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QCD at high energy ( experiments )

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31st International Conference on High Energy Physics

Amsterdam, The Netherlands

24th – 31st July 2002

QCD at high energy (experiments)

K. Long Imperial College London

- 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

- 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-jet rate:

Precision!~1%

High sensitivity to S at LO

O(S3) and NLL resum.

Angular correlations sensitive to colour factors

Simultaneous fit to 4 angles and R4

- 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.

Characterise event shape using:

Thrust - T

and MHBTBWCy23

Breit frame

- NLO (O(S2)) gives reasonable description over wide range of Q2 and EBT,jet
- Fit to extract S

Cone jetsR = 0.7

CDF

Extrapn

Used in fit

Q2

- NC~CC at high Q2 – EW unification
- SM ( QCD & EW) describes data over 6 orders of magnitude - long lever-arm for QCD studies

Neutral current

x

Z interference for:e-p constructivee+p destructive

e-p>e+pat high Q2:

Z-interference

DIS

- Measurement of valence quarks at high Q2
- Needs HERA-II

DIS

e+p

e-p

Charged current

Towards flavour decomposition:

Sensitivity at high x:

e-pu-quarke+pd-quark

- 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

~3%

~10%

- S determined in PDF fits is correlated with gluon density

- 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

- 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

- 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

- 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

- Two techniques:
- Exploit absence of hadronic energy flow in forward direction – ‘rapidity-gap’ selection.

- Tag scattered proton to determine t dependence:

Gap!

Data consistent with ‘Regge’ factorisation:

Q2

Factorisation (Collins)

- Good description of data
- Positive scaling violations ( < 0.6) diffractive PDFs are gluon dominated

Predictive?

Diff. dijets

Diff. D*

First set of diffractive PDFs!

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

Direct: high xobs

Resolved: low xobs

- 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

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

- 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!