p-DVCS and n-DVCS experiment status

1. p-DVCS and n-DVCS experiment status Malek MAZOUZ LPSC Grenoble Hall A • Brief overview of the theory • Experiment setup • Analysis status Hall A Collaboration Meeting June 24th 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. Purely real Brief overview of the theory DVCS: Simplest hard exclusive process involving GPDs Perturbative description pQCD factorization theorem (Bjorken regime) Non perturbative description by Generalized Parton Distributions

4. Proton Target sin(Φ) term Proton Model: Goeke, Polyakov and Vanderhaeghen t=-0.3

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

6. p-DVCS and n-DVCS in Hall A Goal : Measure the absolute cross section of DVCS on proton (3 Q² values: 1.4, 1.9, 2.3 GeV²) and on neutron (Q²=1.9 GeV²) DVCS on the proton : E00-110 Check Handbag dominance & Test factorization Deduce Q² dependence and relative importance of leading twist and higher twists in helicity dependent cross-section Constrain GPD’s …including Re(DVCS) DVCS on the neutron : E03-106 Simplest access to the least known of GPDs: E First constraint of nucleon orbital angular momentum through model of E

7. 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) (fb-1) xBj=0.364 proton neutron Beam polarization was about 78% during the experiment

8. 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 tagger) Check of the recoil nucleon position Only for Neutron experiment Scintillator Array Electromagnetic Calorimeter (photon detection) (Proton Array)

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

10. PMT G=104 x10 electronics High luminosity measurement Up to At ~1 meter from target (Θγ*=15 degrees) Low energy electromagnetic background Requires good electronics

11. 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.

12. 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.

13. Analysis status - What is done All good runs selected and total integrated luminosity extracted Left HRS efficiency (detectors, tracking …) determined Parameters of the wave form analysis and the clustering optimized Calorimeter calibration done for almost all data Coincidence time of all detectors precisely adjusted All geometrical offsets taken into account Proton Array calibration done for a part of the data and still in progress

14. 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

15. Analysis Status – Very preliminary Absolute cross sections necessary to extract helicity dependence of neutron

16. Analysis – Very 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

17. Analysis – Very preliminary Triple coincidence Missing mass2 (background subtracted) LH2 target

18. Analysis – Fermi momentum effect Triple coincidence Very preliminary Missing mass2 (background subtracted) LD2 target

19. π0 electroproduction - preliminary Invariant mass of 2 photons in the calorimeter Sigma = 9.5 MeV Good way to control calorimeter calibration Missing mass2 of epeπ0x Sigma = 0.160 GeV2 2π production threshold 3 possible reactions: epeπ0p epenρ+ , ρ+ π0 π+ epe π0Δ+

20. Analysis – Proton Array Calibration Used to calibrate the Proton Array

21. π0 electroproduction - preliminary Invariant mass of 2 photons in the calorimeter Sigma = 9.5 MeV Good way to control calorimeter calibration Missing mass2 of epeπ0x Sigma = 0.160 GeV2 2π production threshold 3 possible reactions: epeπ0p epenρ+ , ρ+ π0 π+ epe π0Δ+

22. π0 electroproduction - preliminary Background subtraction Accidental π0 π0 Decorrelated photons

23. π0 electroproduction – background subtraction

24. Analysis – work in progress wave form analysis (detection) efficiency (almost done) Proton experiment Proton Array Calibration (almost done) Acquisition trigger efficiency Acceptance calculation Neutron detection efficiency in the Proton Array Neutron experiment Implement neutron tagger analysis Evaluate Fermi motion consequences Study knock-out effects in the tagger (data + simulation)

25. Conclusion • We have demonstrated that in Hall A with High Resolution spectrometer and a good calorimeter, we are able to measure: • Real and Imaginary parts of DVCS•BH interference: Work at precisely defined kinematics: Q2 , s and xBj Work at a luminosity up to But -Requires wave form electronics - 10% of detector components almost unusable as expected after 3 months of data taking Absolute cross sections and cross section difference are determined with the precision of HRS (better than 5%) Analysis is in progress Deep π0 electroproduction cross-section almost finalized