WG-A Summary Working Group A Structure Functions, Low x and Diffraction 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 F 2 new data at low Q 2 and x > from QED Comptons (Lenderman, H1) F L first measurement at HERA using ISR events (J Cole, ZEUS) new extraction at high y and low Q 2 (Lobodzinska, H1) High Q 2  NC  CC – final HERA-I e + p data (Dubak, H1; Rautenberg, ZEUS) -N F 2, xF 3 new data from high stats improved and -bar beams (Naples, NuTeV)

WG-A Summary F L 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 sin 2  W 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 Q 2 Individual fits describe the data very well There are well known worries about the partons at low x and Q 2 – gluon rising less steeply than the sea and eventually going negative MRST study of ‘stability’ against data fit (x min, Q 2 min ) NLO DGLAP not enough - NNLO? - BFKL improvements?

WG-A Summary Alekhin NNLO

WG-A Summary Importance of F L Summary of H1 extraction of F L – now extended to lower Q 2 F L with a precision of present F 2 would do wonders for low x physics! (Lobodzinska)

WG-A Summary 8 orders of magnitude! Highest E T 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 (N chg, E T ) determined by gluons Can this be measured and models tested? YES! (B Cole)

WG-A Summary Low-x Observables in PHENIX (B Cole) Charged Multiplicity Pad Chambers: R PC1 = 2.5 m R PC3 = 5.0 m |  |<0.35,  =  Transverse Energy Lead-Scintillator EMCal: R EMC = 5.0 m |  |<0.38,  = (5/8)  Trigger & Centrality Beam-Beam Counters: 3.0<|  |<3.9,  = 2  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 Q 2 “sees” multiple partons, twist expansion fails i.e. when  >> 1 New scale: Q s 2  Q 2 at which  = 1 –Take cross section  =   s (Q 2 ) / Q 2 –Gluon area density in nucleus   xG(x, Q 2 )  nucleon –Then solve: Q s 2 = [constants]  s (Q s 2 ) xG(x, Q s 2 )  nucleon Observe: Q s depends explicitly on  nucleon –KLN obtain Q s 2 = 2 GeV 2 at center of Au nucleus. –But gluon flux now can now be related to Q s   Q s 2 /  s (Q s 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   ) M 2 = Q s 1 GeV –Compare to PHOBOS data at 130 GeV. –Incredible agreement ?!! Kharzeev and Levin Phys. Lett. B523:79-87, 2001 dN/d  per part. pair dN/d 

WG-A Summary GVDM and Colour Dipole models At low x and Q 2 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 Q 2 inclusive  *p,  *A,  *p to Vp, DVCS - and allow for saturation A combined GVDM + dipole model (Szczurek) gives a good description of low Q 2 inclusive data – to Q 2 = 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 well motivated: Glauber- Mueller dipole cross section (Kowalski)

WG-A Summary t-dependence of the diffractive cross sections determines the b distribution (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 M V and Q 2 choice of scale Q 2 +M 2

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 as Function of Q 2 – 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 /Q 6 |xG(x,Q 2 )| 2  V /  tot  W 2 /b Regge:  V  W 4 (  0  1)  V /  tot  W 2 (  0  1) /b Q 2 +M 2 4 GeV 2

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)

WG-A Summary Tests of factorization – Schaetzel H1 Factorisation works at large Q 2 - but not for photoproduction

WG-A Summary (Terashi CDF) D0 has pots in quads as well

WG-A Summary

Erhan & Schlein test of factorization

WG-A Summary

Summary so far… There is much still to learnt and measured… QCD is good shape at large Q 2 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 Q 2 requires theoretical effort Which leads into part II…