Inclusive Diffraction at HERA Marcella Capua – INFN and Calabria University

1. on behalf of Inclusive Diffraction at HERAMarcella Capua – INFN and Calabria University • experimental methods and diffractiveg*p cross section • Regge phenomenology • measurement of t and xIP dependence • comparison with inclusive ep cross section • QCD analysis of diffraction • - diffractive pdf Small X and Diffraction FNAL Chicago (USA) 17 – 20 September 2003

2. fraction of p momentum: squared4-momentum transfer to p: fraction of the p momentumcarried by the IP: fraction of the IP momentum carried by the struck parton: Diffractive γ*p interactions at HERA Q2 • large kinematic range and cross section: • the order 10 % of DIS is diffractive • ~4p acceptance (important for g* dissociative system) • point-like couplings to probe the Pomeron structure • wide Q2 range: to access the transition region from soft • to hard processes W MY Small X and Diffraction - FNAL September 2003

3. Diffractive peak Large Rapidiy Gap method • X system and e’ measured • System Y not measured, some theoretical and experimental uncertanties • Integrate over t<1GeV2 and MY<1.6 GeV • High acceptance MX method • High acceptance • t measurement not possible • systematics from p-diss • p tag method • Measurement of t • Free of p-diss background • Lower acceptance NB: if scattered proton not detected, background from proton dissociative events Small X and Diffraction - FNAL September 2003

4. Factorization • QCD Hard Scattering factorization (by Collins;Trentadue, Veneziano; Berera, Soper…:) At fixed xIP and t diffractive Parton Densities evolve according to DGLAP • Regge factorization - “resolved IP model”(IP with partonic structure): (Breit frame) Regge motivated pomeron flux Shape of diffractive pdfs independent of xIP and t Small X and Diffraction - FNAL September 2003

5. What we know about t-distribution (pTag method) Exponential fit to t distribution: Regge phenomenology predicts “shrinkage” of the diffractive peak (b rise as xIP→ 0): For xIP < 10 –2, data prevent any firm conclusion We are waiting new data from H1 and ZEUS proton tagger no Q2 dependence Small X and Diffraction - FNAL September 2003

6. Cross section energy dependence (MX method) Fit: As expected for soft IP • MX < 2 GeV (resonance range) weak W dependance • MX > 2 GeVd/dMXrises with Q2adiff typically higher then soft Small X and Diffraction - FNAL September 2003

7. Energy dependence (MX method) diffractive cross section: total cross section: Data (4<MX<8 GeV) consistent with the same W-dependence for the diffractive and the total cross section: value of IP diff higher than soft Pomeron indication of a rise of IPdiff with Q2. Small X and Diffraction - FNAL September 2003

8. Ratio diff/tot (MX method) Regge th: • low MX:strong decrease of diff/tot • with increasing Q2 • high MX:no Q2dependence diff(MX<35 GeV)/tot ~ 20 %Q2= 2.7 GeV  10 %Q2= 27 GeV • at W=220 GeV: In DIS diff/tot independent of W ! Same results from H1 and ZEUS-LPS Small X and Diffraction - FNAL September 2003

9. Diffractive reduced cross section srD • 3-fold differential diffr. struct. func. H1 Measurement of σrD(3) (LRG method) New low and very high Q2 data!!! Agreement between measurements Red sample used for DGLAP QCD interpretation and aIP(0) calculation well described by the fit FL unknown, FL = 0 or FL = F2 few % error Small X and Diffraction - FNAL September 2003

10. b and Q2 dependences large scaling violation except at high b→ gluon dominated similar at all xIP→support for Regge factorization Small X and Diffraction - FNAL September 2003

11. NLO DGLAP QCD fit -Regge factorization -Singlet Σ and gluon g with Q20=3GeV2 -NLO DGLAP evolution -Fit medium Q2 (6 < Q2< 120 GeV2) -Experimental and theor incertainties Diff PDF’s: Extended to large z Gluon dominate Σ well constrained Gluon momentum fraction 75 ±15 % at Q2 = 10 GeV2 and remains large up to high Q2 Small X and Diffraction - FNAL September 2003

12. NLO Comparisons with Diff DIS Jets Test of QCD factorization Good agreement with prediction with NLO fit at medium Q2 Small X and Diffraction - FNAL September 2003

13. Summary • recent data from H1 and ZEUS with improved precision and extended • kinematic range • data prevent any firm conclusion about shrinkage (waiting for new proton • tagged data) • value of diffractive IP higher than soft Pomeron • W-dependence of diffractive and total cross section similar on DIS • similar Q2 and b dependencies to the inclusive DIS at medium b • NLO QCD fit of inclusive diffractive gives a good description of the data: • diffractive pdf dominated by gluons • diffractive pdf predictions well confirmed by tests Small X and Diffraction - FNAL September 2003