1. Harut Avakian (Jlab) DVCS results with unpolarized and polarized target Introduction Event selection MC simulations and radiative corrections DVCS with unpolarised target DVCS with longitudinally polarized target Summary 1 H.Avakian, Paris March 8

2. Deeply Virtual Compton Scattering ep->e’p’  Different GPD combinations accessible as azimuthal moments of the total cross section. DVCS BH  LU  ~ sin  Im{F 1 H +  (F 1 +F 2 ) H +kF 2 E } ~ Polarized beam, unpolarized target: Unpolarized beam, longitudinal target:  UL  ~ sin  Im{F 1 H +  (F 1 +F 2 )( H +.. } ~  = x B /(2-x B ),k = t/4M 2 Kinematically suppressed d4d4 dQ 2 dx B dtd  ~ | T DVCS + T BH | 2 DVCSBH T BH : given by elastic form factors T DVCS : determined by GPDs GPD  UT  ~ cos  m{k 1 ( F 2 H -F 1 E ) +.. } Unpolarized beam, transverse target: Kinematically suppressed 2 H.Avakian, Paris March 8

3. Electroproduction Kinematics  require a finite longitudinal momentum transfer defined by the generalized Bjorken variable    e p

4. Define the procedure to extract GPDs from DVCS data effect of finite bins (prefactor variations) ~10% Define background corrections pion contamination ~10% Radiative corrections GPDs from ep->e’p’  Requirements for precision (<15%) measurements of GPDs from DVCS SSA: A complete simulation of the whole chain from particle detection to GPD extraction, including the DVCS and background (counts, asymmetries) as well as extraction procedure (averaging over kinematic factors) required to ensure the reliability of measured GPDs. 4 H.Avakian, Paris March 8

5. Main experiments in valence region H.Avakian, Paris March 8 5 1)HERMES e  X sample ~1000 events 2)CLAS epX e1c,e1d e1f+e16(~2M events) Dominated by small t, small photon angles: (  <10) 3) Hall A e  X sample 4) Hall-B e1dvcs/e1dvcs2 e  p 5) Hall-B eg1dvcs e  p 6) Hall-A+Hall-B @ 12 GeV e  p 7)COMPASS x~0.1

6. Target Spin Asymmetry: t- Dependence Measurements with polarized target will constrain the polarized GPDs and combined with beam SSA measurements would allow precision measurement of unpolarized GPDs. Unpolarized beam, longitudinal target: Eg1dvcs provides order of magnitude more data compared to published eg1 data(5 CLAS days),  UL  ~ sin  Im{F 1 H +  (F 1 +F 2 )( H +.. }  LL  ~ cos  Re{F 1 H +  (F 1 +F 2 )( H +.. } ~ Kinematically suppressed ~ 6 H.Avakian, Paris March 8

7. GPD extraction from DVCS data H.Avakian, Paris March 8 7 Polarized data is crucial also for GPD-H extraction M.Guidal Phys.Lett.B689:156-162,2010

8.  MC vs Data Kinematic distributions in x,Q 2,t consistent with the CLAS data Region where BH totally dominates (small t, small photon  LAB ) Negligible DVCS x-section, small  0 contamination Rapidly changing prefactors, mainly small , hard to detect photons Large angles Uniform coverage in angle , photon measurement less challenging DVCS x-section non negligible introduce some model dependence)  0 dominates the single photon sample (in particular at low Q 2 ) 8 H.Avakian, Paris March 8

9.  -dependent amplitude Strong dependence on kinematics of prefactor  -dependence, at t≈t col,P 1 (  )→0 Radiative corrections may be significant 5.7 GeV 9 H.Avakian, Paris March 8

10. Radiative corrections 10 H.Avakian, Paris March 8 z 1/2 m defined from minimum photon energy cut, x 1/2 -defined shifted kinematics I. Akushevich true x

11.  -dependent amplitude Depending on the t the correction (the leading term of double bremsstrahlung x-section expanded over the electron mass ) can change the shape. 5.7 GeV 11 H.Avakian, Paris March 8 I. Akushevich

12. 12 CLAS configuration with longitudinally pol. target e  Polarizations:  Beam: ~70%  NH3 proton ~70%  Target position -55cm  Torus +/-2250  Beam energy ~5.7 GeV Longitudinally polarized target ep→e’  X

