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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 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 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
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!