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

Marcella Capua, INFN and Calabria University Small X and Diffraction - FNAL September Diffractive γ*p interactions at HERA squared 4-momentum transfer to p: fraction of p momentum: fraction of the p momentum carried by the IP: fraction of the IP momentum carried by the struck parton: - large kinematic range and cross section: the order 10 % of DIS is diffractive - ~4  acceptance (important for  * dissociative system) - point-like couplings to probe the Pomeron structure - wide Q 2 range: to access the transition region from soft to hard processes Q2Q2 W MYMY

Marcella Capua, INFN and Calabria University Small X and Diffraction - FNAL September Large Rapidiy Gap method  X system and e’ measured System Y not measured, some theoretical and experimental uncertanties Integrate over t<1GeV 2 and M Y <1.6 GeV High acceptance p tag method o Measurement of t o Free of p-diss background o Lower acceptance M X method  High acceptance  t measurement not possible  systematics from p-diss Diffractive peak NB: if scattered proton not detected, background from proton dissociative events

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

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

Marcella Capua, INFN and Calabria University Small X and Diffraction - FNAL September M X < 2 GeV (resonance range) weak W dependance M X > 2 GeV d  /dM X rises with Q 2 a diff typically higher then soft Cross section energy dependence (M X method) As expected for soft IP Fit:

Marcella Capua, INFN and Calabria University Small X and Diffraction - FNAL September Energy dependence (M X method) Data (4<M X <8 GeV) consistent with the same W-dependence for the diffractive and the total cross section: total cross section: diffractive cross section: value of  IP diff higher than soft Pomeron indication of a rise of  IP diff with Q 2.

Marcella Capua, INFN and Calabria University Small X and Diffraction - FNAL September Ratio  diff /  tot (M X method) In DIS  diff /  tot independent of W ! low M X : strong decrease of  diff /  tot with increasing Q 2 high M X : no Q 2 dependence Regge th: at W=220 GeV:  diff (M X <35 GeV)/  tot ~ 20 % Q 2 = 2.7 GeV  10 % Q 2 = 27 GeV Same results from H1 and ZEUS-LPS

Marcella Capua, INFN and Calabria University Small X and Diffraction - FNAL September H1 Measurement of σ r D(3) (LRG method) F L unknown, F L = 0 or F L = F 2  few % error New low and very high Q 2 data!!! Agreement between measurements Red sample used for DGLAP QCD interpretation and  IP (0) calculation well described by the fit 3-fold differential diffr. struct. func. Diffractive reduced cross section  r D

Marcella Capua, INFN and Calabria University Small X and Diffraction - FNAL September  and Q 2 dependences large scaling violation except at high  → gluon dominated similar at all x IP → support for Regge factorization

Marcella Capua, INFN and Calabria University Small X and Diffraction - FNAL September NLO DGLAP QCD fit Gluon momentum fraction 75 ±15 % at Q 2 = 10 GeV 2 and remains large up to high Q 2 -Regge factorization -Singlet Σ and gluon g with Q 2 0 =3GeV 2 -NLO DGLAP evolution -Fit medium Q 2 (6 < Q 2 < 120 GeV 2 ) -Experimental and theor incertainties Diff PDF’s: Extended to large z Gluon dominate Σ well constrained

Marcella Capua, INFN and Calabria University Small X and Diffraction - FNAL September Test of QCD factorization Good agreement with prediction with NLO fit at medium Q 2 NLO Comparisons with Diff DIS Jets

Marcella Capua, INFN and Calabria University Small X and Diffraction - FNAL September 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 Q 2 and  dependencies to the inclusive DIS at medium  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