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Exclusive VM electroproduction

Exclusive VM electroproduction. Aharon Levy Tel Aviv University on behalf of the H1 and ZEUS collaborations. g. g. IP. ‘hard’. ‘soft’. Why are we measuring. Expect  to increase from soft (~0.2, from ‘soft Pomeron’ value) to hard (~0.8, from xg(x,Q 2 ) 2 )

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Exclusive VM electroproduction

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  1. Exclusive VM electroproduction Aharon Levy Tel Aviv University on behalf of the H1 and ZEUS collaborations A. Levy: Exclusive VM, GPD08, Trento

  2. g g IP ‘hard’ ‘soft’ Why are we measuring • Expect  to increase from soft (~0.2, from ‘soft Pomeron’ value) to hard (~0.8, from xg(x,Q2)2) • Expect b to decrease from soft (~10 GeV-2)to hard (~4-5 GeV-2) A. Levy: Exclusive VM, GPD08, Trento

  3.   s0.096 A. Levy: Exclusive VM, GPD08, Trento

  4. g g IP ‘hard’ ‘soft’ Why are we measuring • Expect  to increase from soft (~0.2, from ‘soft Pomeron’ value) to hard (~0.8, from xg(x,Q2)2) • Expect b to decrease from soft (~10 GeV-2)to hard (~4-5 GeV-2) A. Levy: Exclusive VM, GPD08, Trento

  5. Why are we measuring Use QED for photon wave function. Study properties of VM wf and the gluon density in the proton. A. Levy: Exclusive VM, GPD08, Trento

  6. PHP Mass distributions DIS A. Levy: Exclusive VM, GPD08, Trento

  7. Photoproduction process becomes hard as scale (mass) becomes larger. real part of amplitude increases with hardness. A. Levy: Exclusive VM, GPD08, Trento

  8. (W) – ρ0 A. Levy: Exclusive VM, GPD08, Trento

  9. Proton dissociation MC: PYTHIA pdiss: 19± 2(st) ± 3(sys) % A. Levy: Exclusive VM, GPD08, Trento

  10. (W) – ρ0,  A. Levy: Exclusive VM, GPD08, Trento

  11. (W) - , J/,  A. Levy: Exclusive VM, GPD08, Trento

  12.  (Q2+M2)- VM A. Levy: Exclusive VM, GPD08, Trento

  13. Kroll + Goloskokov:  = 0.4 + 0.24 ln (Q2/4) (close to the CTEQ6M gluon density, if parametrized as xg(x)~x-/4) A. Levy: Exclusive VM, GPD08, Trento

  14. 1 10 Q2(GeV2) (Q2) Fit to whole Q2 range gives bad 2/df (~70) A. Levy: Exclusive VM, GPD08, Trento

  15. (Q2) (for Q2 > 1 GeV2) A. Levy: Exclusive VM, GPD08, Trento

  16. IP IP Y Proton vertex factorisation p-dissociative elastic photoproduction proton vertex factorisation Similar ratio within errors for  and   proton vertex factorisation in DIS A. Levy: Exclusive VM, GPD08, Trento

  17. Fit : b(Q2) – ρ0,  A. Levy: Exclusive VM, GPD08, Trento

  18. g g ‘hard’ b(Q2+M2) - VM A. Levy: Exclusive VM, GPD08, Trento

  19. Information on L and T Use 0 decay angular distribution to get r0400 density matrix element using SCHC  - ratio of longitudinal- to transverse-photon fluxes ( <> = 0.996) A. Levy: Exclusive VM, GPD08, Trento

  20. R=L/T (Q2) When r0004 close to 1, error on R large and asymmetric  advantageous to use r0004 rather than R. A. Levy: Exclusive VM, GPD08, Trento

  21. R=L/T (Q2) A. Levy: Exclusive VM, GPD08, Trento

  22. R=L/T (Mππ) Why?? A. Levy: Exclusive VM, GPD08, Trento

  23. R=L/T (Mππ) Possible explanation: example for =1.5 A. Levy: Exclusive VM, GPD08, Trento

  24. Light VM: transverse size of ~ size of proton Heavy VM: size small  cross section much smaller (color transparency) but due to small size (scale given by mass of VM) ‘see’ gluons in the proton   ~ (xg)2  large  large kT small kT large config. small config. Photon configuration - sizes T: large sizesmall size strong color forcescolor screening large cross sectionsmall cross section *: *T, *L *T – both sizes, *L – small size A. Levy: Exclusive VM, GPD08, Trento

  25. L and T same W dependence L  in small configuration T  in small and large configurations small configuration  steep W dep large configuration  slow W dep  large configuration is suppressed L/tot(W) A. Levy: Exclusive VM, GPD08, Trento

