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Deeply Virtual Compton Scattering on the neutron

Deeply Virtual Compton Scattering on the neutron. Malek MAZOUZ. LPSC Grenoble. Hall A. EINN 2005. September 23 rd 2005. Link to form factors (sum rules). Generalized Parton distributions. Link to DIS at x =t=0. Access to quark angular momentum (Ji’s sum rule). Quark correlations !.

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Deeply Virtual Compton Scattering on the neutron

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

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

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

  4. Purely real 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 -High luminosity -High precision measurement

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

  6. Proton Target Proton Model: Goeke, Polyakov and Vanderhaeghen t=-0.3

  7. Neutron Target Neutron Model: neutron Goeke, Polyakov and Vanderhaeghen t=-0.3

  8. Experiment status E00-110 (p-DVCS) was finished in November 2004 (started in September) E03-106 (n-DVCS) was finished in December 2004 (started in November) xBj=0.364 (fb-1) proton neutron Beam polarization was about 75.3% during the experiment

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

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

  11. PMT G=104 x10 electronics High luminosity measurement Up to At ~1 meter from target (Θγ*=18 degrees) Low energy electromagnetic background Requires good electronics

  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 Not all the calorimeter channels are read for each event Calorimeter trigger 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 Sigma=9.5 MeV Invariant mass of 2 photons in the calorimeter Good way to control the calorimeter calibration Missing mass2 with LH2 target

  15. Analysis Status – Very preliminary LH2 target 0.5 GeV2 < missing mass 2 < 1.5 GeV2 α (N+ - N-) φ LD2 target α (N+ - N-) 0.5 GeV2 < missing mass 2 < 1.5 GeV2 φ LD2 – LH2 Possible neutron signal ! α (N+ - N-) Absolute cross sections necessary to extract helicity dependence of neutron φ

  16. Analysis Status – Very preliminary 0.5 GeV2 < missing mass 2 < 1.5 GeV2 α (N+ - N-) Signal φ α (N+ - N-) 1.5 GeV2 < missing mass 2 < 2.5 GeV2 φ No signal α (N+ - N-) 2.5 GeV2 < missing mass 2 < 3.5 GeV2 φ

  17. Conclusion • With High Resolution spectrometer and a good calorimeter, we are able to measure: • Helicity dependence of the neutron using LD2 and LH2 target. Work at precisely defined kinematics: Q2 , s and xBj Work at a luminosity up to Coming soon: Polarized cross sections to extract GPD E Relative asymmetry considering Proton Array and Tagger. Proton preliminary results tomorrow morning

  18. Analysis status – preliminary Sigma = 0.6ns Time difference between the electron arm and the detected photon 2 ns beam structure Selection of events in the coincidence peak Determination of the missing particle (assuming DVCS kinematics) Time spectrum in the predicted block (LH2 target) Sigma = 0.9ns Check the presence of the missing particle in the predicted block (or region) of the Proton Array

  19. Analysis – preliminary Triple coincidence Missing mass2 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

  20. π0 electroproduction - preliminary Invariant mass of 2 photons in the calorimeter Sigma = 9.5 MeV Good way to control calorimeter calibration Sigma = 0.160 GeV2 Missing mass2 of epeπ0x 2π production threshold 2 possible reactions: epeπ0p epenρ+ , ρ+ π0 π+

  21. Missing mass2 with LD2 target

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

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