Low-x physics at HERABrian FosterBristol/DESYCorfu Summer School, 4.9.01Other probes of QCD dynamics @ HERADiffraction and its connection with low-x DISSummary & Outlook

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Where are we now?

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QCD as a description of low xFactorization - hard processes can be regarded as convolutionof sub-process cross section with probability to find participating partons in target & probe - subsequenthadronisation ~ independent process

For DIS can (normally) consider virtual photon as d-function=> s ~ f s where s is sub-process cross section f is parton dist. function, satisfying f/ m ~ f P (m = renormalisation scale, P is a splitting function)

Brian Foster - Corfu lectures

QCD evolutionIn general Ps are perturbative expansions to particularorders, keeping terms most important for particular regions:Leading ln Q2 terms come, in axial gauge, from evolution along parton chain strongly ordered in transverse momenta, LO DGLAP sums up terms - NLO sums terms which arise when two adjacent kts become comparable, losing factor ln Q2.

Brian Foster - Corfu lectures

QCD evolution

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Low-x structure function data s = k+P =energy in the ep c.m.s.Q2 = -(k-k')2 = -q2 = virtuality of the exchanged x = Q2/(2P q) =fraction of proton momentumcarried by the struck quarky = (P q)/(P k) =fraction of beam lepton energytransferred to the photon

W 2 = ys ~ Q2/ x energy in the *p c.m.s.

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BPT F2 (low Q2) from ZEUS

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F2 (low Q2) from ZEUS & H1

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FL from QCD fit from H1

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FL from QCD fit from H1

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FL from QCD fit from H1

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III: Other probes of QCD dynamicsForward p0 @ H1

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Forward jets @ H1

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Forward Jets @ H1

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IV. Interpretation & models

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Logarithms

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More Logarithms

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NLO QCD fitsGRV98Deviation of exp. data from CTEQ fitCTEQ

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PDFs with errorsBotje

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NNLO PDF fitting

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NNLO PDF fitting

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F2 & its derivatives

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F2 & its derivatives

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F2 & its derivatives

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What is happening at low x?

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Dipole Models Inclusive F2

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Dipole Models

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Fits to slopes

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QCD fit pdfs

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QCD fit FL

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F2 derivatives - summary

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Diffraction

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Diffraction

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Diffraction

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Diffraction - vector mesons

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Diffraction - vector mesons

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Diffraction - vector mesons

Brian Foster - Corfu lectures

Diffraction - vector mesons

Brian Foster - Corfu lectures

Diffraction - vector mesons

Brian Foster - Corfu lectures

Diffraction - vector mesons

Brian Foster - Corfu lectures

Diffraction - vector mesons

Brian Foster - Corfu lectures

Diffraction - vector mesons

Brian Foster - Corfu lectures

Diffraction - vector mesons

Brian Foster - Corfu lectures

Diffraction - vector mesons

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Deeply virtual Compton scattering Measures Re part of a QCD amplitudeMeasures skewed parton distributions -generalisation on normal proton pdfs.

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Deeply virtual Compton scattering DVCS process clearly necessary - extract cross-section

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Deeply virtual Compton scattering

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V Summary & Outlook

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Summary & Outlook

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Summary & Outlook

Brian Foster - Corfu lectures