauss,Schönherr,Siegert 13, Alwall,Frederix,Frixione,Hirschi,Maltoni,Mattelaer,Shao,Stelzer,Torrielli,Zaro 14] Combine LO/NLO & PS Mad5_aMC@NLO, MEPS@NLO Merging: Use appropriate Description in specific phase space region Remove double counting through matching à la MC@NLO, Powheg LO NLO Parton shower Fixed order  Unified description

  13. Top-quark pair production in association with jets [Höche,Krauss,Maierhöfer,Pozzorini,Schönherr,Siegert ‘14]  much improved uncertainties, unified description Progress possible due to very advanced one-loop technology and high degree of automation Matching and merging: de facto standard for future predictions ?

  14. Single top-quark production Interesting: V_tb, b-pdf, source of polarized top-quarks,… Born processes for single top-quark production: s-channel t-channel Wt production NLO corrections for the t-channel: + … Cancel in interference with Born due to color

  15. Single top-quark production in NLO [Hathor-2.1] Small NLO corrections Scale uncertainties very small PDF uncertainties small scale var. up and down PDF uncert. However: Sizeable cancellations in t-channel NLO corrections Box diagrams don’t contribute Size of the NLO corrections accidentally small ?

  16. Single top-quark production: Steps towards NNLO [Brucherseifer,Caola, Melnikov ‘14] +… + +…+ (neglected) NNLO Results for top-quark production: “Factorizable” NNLO corrections under control Fully differential What about double-box contributions? ( may affect distributions…)

  17. Disentangling tW and tt production tW production in NLO ─ real corrections In NLO tW and tt production interfere For an overview of different strategies see [Frixione,Laenen, Motylinski, Webber, White 08] ( gauge invariance, realistic cuts…) Recent progress: [Cascioli,Kallweit,Maierhöfer,Pozzorini 14, Frederix 14] Study in the 4-flavour scheme keeping top-quark / W-boson off-shell effects and a finite b-quark mass

  18. Disentangling tW and tt production Technically very challenging, possible only due to recent progress in one-loop calculations + automation Full calculation includes all interference terms allows gauge invariant separation in • narrow width limit  on-shell top-quark pair prod. • finite remainder  off shell contr., single-top, …. Corrections to inclusive cross sections well behaved

  19. Disentangling tW and tt production b-jet multiplicity Complete cross section Finite width contribution Finite b-quark mass allows full phase coverage Interesting to see how this will affect realistic analysis

  20. Top-quark mass measurements […my view.] Measurements based on established methods start to reach intrinsic limitations due to • Ambiguity MC mass  Pole mass • Sometimes used leading order analysis • Intrinsic uncertainties of the pole mass (and the fact that the measurements are getting incredible precise…) Very active field, new techniques and improved known methods under investigation New methods follow the same theme: Fit top mass through measurement of theoretically welldefined observable (holds also for distributions)

  21. New methods ─ mt from tt+1-jet Pert. theory reliable High sensitivity [Fuster et al 13] Requirements for sub percent level accuracy Further improvements to the already very precise measurements very challenging. Use complementary methods with comparable uncertainties to cross check measurements (Mlb, ttj, lepton-spectra, endpoint analysis) Small experimental uncertainties Insensitiv to PDFs Small impact of non-pert. modelling

  22. Constraining non-perturbative input of the SM Top-quark pair production sensitive to gluon distribution  Use top-quark pair production to constrain the gluon luminosity [Czakon, Mangano, Mitov, Rojo 13] X-Range: x = 0.06…0.5

  23. Determining the gluon PDF, mt and as in a global fit [Alekhin, Blümlein,Moch 14] Incl. of tt data leads to consistent results ABM11   ABM12 Top-quark mass determination: (correlations taken into account by the fit) As usual: Be careful that we don’t absorb New Physics into the PDFs Only a handful data points for inclusive cross section used so far, no distributions included, more to come

  24. Conclusion / Summary Tremendous progress in the theoretical description of top-quark physics Possible, due to many synergies in recent developments Per cent level accuracy for inclusive quantities (10%) level accuracy for exclusive processes even for “large” multiplicities Many recent results publicly available for the analysis in form of Computer code The time of “predicted discoveries” might be over for a while, remember method successfully applied over hundreds of years: Turn every stone and look below Top-quark physics is a very big one, many things may hide below

  25. Precision physics with tops ─ Future directions Further test top-quark physics at the quantum level Nice example: Top-quark charge asymmetry virt. corr. to tt t @ NLO real corr. to tt • SM predictions typically small, any significant deviation may point to new physics Alternative observables: transversal polarisation, specific spin correlations in tt (mw-mt-mh, vacuum stability are of the same type…)

  26. Inclusive top-quark pair production in NNLO Residual scale dependence [Top++ 2.0, Hathor 2.1] restricted scale variation to avoid large ratios [Top++ 2.0]

  27. Top-quark pair production ─ theory status Some milestones 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

  28. 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” My feeling: Large effects unlikely From different studies:

  29. Testing the SM with top-quarks ─ examples Consistency of the SM Vacuum stability [Degrassi, Di Vita, Elias-Miro,Spinosa,Giudici ’12, Alekhin, Djouadi, Moch ’12]  Precise theoretical predictions required

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