13. H.Avakian, Paris March 8 13 CLAS DVCS experiments ( eg1-dvcs/e1dvcs2) Helium tube Polarized target Inner Calo DVCS solenoid 15 o 18 o Extended cell

14. IC simulation and fiducial cuts H.Avakian, Paris March 8 14 Photons in IC Detailed simulation (Ahmed) is crucial for the x-section analysis

15. All single photons DVCS MC DVCS data Angular cut on difference between calculated and measured photons used to identify DVCS events 15 H.Avakian, Paris March 8 ep  DVCS  ep    DVCS identification cuts

16. 16 H.Avakian, Paris March 8 Missing energy Angular cut on difference between calculated and measured photons practically eliminates the background F.X. Girod DVCS identification cuts

17. JLab DPWG, May 19 17 Nuclear background NH3 Carbon Dilution for  X <1 degree f=0.87 ep  0 ep  eg1-dvcs Angular cut cleans up also the nuclear background

18. DVCS:  0 –background  Use ep    to estimate the contribution of  0 in the epX, ep  sample. ep → ep   ~70000 exclusive  0 s contamination by π 0 photons π 0 SSA.

19. DVCS kinematics H.Avakian, Paris March 8 19 F.X. Girod

20. DVCS x-sections from e1dvcs H.Avakian, Paris March 8 20 F.X. Girod Hyon-Suk Jo Alex Kubarovski CLAS PRELIMINARY F.X. Girod Radiative corrections and  0 contamination accounted, waiting for cross check

21. Radiative corrections comparison H.Avakian, Paris March 8 21 F.X Girod I. Akushevich Good agreement for the leading contribution

22. Polarized DVCS kinematics H.Avakian, Paris March 8 22 E. Seder Longitudinal target SSA will be extracted in bins in x and t

23. H.Avakian, Paris March 8 23 Summary CLAS e1dvcs experiment 1/2 provides precision data, crucial for extraction of GPDs in a wide kinematical range. CLAS experiment with longitudinally polarized NH3 and ND3 targets (eg1dvcs) provides superior sample of events allowing for detailed studies of single and double spin asymmetries using multidimensional bins. Combination of DVCS measurements with unpolarized and polarized targets would allow precision measurement of GPDs H and H~. Radiative corrections are important for precision measurement of CFFs from final observables

24. H.Avakian, Paris March 8 24 Support slides….

25. Radiative corrections 25 H.Avakian, Paris March 8

26. 26 H.Avakian, Paris March 8

27. BH cos  moment BH cos  moment can generate ~3% sin2  in the A LU 27 H.Avakian, Paris March 8

28. Collinearity kinematics Strong dependence of collinearity kinematics changes region of enhanced t as afunction of beam energy HERMESCLAS-5.7 28 H.Avakian, Paris March 8

29. All single photons DVCS MC DVCS data  MC vs Data Exclusive photon production simulated using a realistic MC Kinematic distributions in x,Q 2,t consistent with the CLAS data 29 H.Avakian, Paris March 8

30. H.Ava kian Deep Proce sses Meeti ng March 3 JLab 30  -dependent amplitude Strong dependence on kinematics of prefactor  -dependence, at y=y col P 1 (  )=0 Fraction of pure DVCS increases with t and   =0  =45  =90 BH DVCS x=0.25 5.7 GeV

31. H.Avakian, Paris March 8 31 eg1-dvcs: Monitoring polarizations Monitoring the time dependence of the product of target and beam polarizations using the elastic asymmetry Monitoring the time dependence of the beam polarization using the single spin asymmetry in ep→e’  X HWP→IN HWP→OUT

32. Deeply Virtual Compton Scattering ep→e’p’  Interference responsible for SSA, contain the same lepton propagator P 1 (  ) as BH Way to access to GPDs GPD combinations accessible as azimuthal moments of the total cross section. 32 H.Avakian, Paris March 8

33. Define the procedure to extract GPDs from A LU effect of finite bins ~10% Define background corrections pion contamination ~10% radiative background GPDs from ep->e’p’  Requirements for precision (<10%) measurements of GPDs from DVCS SSA:  0 dominates the single photon sample at low Q 2 in the kinematics where BH is small VGG-99 33 H.Avakian, Paris March 8