1. Deeply Virtual Compton Scattering on the neutron Malek MAZOUZ LPSC Grenoble EINN 2005September 23 rd 2005

2. GPDs properties, link to DIS and elastic form factors Generalized Parton distributions Link to DIS at  =t=0 Link to form factors (sum rules) Access to quark angular momentum (Ji’s sum rule) Quark correlations !

3. Brief overview of the theory k k’ q’ GPDs pp’ Simplest hard exclusive process involving GPDs pQCD factorization theorem Perturbative description (High Q² virtual photon) Non perturbative description by Generalized Parton Distributions Bjorken regime X. Ji, Phys. Rev. DS56 (1997) 5511 A. Radyushkin, Phys. Lett. B380 (1996) 417 fraction of longitudinal momentum amplitude

4. What can be done at JLab Hall A Using a polarized electron beam: Asymmetry appears in Φ K. Goeke, M.V. Polyakov and M. Vanderhaeghen Direct handle on the imaginary part of the DVCS amplitude Enhanced by the full magnitude of the BH amplitude Purely real -High luminosity -High precision measurement

5. cross-section difference in the handbag dominance Γ contains BH propagators and some kinematics B contains higher twist terms A is a linear combination of three GPDs evaluated at x=ξ

6. 0.11.340.810.380.04 0.30.820.560.240.06 0.50.540.420.170.07 0.70.380.330.130.07 Proton Target Proton t=-0.3 Target Proton1.130.700.98 Goeke, Polyakov and Vanderhaeghen Model:

7. 0.1-1.46-0.01-0.26-0.04 0.3-0.91-0.04-0.17-0.06 0.5-0.6-0.05-0.12-0.08 0.7-0.43-0.06-0.09-0.08 Neutron Target Neutron t=-0.3 Target Proton0.81-0.071.73 Goeke, Polyakov and Vanderhaeghen Model: neutron

8. Experiment status s (GeV²) Q² (GeV²) P e (Gev/c) Θ e (deg) -Θ γ* (deg) 4.942.322.3523.9114.805832 4.221.912.9519.3218.254365 3.51.53.5515.5822.293097 4.221.912.9519.3218.2524000 proton neutron x Bj =0.364 Beam polarization was about 75.3% during the experiment E00-110 (p-DVCS) was finished in November 2004 (started in September) E03-106 (n-DVCS) was finished in December 2004 (started in November) (fb -1 )

9. Left High Resolution Spectrometer LH2 or (LD2) target Polarized beam Electromagnetic Calorimeter (photon detection) Scintillator Array (Proton Array) Experimental method (proton veto) Scintillating paddles scattered electron photon Proton: (E00-110) Neutron: (E03-106) Only for Neutron experiment Check of the recoil nucleon position recoil nucleon Reaction kinematics is fully defined

10. Calorimeter in the black box (132 PbF2 blocks) Proton Array (100 blocks) Proton Tagger (57 paddles)

11. High luminosity measurement Up to At ~1 meter from target (Θ γ* =18 degrees) Requires good electronics Low energy electromagnetic background PMT G=10 4 x10electronics

12. Electronics 1 GHz Analog Ring Sampler (ARS) x 128 samples x 289 detector channels Sample each PMT signal in 128 values (1 value/ns) Extract signal properties (charge, time) with a wave form Analysis. Allows to deal with pile-up events.

13. Electronics Calorimeter trigger Not all the calorimeter channels are read for each event Following HRS trigger, stop ARS. 30MHz trigger FADC digitizes all calorimeter signals in 85ns window. - Compute all sums of 4 adjacent blocks. - Look for at least 1 sum over threshold - Validate or reject HRS trigger within 340 ns Not all the Proton Array channels are read for each event

14. Analysis Status - Preliminary Invariant mass of 2 photons in the calorimeter Sigma=9.5 MeV Good way to control the calorimeter calibration Missing mass 2 with LH 2 target

15. Analysis Status – Very preliminary φ φ φ α (N + - N - ) LH 2 target LD 2 target LD 2 – LH 2 Possible neutron signal ! 0.5 GeV 2 < missing mass 2 < 1.5 GeV 2 Absolute cross sections necessary to extract helicity dependence of neutron

16. Analysis Status – Very preliminary φ φ φ α (N + - N - ) 0.5 GeV 2 < missing mass 2 < 1.5 GeV 2 1.5 GeV 2 < missing mass 2 < 2.5 GeV 2 2.5 GeV 2 < missing mass 2 < 3.5 GeV 2 No signal Signal

17. Conclusion With High Resolution spectrometer and a good calorimeter, we are able to measure: Helicity dependence of the neutron using LD 2 and LH 2 target. Work at precisely defined kinematics: Q 2, s and x Bj Polarized cross sections to extract GPD E Relative asymmetry considering Proton Array and Tagger. Work at a luminosity up to Proton preliminary results tomorrow morning Coming soon:

19. Analysis status – preliminary Sigma = 0.6ns 2 ns beam structure Time difference between the electron arm and the detected photon Selection of events in the coincidence peak Determination of the missing particle (assuming DVCS kinematics) Check the presence of the missing particle in the predicted block (or region) of the Proton Array Sigma = 0.9ns Time spectrum in the predicted block (LH 2 target)

20. Analysis – preliminary Triple coincidence Missing mass 2 of H(e,e’ γ)x for triple coincidence events Background subtraction with non predicted blocks Proton Array and Proton Veto are used to check the exclusivity and reduce the background

21. π 0 electroproduction - preliminary Invariant mass of 2 photons in the calorimeter Good way to control calorimeter calibration Missing mass 2 of ep  eπ 0 x 2 possible reactions: ep  eπ 0 p ep  enρ +, ρ +  π 0 π + 2π production threshold Sigma = 0.160 GeV 2 Sigma = 9.5 MeV

22. Missing mass 2 with LD 2 target

23. Time spectrum in the tagger (no Proton Array cuts)