1. Marta Ruspa, "Inclusive diffraction", DIS 20041 Inclusive diffraction Diffractive cross section and diffractive structure function Comparison with colour dipole models NLO QCD fit Marta Ruspa Univ. of Eastern Piedmont-Novara and INFN-Torino (Italy) XII International Workshop on Deep Inelastic Scattering Strbske Pleso, High Tatras, Slovakia April 14-18, 2004 on behalf of

2. Marta Ruspa, "Inclusive diffraction", DIS 20042 IP Q2Q2 W MXMX e’ p’ ** e p Q 2 = virtuality of photon = = (4-momentum exchanged at e vertex) 2 t = (4-momentum exchanged at p vertex) 2 typically: |t|<1 GeV 2 W = invariant mass of photon-proton system M X = invariant mass of photon-Pomeron system x IP = fraction of proton’s momentum taken by Pomeron ß = Bjorken’s variable for the Pomeron = fraction of Pomeron’s momentum carried by struck quark = x/x IP x IP t Inclusive diffraction γ * p  Xp  Exchange of an object with the vacuum q. n.  Proton almost intact after the collision

3. Marta Ruspa, "Inclusive diffraction", DIS 20043 (Breit frame) Diffractive DIS in the Breit frame Diffractive Deep Inelastic Scattering probes the diffractive PDFs of the proton relevant when the vacuum quantum numbers are exchanged f i/p D (z,Q 2,x IP,t): probability to find in a proton, with a probe of resolution Q 2, parton i with momentum fraction z, under the condition that the proton remains intact and emerges with small energy loss, x IP, and momentum transfer,t HARD SCATTERING FACTORISATION  DIS of a pointlike virtual photon off the exchanged object  PDFs

4. Marta Ruspa, "Inclusive diffraction", DIS 20044 Diffractive DIS in the colour dipole picture We can learn more about the structure of the proton by studying DDIS in a frame in which the virtual photon is faster than the proton (γ* much faster than p) Lifetime of dipoles very long due to large γ boost (E γ ~ W 2 ~ 1/x  50TeV ! )  it is the dipole that interacts with the proton ! Transverse size of dipoles proportional to  can be so small that the strong interaction with proton can be treated perturbatively ! 2 gluon exchange: LO QCD realisation of vacuum q.n.

5. Marta Ruspa, "Inclusive diffraction", DIS 20045 Diffractive DIS in the colour dipole picture BEKW model : at medium β; at small β saturation model : (colour transparency) as Q 2  0, growth tamed by saturating We can learn more about the structure of the proton by studying DDIS in a frame in which the virtual photon is faster than the proton (γ* much faster than p) 2 gluon exchange: LO QCD realisation of vacuum q.n.

6. Marta Ruspa, "Inclusive diffraction", DIS 20046 e p Exchange of color singlet producing a GAP in the particle flow Inclusive diffraction γ * p  Xp  No activity in the forward direction  Proton suffers only a small energy loss M X method

7. Marta Ruspa, "Inclusive diffraction", DIS 20047 Diffr. Non-diffr. c, b from fit  n.d. events subtracted contamination from reaction ep  eXN Selection of events γ * p  Xp with M x method Properties of M x distribution: - exponentially falling for decreasing M x for non- diffractive events - flat vs ln M x 2 for diffractive events Forward Plug Calorimeter (FPC): CAL acceptance extended by 1 unit in pseudorapidity from η=4 to η=5  higher M x and lower W  if M N > 2.3 GeV deposits E FPC > 1 GeV recognized and rejected! Diffr. Non-diffr.

