1. Deeply Virtual Compton Scattering from the Proton and the Neutron ( E00-110 & E03-106: DVCS/nDVCS ) Hall A Collaboration MeetingDecember 6, 2005 Spokespersons: Pierre-Yves Bertin, Charles Hyde-Wright, Ron Ransome, Franck Sabatie, and Eric Voutier Elena Kuchina Rutgers, The State University of New Jersey

2. Hall A DVCS at 6 GeV: Methods and Goals E00-110: Measure the absolute cross section of DVCS on proton E00-110 (3 Q² values: 1.4, 1.9, 2.3 GeV²) and on neutron E03-106 (Q²=1.9 GeV²) E03-106: Simplest access to the least known of GPDs: E First constraint of nucleon orbital angular momentum through model of E Check Handbag dominance & Test factorization of DVCS Deduce Q² dependence and relative importance of leading twist and higher twists in helicity dependent cross-section Measure linear combination of GPDs …including Re (DVCS) Unique characteristics: High luminosity (up to 4·10 37 cm -2 s -1 ) Well defined kinematics (high resolution detection of e - and γ) Method: Measure weighted cross-section differences as function of φ angle Deduce Q 2 dependence & relative importance of twist-2 and higher twists Goal:

3. DVCS experimental setup

4. contains BH propagators and some kinematics B contains twist-3 terms A is a linear combination of three GPDs evaluated at x=ξ Cross-section difference assuming handbag dominance

6. 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 E00-110 p-DVCS 9-11 04 E03-106 n-DVCS 11-12 04 x Bj =0.364 Kinematics is fully defined Beam polarization was about 75.3% during the experiment (fb -1 ) 5.7572 GeV polarized 110 cm LH2 (LD2) e' (LHRS) p'(n') Experiment kinematic settings

7. Analysis Method MC includes radiative corrections (both external and internal).

8. Calorimeter acceptance Acceptance in φ is very dependent on t for large -t

9. Preliminary results on p-DVCS Difference if Yields N + -N - : 3 bins in Q 2 x 5 bins in t (x 50 bins in φ) 0.5< M x ² < 1.2 GeV 2 relatively small contribution of the twist-3 term Q 2 dependence of the twist-2 and twist-3 terms: Twist-2 term (high precision) ~ 12% variation Twist-3 term (20-30% uncertainty) is ~ 0 handbag dominance test Twist-2 absolute value & t-dependence is well reproduced by VGG model.

10. φ LD 2 target – LH 2 target Possible neutron signal ! 0.5 GeV 2 < missing mass 2 < 1.5 GeV 2 Absolute cross sections necessary to extract helicity dependence of neutron n-DVCS – Very preliminary 1.5 GeV 2 < missing mass 2 < 2.5 GeV 2 2.5 GeV 2 < missing mass 2 < 3.5 GeV 2 NO signal

11. To do Acceptance checks Virtual radiative corrections Contribution from πº Total cross-section: access to real part of DVCS and other cross-section coefficients Polarized cross sections to extract GPD E Relative asymmetry considering Proton Array and Tagger

12. Summary and Conclusions Very exciting times where first high precision data is coming out. Test of handbag dominance t-dependence and access to linear combinations of GPDs DVCS is a golden probe. New exclusive experiments in different kinematic domains and with different observables (BSA, BCA,TSA…) are needed in order to fully map out GPDs.