Working Group A Structure Functions, Low x and Diffraction

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Working Group AStructure Functions, Low x
andDiffraction

• Robin Devenish
• Victor Fadin
• (Yuri Kovchegov)

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OUTLINE pt I (RD)

1. Inclusive DIS Data
2. QCD fits and parton densities
3. RHIC A-A collisions and low x
4. GVDM and Colour Dipole models
5. DVCS and vector mesons
6. Hard Diffraction

Cannot do justice to all the 60 talks and
discussion.
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Inclusive DIS Data
e-p F2 new data at low Q2 and x gt 0.001 from
QED Comptons (Lenderman, H1) FL first
measurement at HERA using ISR events (J
Cole, ZEUS) new extraction at high y and
low Q2 (Lobodzinska, H1) High Q2 sNC sCC final
HERA-I ep data
(Dubak, H1 Rautenberg, ZEUS) n-N F2, xF3
new data from high stats improved n and n-bar
beams (Naples, NuTeV)
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FL from ISR 36 pb-1 (J Cole)
Need variable s and high y lower E'e
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QCD fits and parton densities

• Updates by ZEUS (Rautenberg) and H1 (Reisert) of
  NLO QCD fits
• Detailed study (Portheault) on s-quark asymmetry
  and effect on NuTeV sin2qW
• MRST (Martin) on uncertainties
• CTEQ (Tung) on recent improvements
• Beyond NLO (Alekhin) very careful study using
  NNLO extensions, target mass and higher twist
• Technical improvement in fitting procedure -
  diagonalising Hessian matrix gives numerical
  stability and enables PDF uncertainties (from
  exp. sys errs) to be propagated to observables.
  Now well established thanks to work of CTEQ.

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Problems with at low x and Q2

• Individual fits describe the data very well
• There are well known worries about the partons
  at low x and Q2 gluon rising less steeply than
  the sea and eventually going negative
• MRST study of stability against data fit
  (xmin, Q2min)
• NLO DGLAP not enough
• NNLO?
• BFKL improvements?

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Alekhin NNLO
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Importance of FL
Summary of H1 extraction of FL now extended to
lower Q2 FL with a precision of present F2 would
do wonders for low x physics!
(Lobodzinska)
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Comparison prediction vs. first data from
Tevatron IICDF La Thuile and Moriond

• 8 orders of magnitude!
• Highest ET jets ever!

(Tung)
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Importance of D-Y data in tightening flavour
constraints
(Tung)

• Cross-section is multiplicatively dependent on
  anti-quarks, i.e. uu, dd, compared to
  additively dependent, i.e. ud ud in DIS.
• E605 (1989, pA collision) has been the main
  source of experimental input to global analysis
• pp/pd measurement of NA51 and E866 revolutionized
  the determination of d/u quark distribution ratio
  several years ago.
• Recent E866 data on separate pp and pd
  cross-sections will make the d/u discrimination
  much more quantitative.

The d/u ratio at large x is more directly probed
by the CC ep / e-p (and ep / ed) measurements
at HERA.
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RHIC A-A collisions and low x
Spectators
(B Cole)
Impact parameter (b)

• Central collisions (small b) produce high
  density gluons
• Universality of low x gluon dynamics (input
  from HERA)
• Produced particle properties (Nchg, ET)
  determined by gluons
• Can this be measured and models tested?
• YES!

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Low-x Observables in PHENIX (B Cole)

• Charged Multiplicity
• Pad Chambers
• RPC1 2.5 m
• RPC3 5.0 m
• ?lt0.35, ???
• Transverse Energy
• Lead-Scintillator EMCal
• REMC 5.0 m
• ?lt0.38, ?? (5/8)?
• Trigger Centrality
• Beam-Beam Counters
• 3.0lthlt3.9, ?? 2?
• 0º Calorimeters
• h gt 6, Z18.25 m

Collision Region (not to scale)
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Saturation in Heavy Ion Collisions

• Kharzeev, Levin, Nardi Model
• Large gluon flux in highly boosted nucleus
• When probe w/ resolution Q2 sees multiple
  partons, twist expansion fails
• i.e. when ?? gtgt 1
• New scale Qs2 ? Q2 at which ?? 1
• Take cross section ? ? ?s(Q2) / Q2
• Gluon area density in nucleus ? ? xG(x, Q2)
  ?nucleon
• Then solve Qs2 constants ?s (Qs2) xG(x,
  Qs2) ?nucleon
• Observe Qs depends explicitly on ?nucleon
• KLN obtain Qs2 2 GeV2 at center of Au nucleus.
• But gluon flux now can now be related to Qs
• ? ? Qs2 / ?s (Qs2)

