Inclusive diffraction

1. Inclusive diffraction 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 • Diffractive cross section and diffractive structure function • Comparison with colour dipole models • NLO QCD fit

2. Inclusive diffraction γ*p  Xp Q2 = virtuality of photon = = (4-momentum exchanged at e vertex)2 t = (4-momentum exchanged at p vertex)2 typically: |t|<1 GeV2 W = invariant mass of photon-proton system MX = invariant mass of photon-Pomeron system xIP = 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/xIP e’ Q2 e g* W MX xIP IP p p’ t Exchange of an object with the vacuum q. n.  Proton almost intact after the collision

3. Diffractive DIS in the Breit frame (Breit frame) • DIS of a pointlike virtual photon off the exchanged object • PDFs HARD SCATTERING FACTORISATION fi/pD(z,Q2,xIP,t): probability to find in a proton, with a probe of resolution Q2, parton i with momentum fraction z, under the condition that the proton remains intact and emerges with small energy loss, xIP, and momentum transfer,t Diffractive Deep Inelastic Scattering probes the diffractive PDFs of the proton relevant when the vacuum quantum numbers are exchanged

4. Lifetime of dipoles very long due to large γ boost (E γ ~ W2 ~ 1/x  50TeV ! )it is thedipole that interactswith theproton ! • Transverse size of dipoles proportional to  can be so small • that the strong interaction with proton can be treatedperturbatively ! 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) 2 gluon exchange: LO QCD realisation of vacuum q.n.

5. 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) 2 gluon exchange: LO QCD realisation of vacuum q.n. • BEKW model : at medium β; at small β • saturation model : (colour transparency) • as Q2  0, growth tamed by saturating

6. Inclusive diffraction γ*p  Xp Exchange of color singlet producing a GAP in the particle flow e p • No activity in the forward direction MX method • Proton suffers only a small energy loss

7. Selection of events γ*p  Xp with Mx method Properties of Mx distribution: - exponentially falling for decreasing Mx for non-diffractive events - flat vs ln Mx2 for diffractive events Diffr. Non-diffr. Non-diffr. Diffr. • Forward Plug Calorimeter (FPC): • CAL acceptance extended by 1 unit in pseudorapidity from η=4 to η=5 • higher Mx and lower W • if MN > 2.3 GeV deposits EFPC > 1 GeV recognized and rejected! c, bfrom fit n.d. events subtracted contamination fromreaction epeXN

8. Inclusive diffraction γ*p  Xp Exchange of color singlet producing a GAP in the particle flow e p • No activity in the forward direction MX method • Proton suffers only a small energy loss LPS method

9. Selection of events γ*p  Xp with LPS Diffractive peak • Free of p-diss background • Low acceptance •  low statistics

10. 97 LPS sample 0.03 < Q2 < 100 GeV2 25 < W < 280 GeV 1.5 < Mx < 70 GeV xIP< 0.1 Higher xIP region 99-00 FPC sample (Mx method) 22 < Q2 < 80 GeV2 37 < W < 245 GeV Mx < 35 GeV MN < 2.3 GeV Higher β region Data samples

11. Cross section and structure function • xIP dependence of F2D(3) and W dependence of dσ/dMX • - extraction of αIP • - Regge factorisation • Q2 dependence of F2D(3) and dσ/dMX • -sensitivity to diffractive • PDFs • comparison to BEKW model and to saturation model diffractiveγ*p cross section • diffractive structure function (assumes )

12. F2D(3)xIPdependence (LPS) Regge fit (xIP<0.01): with Data agree with Regge factorisation assumptionin the region of the fit xIPdep. of F2D(3) equivalent to W dep. of dσ/dMx

13. Cross section W dependence (Mx method) p-dissociation events with MN<2.3 GeVincluded MX< 2 GeV: weak W dep. MX> 2 GeV: d/dMX rises rapidly with W power-like fit

14. αIP from diffractive and total γ*p scattering (Mx method) fit to diffractive cross section data: • IPdiff higher than soft Pomeron • Evidence of a rise of IPdiff with Q2  mild Regge factorisation violation. fit to total cross section data: • Similar W dep. of diffractive and total cross section

15. diff(MX<35 GeV)/tot ~ 20 %Q2= 2.7 GeV  10 %Q2= 27 GeV • at W=220 GeV: σdiff/ σtot W dependence (Mx method) Regge expectation: • BUT • low MX :strong decrease of diff/tot with increasing Q2 • high MX :no Q2dependence ! ratio ~ flat in W Explained by saturation model

16. Cross section Q2 dependence (LPS) Transition to a constant cross section as Q20 (similar to total cross section ) Main features of the data described by BEKW parametrization (xIP<0.01) (Bartels, Ellis, Kowalski and Wüsthoff) medium β small β qqg fluctuations dominant at low Q2

17. F2D(3) Q2 dependence (LPS) Data well described by BGK saturation model (xIP<0.01) Positive scaling violation at all values of β QCD fit

18. NLO QCD fit on LPS+charm data • xIP <0.01 • QCDNUM • Regge factorisation assumption possible for this small data set • DL flux • initial scale Q2=2 GeV2 • zf(z)=ΣPi(1-x)aat initial scale • other PDFs parametrisation tried • Thorne-Robert variable-flavour-number-scheme (LPS) QCD fit describes data fractional gluon momentum is at initial scale [F2D(3)cc from DESY-03-094, see N. Vlasov talk]

19. LPS QCD fit compared to Mxdata ZEUS (MX method) NB: fits scaled by 0.69 to account for p-diss background in Mx data Mx method data described by the fit in the region of overlap LPS-Mxmethod Main discrepancies at high β, where no LPS data available

20. xIP.F2D(3)/F2Q2 and xBJ dependences (LPS) (LPS)

21. Summary • 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 αIP increases with Q2 in diffraction • W dep. of diffractive and total cross section similar at high Q2