WG-A Summary

Working Group A

Structure Functions, Low x andDiffraction

Robin Devenish

Victor Fadin

(Yuri Kovchegov)

WG-A Summary

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….

WG-A Summary

Inclusive DIS Data

e-p

F2 new data at low Q2 and x > 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 NC CC – final HERA-I e+p data (Dubak, H1; Rautenberg, ZEUS)

-NF2, xF3 new data from high stats improved and -bar beams (Naples, NuTeV)

WG-A Summary

FL from ISR ~36 pb-1 (J Cole)

Need variable s and high y – lower E'e

WG-A Summary

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 sin2W

• 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.

WG-A Summary

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, Q2

min)•NLO DGLAP not enough

- NNLO?- BFKL improvements?

WG-A Summary

Alekhin NNLO

WG-A Summary

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)

WG-A Summary

• 8 orders of magnitude!

• Highest ET jets ever!

Comparison: prediction vs. first data from Tevatron IICDF – La Thuile and Moriond

(Tung)

WG-A Summary

– Cross-section is multiplicatively dependent on anti-quarks, i.e. u·u, d·d, …compared to additively dependent, i.e. u+d + u+d … 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 e+p / e-p (and ep / ed) measurements at HERA.

- ---

Importance of D-Y data in tightening flavour constraints

(Tung)

WG-A Summary

RHIC A-A collisions and low x

Spectators

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!

(B Cole)

WG-A Summary

Low-x Observables in PHENIX (B Cole)

Charged MultiplicityCharged MultiplicityPad Chambers:Pad Chambers:RRPC1PC1 = 2.5 m = 2.5 mRRPC3PC3 = 5.0 m = 5.0 m||<0.35, =

Transverse EnergyTransverse EnergyLead-Scintillator Lead-Scintillator EMCal:EMCal:RREMCEMC = 5.0 m = 5.0 m||<0.38, = (5/8)

Trigger & CentralityTrigger & Centrality Beam-Beam Counters:Beam-Beam Counters:3.0<||<3.9, = 20º Calorimeters:0º Calorimeters:|| > 6, |Z|=18.25 m

Collision Region (not to scale)

WG-A Summary

•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 >> 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 (Qs

2) 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 (Qs

2)

Saturation in Heavy Ion Collisions

WG-A Summary

dN/d Saturation Model Comparisons

• 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 ?!!

Kharzeev and Levin Phys. Lett. B523:79-87, 2001

dN

/d

per

part

. p

air

dN

/d

WG-A Summary

GVDM and Colour Dipole models

• At low x and Q2 a partonic approach may not make sense – return to an older picture for *p – vector dominance

• Talks by Ingelman and Schildknecht reviewed this approach.• Schildknecht emphasised the wider applicability of the GVDM approach – consistent picture of inclusive *p, *p 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 *p, *A, *p 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)

WG-A Summary

Proton

b – impact parameter

Impact Parameter Dipole Saturation Model

T(b) - proton shape 1)( 2

0

bdbT

well motivated:

Glauber- Mueller dipole cross section

))()/,(32

exp(12 ),( 2

022

2

2bTQrCxxgr

bd

rxds

qq

(Kowalski)

WG-A Summary

d)-(2 parameter impact - d)-(2 momentum transv.- 2 bt

t-dependence of the diffractive cross sections determines the b distribution

22222

2*1

0

22 |),,())(),(32

exp(1),,(|16

1*

rzQbTxxgrrzQdzebdrddt

dsVM

bip

VM

)2/exp(~)(

)exp(~

2 BbbT

tBdt

d diff

)/()2/)(exp()(

)2/exp()(

022

2

EGGY

GG

wbKwbbbdbT

wbbT

(Kowalski)

WG-A Summary

DVCS and vector mesons

• New measurement of DVCS by ZEUS (Abramowicz)

• New results at large |t| for inelastic J/ from H1 (Beckingham)

• New results on elastic and J/ from H1 (Fleischmann)

• New results from ZEUS on and J/ (Tandler)

• First look at vector mesons with COMPASS (Korzenev)

• New data on gg to rr from L3 at LEP (Fedin)

• Issues• energy dependence W as function of MV and Q2

• choice of scale Q2+M2

WG-A Summary

Deeply Virtual Compton Scattering

The process

The background

Data and dipole models

(Abramowicz)

WG-A Summary

H1 VM results

(Beckingham)Change from exponential to (-t)-n

With n ~ 3

(Fleischmann)Energy dependence asFunction of Q2 – elastic

WG-A Summary

ZEUS VM (Tandler) - Ratio V/tot

• W independence for cannot be explained by pQCD or Regge

still unknown soft physics?

J/

pQCD:

V S/Q6 |xG(x,Q2)|2

V/tot W2/b

Regge:

V W4 (01)

V/tot W2 (01) /b

Q2+M2

4 GeV2

WG-A Summary

COMPASS experiment at CERN

WG-A Summary

Vector mesons with COMPASS

(Korzenev)

Will very soon be provinding a lot of high statistics results – with advantage of polarized beam and target

WG-A Summary

Cross section for

Broad enhancement near threshold of

L3 at LEP (Fedin)

WG-A Summary

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)

WG-A Summary

Ratio of hard diffraction to total DIS - ZEUS

(Lim)

WG-A Summary

Energy Dependence of Hard diffraction

(Lim)

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Tests of factorization – Schaetzel H1

Factorisation works at large Q2 - but not for photoproduction

WG-A Summary

(Terashi CDF)

D0 has pots in quads as well

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Erhan & Schlein test of factorization

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WG-A Summary

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…