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dN/d? Saturation Model Comparisons
Kharzeev and Levin Phys. Lett. B52379-87, 2001

• Additional model input
• x dependence of G(x) outside saturation region
• xG(x) x-? (1-x)4
• GLR formula for inclusive gluon emission
• To evaluate yield when one of nuclei is out
  of saturation.
• Assumption of gluon mass (for y ? ?)
• M2 Qs 1 GeV
• Compare to PHOBOS data at 130 GeV.
• Incredible agreement ?!!

dN/d? per part. pair
dN/d?
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GVDM and Colour Dipole models

• At low x and Q2 a partonic approach may not make
  sense return to an older picture for gp
  vector dominance
• Talks by Ingelman and Schildknecht reviewed this
  approach.
• Schildknecht emphasised the wider applicability
  of the GVDM approach consistent picture of
  inclusive gp, gp to Vp and DVCS and close
  connection to colour dipole models
• Dipole models - very flexible and powerful
  framework for describing low x low Q2 inclusive
  gp, gA, gp to Vp, DVCS - and allow for
  saturation
• A combined GVDM dipole model (Szczurek) gives
  a good description of low Q2 inclusive data to
  Q2 0 limit
• Crucial extension from early GBW approach is to
  add transverse profile in form of impact
  parameter (Kowalski)

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Impact Parameter Dipole Saturation Model
(Kowalski)
Proton
b impact parameter
well motivated
Glauber- Mueller dipole cross section
T(b) - proton shape
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(Kowalski)
t-dependence of the diffractive cross sections
determines the b distribution
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DVCS and vector mesons

• New measurement of DVCS by ZEUS (Abramowicz)
• New results at large t for inelastic J/y from
  H1 (Beckingham)
• New results on elastic r and J/y from H1
  (Fleischmann)
• New results from ZEUS on r and J/y (Tandler)
• First look at vector mesons with COMPASS
  (Korzenev)
• New data on gg to rr from L3 at LEP (Fedin)
• Issues
• energy dependence Wd as function of MV and Q2
• choice of scale Q2M2

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Deeply Virtual Compton Scattering
(Abramowicz)
The process
The background
Data and dipole models
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H1 VM results
(Fleischmann) Energy dependence as Function of
Q2 elastic r
(Beckingham) Change from exponential to
(-t)-n With n 3
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ZEUS VM (Tandler) - Ratio sV/stot
J/y
pQCD sV ? aS/Q6 xG(x,Q2)2 sV/stot ? W2l/b
Q2M2 4 GeV2
Regge sV ? W4 (a(0) - 1) sV/stot ? W2 (a(0) -
1) /b

• W independence for r cannot be explained by pQCD
  or Regge
• ? still unknown soft physics?

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COMPASS experiment at CERN
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Vector mesons with COMPASS
(Korzenev)
Will very soon be provinding a lot of high
statistics results with advantage of polarized
beam and target
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Cross section for ?? ? ??
L3 at LEP (Fedin)
? Broad enhancement near threshold of ?? ? ??
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Hard Diffraction

• New results on inclusive hard diffraction in DIS
  from H1 (Coppens) and ZEUS (Lim)
• New results on hard diffractive final state
  (Schaetzel)
• New results on leading baryons from ZEUS
  (Soares)
• First look at Diffraction at Tevatron Run II
  (Terashi, CDF)
• D0 showed first diffractive Z0 (Stevenson)
• Issues
• Energy dependence (hard or soft Pomeron)
• Factorisation within ep and between ep and ppbar
  (Schlein)

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Ratio of hard diffraction to total DIS - ZEUS
(Lim)
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Energy Dependence of Hard diffraction
(Lim)
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Tests of factorization Schaetzel H1
Factorisation works at large Q2 - but not for
photoproduction
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(Terashi CDF)
D0 has pots in quads as well
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Erhan Schlein test of factorization
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Summary so far

• There is much still to learnt and measured
• QCD is good shape at large Q2 and hard scales
• Partons are becoming a precision tool
• Prospect of connecting HERA to heavy ions is
  exciting wider universality of gluon dynamics?
• But low x and low Q2 requires theoretical effort
• Which leads into part II