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QCD at high energy ( experiments )

31 st International Conference on High Energy Physics. Amsterdam, The Netherlands 24 th – 31 st July 2002. QCD at high energy ( experiments ). K. Long Imperial College London. Mission statement. QCD: Theory of quarks and gluons

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QCD at high energy ( experiments )

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  1. 31st International Conference on High Energy Physics Amsterdam, The Netherlands 24th – 31st July 2002 QCD at high energy (experiments) K. Long Imperial College London

  2. Mission statement • QCD: Theory of quarks and gluons • Ambition: develop complete and quantitative understanding of QCD in all kinematic regions • Precise measurement of S and colour factors • Proton structure and pQCD: Structure functions, evolution; gluons  Pomeron transition • Compile extensive ‘library’ of precise measurements to compare with predictions and catalyze theoretical developments

  3. Plan of the talk: • Measurement of S and colour factors • Proton and photon structure • Diffraction • Dijets in photoproduction Apology: Short talk covering broad, vibrant field: I apologise for not being able to show all important results.

  4. S from 4-jet rate in e+e- 4-jet rate: Precision!~1% High sensitivity to S at LO O(S3) and NLL resum.

  5. S and colour factors from e+e- Angular correlations sensitive to colour factors Simultaneous fit to 4 angles and R4

  6. S and colour factors from e+e- • Additional constraints: • Ratio of multiplicity in gluon and quark jets (DELPHI) • -function determined in RGI analysis (DELPHI) Note: DELPHI R4 constraint is included in ‘DELPHI combined’. Not shown for clarity.

  7. S from event shapes in e+e- Characterise event shape using: Thrust - T and MHBTBWCy23

  8. S from event shapes in e+e-

  9. S from inclusive jet production in ep Breit frame • NLO (O(S2)) gives reasonable description over wide range of Q2 and EBT,jet • Fit to extract S

  10. S from jet rates in ep

  11. S from jet rates in pp Cone jetsR = 0.7 CDF Extrapn Used in fit

  12. Deep inelastic scattering @ high Q2 Q2 • NC~CC at high Q2 – EW unification • SM ( QCD & EW) describes data over 6 orders of magnitude - long lever-arm for QCD studies

  13. DIS at high Q2 Neutral current x Z interference for:e-p constructivee+p destructive e-p>e+pat high Q2: Z-interference

  14. Proton structure DIS • Measurement of valence quarks at high Q2 • Needs HERA-II

  15. DIS Proton structure e+p e-p Charged current Towards flavour decomposition: Sensitivity at high x: e-pu-quarke+pd-quark

  16. QCD anal. of Sfns • Data well described by NLO QCD: DGLAP evolution: • Fit to determine PDFs and S:Emphasis on full treatment of errors:- Correlated exp. Errors- Theory/param. errors

  17. PDFs from NLO QCD analysis of S.Fns ~3% ~10%

  18. S and gluon from PDF fits • S determined in PDF fits is correlated with gluon density

  19. Search for limit of validity of DGLAP • Parameterise F2: • Fit for  at fixed Q2 • Change in dependence of  on Q2 for Q2 ~ 1 GeV2 … • But at fixed Q2,  shows no dependence on x

  20. Inclusive jet cross section in pp • Tevatron jets: • QCD at very high scales • Partons at high x and very high Q2 • D0: inclusive jet cross section as a function of pseudorapidity • NLO QCD gives good description of ET and  dependence

  21. Inclusive jets in pp • Main difference in new fits is enhanced gluon at high x • CTEQ/MRST groups: inclusion of D0 data in global fit to determine parton distributions pt CTEQ6 • Good description over full pt and  range

  22. Photon structure • Fit to F2 for S: • [Albino et al.] • PDF (g) uncertainty? Q2 x P2~0 • QCD evaluated with NLO PDFs (GRV-HO) describe data at ~20% level

  23. Diffraction in ep DIS • Two techniques: • Exploit absence of hadronic energy flow in forward direction – ‘rapidity-gap’ selection. • Tag scattered proton to determine t dependence: Gap!

  24. Inclusive diffraction: interpretation Data consistent with ‘Regge’ factorisation: Q2 Factorisation (Collins)  • Good description of data • Positive scaling violations ( < 0.6)  diffractive PDFs are gluon dominated

  25. Diffractive parton densities Predictive? Diff. dijets Diff. D* First set of diffractive PDFs!

  26. Inclusive diffraction: transition to low Q2 Model Pomeron as two gluon system • Model gives reasonable description of data • qqg contribution dominates • In proton rest frame: • Virtual photon fluctuates into a qq (or qqg) state • qq (or qqg) dipole scatters elastically from proton • Xsect. plateaus when dipole size ~ proton size

  27. Dijet production in photoproduction Direct: high xobs Resolved: low xobs

  28. Dijet production in p • ZEUS data falls more slowly with ETjet1 than NLO QCD Sensitivity to -PDFs? H1 • Data described by NLO QCD for x < 0.8 in both ET,max bins

  29. Dijet production in p ZEUS H1 Sensitivity to jet cuts • Dijet cross section: resolved enriched versus ETjet2,cut • LL MC (HERWIG) describes SHAPE • NLO calculation gives reasonable normalisation, but SHAPE requires NNLO

  30. Conclusions • New and updated meas. of S  1-5%. Many techniques. • Fundamentals of QCD ‘check out’: S(Q2), colour factors. • NLO QCD analyses have determined PDFs with precision. Data and theory (nearly) ready for NNLO • Low Q2, low x inclusive DIS and diffraction – fertile fields. New data, many new ideas.

  31. Conclusions • Eagerly awaiting large data sets from HERA, TeVatron • Now, more than ever, require diverse programme of measurement and interpretation Many thanks to all who helped me prepare this talk. Especially, K. Nagano, O. Gonzalez, A. Tapper and E. Laenen. • Jet and heavy flavour production: NLO gives good qualitative description, require exptl and theorl progress to extract quantitative information In partnership we (theorists, phenomenologists and experimentalists) can achieve our ambition!

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