8. Marta Ruspa, "Inclusive diffraction", DIS 20048 e p Exchange of color singlet producing a GAP in the particle flow Inclusive diffraction γ * p  Xp  No activity in the forward direction  Proton suffers only a small energy loss LPS method M X method

9. Marta Ruspa, "Inclusive diffraction", DIS 20049 Free of p-diss background Low acceptance  low statistics Selection of events γ * p  Xp with LPS Diffractive peak

10. Marta Ruspa, "Inclusive diffraction", DIS 200410 97 LPS sample 0.03 < Q 2 < 100 GeV 2 25 < W < 280 GeV 1.5 < M x < 70 GeV x IP < 0.1 Higher x IP region 99-00 FPC sample (M x method) 22 < Q 2 < 80 GeV 2 37 < W < 245 GeV M x < 35 GeV M N < 2.3 GeV Higher β region Data samples

11. Marta Ruspa, "Inclusive diffraction", DIS 200411 diffractive γ * p cross section diffractive structure function (assumes ) Cross section and structure function  x IP dependence of F 2 D(3) and W dependence of dσ/dM X - extraction of α IP - Regge factorisation  Q 2 dependence of F 2 D(3) and dσ/dM X -sensitivity to diffractive PDFs  comparison to BEKW model and to saturation model

12. Marta Ruspa, "Inclusive diffraction", DIS 200412 x IP dep. of F 2 D(3) equivalent to W dep. of dσ/dM x F 2 D(3) x IP dependence Data agree with Regge factorisation assumption in the region of the fit (LPS) Regge fit (x IP <0.01) : with

13. Marta Ruspa, "Inclusive diffraction", DIS 200413 p-dissociation events with M N <2.3 GeV included M X < 2 GeV: weak W dep. M X > 2 GeV: d  /dM X rises rapidly with W Cross section W dependence (M x method) power-like fit

14. Marta Ruspa, "Inclusive diffraction", DIS 200414 fit to total cross section data: fit to diffractive cross section data: Evidence of a rise of  IP diff with Q 2  mild Regge factorisation violation. α IP from diffractive and total γ * p scattering  IP diff higher than soft Pomeron Similar W dep. of diffractive and total cross section (M x method)

15. Marta Ruspa, "Inclusive diffraction", DIS 200415 BUT low M X : strong decrease of  diff /  tot with increasing Q 2 high M X : no Q 2 dependence ! ratio ~ flat in W Regge expectation: at W=220 GeV:  diff (M X <35 GeV)/  tot ~ 20 % Q 2 = 2.7 GeV  10 % Q 2 = 27 GeV σ diff / σ tot W dependence (M x method) Explained by saturation model

16. Marta Ruspa, "Inclusive diffraction", DIS 200416 Main features of the data described by BEKW parametrization (x IP <0.01) Cross section Q 2 dependence Transition to a constant cross section as Q 2  0 (similar to total cross section ) qqg fluctuations dominant at low Q 2 (Bartels, Ellis, Kowalski and Wüsthoff) medium β small β (LPS)

17. Marta Ruspa, "Inclusive diffraction", DIS 200417 F 2 D(3) Q 2 dependence (LPS) Data well described by BGK saturation model (x IP <0.01) Positive scaling violation at all values of β QCD fit

18. Marta Ruspa, "Inclusive diffraction", DIS 200418 QCD fit describes data fractional gluon momentum is at initial scale NLO QCD fit on LPS+charm data [F 2 D(3)cc from DESY-03-094, see N. Vlasov talk] x IP <0.01 QCDNUM Regge factorisation assumption possible for this small data set DL flux initial scale Q 2 =2 GeV 2 zf(z)=ΣP i (1-x) a at initial scale other PDFs parametrisation tried Thorne-Robert variable-flavour- number-scheme (LPS)

19. Marta Ruspa, "Inclusive diffraction", DIS 200419 LPS QCD fit compared to M x data Main discrepancies at high β, where no LPS data available NB: fits scaled by 0.69 to account for p-diss background in M x data M x method data described by the fit in the region of overlap LPS-M x method ZEUS (M X method)

20. Marta Ruspa, "Inclusive diffraction", DIS 200420 x IP.F 2 D(3) /F 2 Q 2 and x BJ dependences (LPS)

21. Marta Ruspa, "Inclusive diffraction", DIS 200421 Recent data from ZEUS with improved precision and extended kinematic range Data described by colour dipole models (BEKW, saturation) Data described by a NLO QCD fit (+model) Possible indication that α IP increases with Q 2 in diffraction W dep. of diffractive and total cross section similar at high Q 2 Summary