Qcd at high energy experiments
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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

31st International Conference on High Energy Physics

Amsterdam, The Netherlands

24th – 31st July 2002

QCD at high energy (experiments)

K. Long Imperial College London

Mission statement

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

Plan of the talk

Plan of the talk:

  • Measurement of S and colour factors

  • Proton and photon structure

  • Diffraction

  • Dijets in photoproduction


Short talk covering broad, vibrant field: I apologise for not being able to show all important results.

S from 4 jet rate in e e

S from 4-jet rate in e+e-

4-jet rate:


High sensitivity to S at LO

O(S3) and NLL resum.

S and colour factors from e e

S and colour factors from e+e-

Angular correlations sensitive to colour factors

Simultaneous fit to 4 angles and R4

S and colour factors from e e1

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.

S from event shapes in e e

S from event shapes in e+e-

Characterise event shape using:

Thrust - T

and MHBTBWCy23

S from event shapes in e e1

S from event shapes in e+e-

S from inclusive jet production in ep

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

S from jet rates in ep

S from jet rates in ep

S from jet rates in pp

S from jet rates in pp

Cone jetsR = 0.7



Used in fit

Deep inelastic scattering @ high q 2

Deep inelastic scattering @ high Q2


  • NC~CC at high Q2 – EW unification

  • SM ( QCD & EW) describes data over 6 orders of magnitude - long lever-arm for QCD studies

Dis at high q 2

DIS at high Q2

Neutral current


Z interference for:e-p constructivee+p destructive

e-p>e+pat high Q2:


Proton structure

Proton structure


  • Measurement of valence quarks at high Q2

  • Needs HERA-II

Proton structure1


Proton structure



Charged current

Towards flavour decomposition:

Sensitivity at high x:


Qcd anal of sfns

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

Pdfs from nlo qcd analysis of s fns

PDFs from NLO QCD analysis of S.Fns



S and gluon from pdf fits

S and gluon from PDF fits

  • S determined in PDF fits is correlated with gluon density

Search for limit of validity of dglap

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

Inclusive jet cross section in pp

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

Inclusive jets in pp

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



  • Good description over full pt and  range

Photon structure

Photon structure

  • Fit to F2 for S:

  • [Albino et al.]

  • PDF (g) uncertainty?




  • QCD evaluated with NLO PDFs (GRV-HO) describe data at ~20% level

Diffraction in ep dis

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:


Inclusive diffraction interpretation

Inclusive diffraction: interpretation

Data consistent with ‘Regge’ factorisation:


Factorisation (Collins)

  • Good description of data

  • Positive scaling violations ( < 0.6)  diffractive PDFs are gluon dominated

Diffractive parton densities

Diffractive parton densities


Diff. dijets

Diff. D*

First set of diffractive PDFs!

Inclusive diffraction transition to low q 2

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

Dijet production in photoproduction

Dijet production in photoproduction

Direct: high xobs

Resolved: low xobs

Dijet production in p

Dijet production in p

  • ZEUS data falls more slowly with ETjet1 than NLO QCD

Sensitivity to -PDFs?


  • Data described by NLO QCD for x < 0.8 in both ET,max bins

Dijet production in p1

Dijet production in p



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



  • 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.



  • 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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