DVCS Beam Spin Asymmetries with CLAS – e1dvcs

1. DVCS Beam Spin Asymmetries with CLAS – e1dvcs P. Konczykowski CEA Saclay Sept. 28th 2010 On behalf of the CLAS Collaboration Introduction e1dvcs particle selection exclusivity cuts e1dvcs-1 results Preliminary results of e1dvcs-2

2. Nucleon study Form Factors (FFs) Parton Distribution Functions (PDFs) Generalized Parton Distributions (GPDs) Correlation between transversal position and longitudinal momentum fraction Parton longitudinal momentum fraction distribution Quark transversal spatial distribution

3.  Leptonic plane Hadronic plane Access to GPDs with DVCS reaction • Deeply Virtual Compton Scatering (DVCS): simplest way to have an access to GPDs • Electromagnetic probe: insensitive to nuclear strong interaction and no internal structure Bjorken limit: Q² → , n →  et xB = Q²/2pq fixed hard soft Momentum transfer Asymmetry variable GPD: parametrize the non- perturbative content of the nucleon

4. 2 + + Beam spin asymmetries DVCS BH BH Interference between the DVCS and Bethe-Heitler processes In the JLab kinematical regim, BH process at least of the same order of magnitude as DVCS

5. 2 + + Beam spin asymmetries DVCS BH BH Interference between the DVCS and Bethe-Heitler processes • In the JLab kinematical regim, BH process at least of the same order of magnitude as DVCS • beam spin asymmetry is proportional to the interference term BH-DVCS and depend on the GPDs at the first order in 1/Q

6. The e1dvcs experiment with CLAS e1dvcs1 • Two parts: e1dvcs1(2005) and e1dvcs2(2009) • E = 5.8 GeV • Average beam polarisation: 80% (e1dvcs1), 85% (e1dvcs2) • Average current: 25 nA • Target: lH2 2.5 cm (e1dvcs1) and 5 cm (e1dvcs2) • LH2 = 1.7 x 1034 cm-2.s-1 • # triggers: 8 M events (e1dvcs1) and 12 M events (e1dvcs2, different trigger scheme) e1dvcs2

7. The CLAS spectrometer toroïdal magnet Electromagnetical Calorimeter (EC, LAC) Drift Chambers (DC) Detection of particle’s spray and also neutral particles Charged particles trajectories angular acceptance: Time Of Flight (TOF) Inner Calorimeter (IC) Angular acceptance: Cerenkov Counter e/π discrimination Photon detector (DVCS)

8. Inner Calorimeter • 424 PbWO4 cristals (16 cm over 1.3 cm² to 1.6 cm²) • Light collection by APDs • Temperature control system • Laser monitoring system

9. DVCS event selection e p → e p g Triple coincidence for full exclusivity

10. Electron selection e p → e p g e-/p- discrimination EC (Ein) Drift Chambers + EC (E/p) Cerenkov (Nphe) Vertex cut Geometry cuts

11. Proton selection e p → e pg p/p+ discrimination Drift Chambers + TOF (b) Geometry cuts

12. Photon selection e p → e p g EC (b) or IC (1 cluster) E>1GeV Geometry cuts

13. (e,p) selection e p → e pg Define (xB,Q2) Same vertex Vz (electron) • Q² > 1 GeV² • W² > 4 GeV² → W² = (q+p)² • 0.1 < xB < 0.58 Vz (proton)

14. DVCS event selection e p → e p g Exclusivity cuts Coplanar angle Missing momentum and energy Cone angle (g,Y)

15. DVCS event selection e p → e p g EC IC cone angle Missing energy

16. p0Backgroundsubtraction e p → e p g e p → e p p° → e p g (g)

17. p0Background subtraction Principles: Simulations & exclusive datas Evaluate for each bin

18. Asymmetrycalculation f f f

19. Asymmetry calculation f f 0.20<xB<0.30 f

20. Asymmetries as a function ofF • Canvas centered on <Q²> et <xB> • Position of the F point weighted by the number of events in each bin

21. abehaviour vs. t • Overshoot at low |t| • Good qualitative and quantitative agreement F.-X. Girod et al., PRL 100 (2008) 162002

22. Comparison between the 2 parts • BSA(e1dvcs2) < BSA(e1dvcs1) • More exclusive cuts (only 2 charged particles) • p0contamination bigger at large -t • Comparison of the statistic between the 2 parts ongoing

23. Conclusion • e1dvcs finished (100 mC collected, 20M triggers) and gives good results • Full Exclusivity achieved and p0 substraction in 2nd part to come • BSA measured in a large kinematical region in (xB,Q2,t), important constraints for the models and parameterisations

24. Outlook • New results with a longitudinal polarized target (eg1dvcs) and data with a transversal polarized target (HD-Ice) to come • Jefferson upgrade at 12 GeV: huge experimental program with a larger kinematical region in (x,Q2,t) and higher statistic. • GPD extraction procedures are still in the works by theorists but great progress is expected soon.

25. ANNEXES

26. Etot/p cuts (by sector): Electron selection Sampling fraction = Emes/Eréelle

27. TOF DC Proton selection

28. g in EC Photon selection g in IC

29. Q² > 1 GeV² → Bjorken limit • W² > 4 GeV² → W² = (q+p)² DVCS events selection • 1 electron, 1 proton and 1 photon (E>1GeV either EC or IC) • no other charged particle, no other photon E>300MeV Kinematical cuts Exclusivity cuts

30. Before exclusivity cuts After exclusivity cuts ep → epγX ep → epX’ EC exclusivity cuts

31. Before exclusivity cuts After exclusivity cuts IC exclusivity cut PX (etPy) in the IC ot centered at 0 (due to the alignement and/or calibration problems)

32. Missing energy Ex IC EC As for Px et Py, missing energy Ex in the IC not centered at 0 → ~0.2 GeV deviation

33. Kinematical distribution and binning e1dvcs1 e1dvcs2 Target position (from the IC front face): 66.5 cm (e1dvcs1) → 57.5 cm (e1dvcs2)

34. BSA integrate over -t a*sin(Φ)/(1+b*cos(Φ)) • Canvas centered on <Q²> et <xb> • Position of the F point weighted by the number of events in each bin

35. Sélection des électrons • q < 0 • Coupure au vertex: -62 cm <vz< -53 cm • Ein > 0.06 GeV • EC est un empilement de plaques de plomb et de plans de scintillateurs disposés suivant 3 orientations différentes U,V et W • 39 couches (scintillateurs+plomb): 15 premières (inner) et • 24 dernières (outer)

36. Sélection d’électrons: coupure dans EC

37. DVCS event selection • The first runs corresponding to a largeP┴ deviation don’t used for the first analysis (~4% of the datas) • IC_timing problems in this region • Next pass should resolve the problem