ZEUS results on inclusive diffraction

1. ZEUS results on inclusive diffraction Marta Ruspa Torino University XXXIII International Symposium on Multiparticle Dynamics Krakow, Poland September 5-11, 2003 on behalf of • Diffractive cross section Q2 and W dependences • Diffractive structure function Q2 and β dependences

2. Inclusive diffraction γ*p  Xp Exchange of color singlet producing a GAP in the particle flow e p  No activity in the forward direction  Proton almost intact after the collision e’(k’) Q2 diffractiveγ*p cross section e (k) *(q)  X W • diffractive structure function (assumes ) IP xIP P (p) P’(p’) t

3. 97 LPS sample 0.03 < Q2 < 100 GeV2 25 < W < 280 GeV 1.5 < Mx < 70 GeV xL > 0.9 Higher xIP region 99-00 FPC sample (Mx method) 22 < Q2 < 80 GeV2 37 < W < 245 GeV Mx < 35 GeV Higher β region Data samples

4. 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 Mx2for 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! D, c, bfrom a fit to data contamination fromreaction epeXN

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

6. 99-00 FPC sample (Mx method) 22 < Q2 < 80 GeV2 37 < W < 245 GeV Mx < 35 GeV Higher β region 97 LPS sample 0.03 < Q2 < 100 GeV2 25 < W < 280 GeV 1.5 < Mx < 70 GeV xL > 0.9 Higher xIP region Data samples

7. 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

8. 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

9. 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. fit to total cross section data: • Same W dep. of diffractive and total cross section

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

11. F2D(3) β dependence (LPS) Different β dep. at low and high xIP Data well described by BGK saturation model (xIP<0.01)

12. F2D(3) at fixed xIP (Mx method) Maximum near β=0.5 consistent with a β(1- β) behaviour suggesting main contribution from a quark-antiquark state For high β F2D(2) decrease with rising Q2 As β  0 F2D(2) rises. The rise becomes stronger as Q2 increases Evidence for pQCD evolution

13. Summary • Recent data from ZEUS with improved precision and extended kinematic range • Data described by dipole models (BEKW, saturation) • W dep. of diffractive and total cross section similar at high Q2 • Indication that αIP increases with Q2 in diffraction • F2D(3) scaling violations at all xIP • Different β dependences of F2D(3) in at low and high xIP • F2D(2) pQCD-like evolution with β and Q2