1. December 16 20081 Extraction of exclusive neutral pions electro-production cross sections in Hall A @ Jefferson Laboratory. Eric FUCHEY Ph.D Student @ Laboratoire de Physique Corpusculaire UMR 6533 CNRS/IN2P3 Université Blaise Pascal Clermont-Ferrand

2. December 16 20082 Outline 1– Motivations 2– 0 electroproduction formalismes –Kinematical variables –Formalismes 3– Experimental layout 4– Cross sections extraction –Calibration –Extraction method –Selection cuts 5– Conclusion and perspectives

3. December 16 20083 Outline 1– Motivations 2– 0 electroproduction formalismes –Kinematical variables –Formalismes 3– Experimental layout 4– Cross sections extraction –Calibration –Extraction method –Selection cuts 5– Conclusion and perspectives

4. December 16 20084 0 interest DVCS is the smartest way to get access to GPDs theoretically. In most of DVCS experiments, 0 production ( with ) is an important by-product (Pollution for DVCS analysis)

5. December 16 20085 Outline 1– Motivations 2– 0 electroproduction formalismes –Kinematical variables –Formalismes 3– Experimental layout 4– Cross sections extraction –Calibration –Extraction method –Selection cuts 5– Conclusion and perspectives

6. December 16 20086 Kinematic variables Virtuality: Momentum fraction: Quadri transfert to proton: *- azimuthal angle: *-p CM energy: Transverse momentum:

7. December 16 20087 Formalismes Drechsel-Tiator (Nucl.Part.Phys.18(1992)449-497): with and More « intuitive » formalisme

8. December 16 20088 Formalismes Levelt-Mulders (Phys.Rev.D.49(1994)96-113) : Show an explicit dependence in transverse momentum

9. December 16 20089 Formalismes: comparison Interest: Being able to get easily cross sections from Drechsel- Tiator as function of pion transverse momentum: With :

10. December 16 200810 Outline 1– Motivations 2– 0 electroproduction formalismes –Kinematical variables –Formalismes 3- Experimental layout 4– Extraction des section efficaces –Calibration –Extraction method –Selection cuts 5– Conclusion and perspectives

11. December 16 200811 DVCS experiment Incident beam : E = 6 GeV, L ~ 10 37 cm -2 s -1 during experiment –Scattered electron measured by HRS –2 photons issued from 0 measured by 132 PbF 2 blocks calorimeter –Recoil proton detected by proton array -> not used

12. December 16 200812 Outline 1– Motivations 2– 0 electroproduction formalismes –Kinematical variables –Formalismes 3– Experimental layout 4– Cross sections extraction –Calibration –Extraction method –Selection cuts 5– Conclusion and perspectives

13. December 16 200813 Calibration Goal : searching coefficients to apply to calorimeter blocks in order to: –bring their reconstructed missing mass squared value to the proton missing mass squared. –Bring then simulated missing mass mean and resolution to real ones Mp2Mp2

14. December 16 200814 Calibration Calibration has been performed for set of Kin3 0 data and our set of Kin3 0 simulated.

15. December 16 200815 Extraction method Kinematical domains : –Kin2: x Bj = 0.36, Q 2 = 1.9 GeV 2 –Kin3: x Bj = 0.36, Q 2 = 2.3 GeV 2 Analysis made according to t min -t. Fitting real data with simulated to take into account of acceptance effect.

16. December 16 200816 Extraction method Principle: compare expected number of counts for data to expected number of counts for simulated No access What we want to extract

17. December 16 200817 Selection cuts Missing mass cut to ensure exclusivity Each photon is in a calibrated block, and E > 1.0 GeV k = pHRS +/- 4.5% ; r-function > 0.005. -6.0 cm < z vtx < 7.5 cm.

18. December 16 200818 Fit results Fit has been performed for helicity-independent cross section and helicity-dependent cross section However, still not able to extract cross sections (debugging…) 2 /NDF = 2.77 2 /NDF = 0.76

19. December 16 200819 Outline 1– Motivations 2– 0 electroproduction formalismes –Kinematical variables –Formalismes 3– Experimental layout 4– Cross sections extraction –Calibration –Extraction method –Selection cuts 5– Conclusion and perspectives

20. December 16 200820 4 – Conclusion / perspectives 0 electroproduction cross section extraction in Hall A is in progress. Calibration has been performed (see talk on calibration this afternoon) Once first results obtained: –Optimizing some selection cuts –Study evolution in other variables: x Bj, Q 2, W