  26.  size of *L  *T  large configuration suppressed L/tot(t) A. Levy: Exclusive VM, GPD08, Trento

  27. (W) - DVCS Final state  is real  T using SCHC  initial * is *T but W dep of  steep large *T configurations suppressed A. Levy: Exclusive VM, GPD08, Trento

  28. Get effective Pomeron trajectory from d/dt(W) at fixed t Regge: Effective Pomeron trajectory ρ0photoproduction A. Levy: Exclusive VM, GPD08, Trento

  29. Effective Pomeron trajectory ρ0 electroproduction A. Levy: Exclusive VM, GPD08, Trento

  30. Comparison to theory • All theories use dipole picture • Use QED for photon wave function • Use models for VM wave function – usually take a Gaussian shape • Use gluon density in the proton • Some use saturation model, others take sum of nonperturbative + pQCD calculation, and some just start at higher Q2 • Most work in configuration space, MRT works in momentum space. Configuration space – puts emphasis on VM wave function. Momentum space – on the gluon distribution. • W dependence – information on the gluon • Q2and R – properties of the wave function A. Levy: Exclusive VM, GPD08, Trento

  31. ρ0 data (ZEUS) - Comparison to theory • Martin-Ryskin-Teubner (MRT) – work in momentum space, use parton-hadron duality, put emphasis on gluon density determination. Phys. Rev. D 62, 014022 (2000). • Forshaw-Sandapen-Shaw (FSS) – improved understanding of VM wf. Try Gaussian and DGKP (2-dim Gaussian with light-cone variables). Phys. Rev. D 69, 094013 (2004). • Kowalski-Motyka-Watt (KMW) – add impact parameter dependence, Q2 evolution – DGLAP. Phys. Rev. D 74, 074016 (2006). • Dosch-Ferreira (DF) – focusing on the dipole cross section using Wilson loops. Use soft+hard Pomeron for an effective evolution. Eur. Phys. J. C 51, 83 (2007). A. Levy: Exclusive VM, GPD08, Trento

  32. ρ0 data (H1) - Comparison to theory • Marquet-Peschanski-Soyez (MPS): Dipole cross section from fit to previous ,  and J/ data. Geometric scaling extended to non-forward amplitude. Saturation scale is t-dependent. • Ivanov-Nikolaev-Savin (INS): Dipole cross section obtained from BFKL Pomeron. Use kt-unintegrated PDF and off-forward factor. • Goloskokov-Kroll (GK): Factorisation of hard process and proton GPD. GPD constructed from standard PDF with skewing profile function. A. Levy: Exclusive VM, GPD08, Trento

  33. Q2 KMW – good for Q2>2GeV2 miss Q2=0 DF – miss most Q2 FSS – Gauss better than DGKP A. Levy: Exclusive VM, GPD08, Trento

  34. Q2 Data seem to prefer MRST99 and CTEQ6.5M A. Levy: Exclusive VM, GPD08, Trento

  35. W dependence KMW - close FSS: Sat-Gauss – right W-dep. wrong norm. MRT: CTEQ6.5M – slightly better in W-dep. A. Levy: Exclusive VM, GPD08, Trento

  36. L/tot(Q2) A. Levy: Exclusive VM, GPD08, Trento

  37. L/tot(W) All models have mild W dependence.None describes all kinematic regions. A. Levy: Exclusive VM, GPD08, Trento

  38. L,T (Q2+M2) • Different Q2+M2 dependence of L and T (L0 at Q2=0) • Best description of L by GK; T not described. A. Levy: Exclusive VM, GPD08, Trento

  39. Density matrix elements - ,  • Fair description by GK • r500 violates SCHC A. Levy: Exclusive VM, GPD08, Trento

  40. VM/tot A. Levy: Exclusive VM, GPD08, Trento

  41. Summary and conclusions • New high statistics measurements on ρ0,  electroproduction and on DVCS. • The Q2 dependence of (*pρp) cannot be described by a simple propagator term. • The cross section is rising with W and its logarithmic derivative in W increases with Q2. • The exponential slope of the t distribution decreases with Q2 and levels off at about b = 5 GeV-2. • Proton vertex factorisation observed also in DIS. • The ratio of cross sections induced by longitudinally and transversely polarised virtual photons increases with Q2, but is independent of W and t. • The effective Pomeron trajectory has a larger intercept and smaller slope than those extracted from soft interactions. • All these features are compatible with expectations of perturbative QCD. • None of the models which have been compared to the measurements are able to reproduce all the features of the data. A. Levy: Exclusive VM, GPD08, Trento

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