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DVCS results with unpolarized and polarized target

DVCS results with unpolarized and polarized target. Harut Avakian (Jlab). Introduction Event selection MC simulations and radiative corrections DVCS with unpolarised target DVCS with longitudinally polarized target Summary. d 4 . ~ | T DVCS + T BH | 2. dQ 2 dx B dtd .

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DVCS results with unpolarized and polarized target

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

  2. d4 ~ |TDVCS + TBH|2 dQ2dxBdtd Deeply Virtual Compton Scattering ep->e’p’g DVCS BH GPD TBH: given by elastic form factors TDVCS: determined by GPDs Polarized beam, unpolarized target: ~ DsLU~ sinfIm{F1H+ x(F1+F2)H+kF2E} DVCS Kinematically suppressed Unpolarized beam, longitudinal target: BH ~ DsUL~ sinfIm{F1H+x(F1+F2)(H+.. } Kinematically suppressed x = xB/(2-xB ),k = t/4M2 Unpolarized beam, transverse target: • Different GPD combinations accessible as azimuthal moments of the total cross section. DsUT~ cosfIm{k1(F2H-F1E)+.. } Kinematically suppressed H.Avakian, Paris March 8

  3. Electroproduction Kinematics g e q p g*->g require a finite longitudinal momentum transfer defined by the generalized Bjorken variable x

  4. GPDs from ep->e’p’g Requirements for precision (<15%) measurements of GPDs from DVCS SSA: • Define the procedure to extract GPDsfromDVCS data • effect of finite bins (prefactor variations) ~10% • Define background corrections • pion contamination ~10% • Radiative corrections 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. H.Avakian, Paris March 8

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

  6. ~ DsUL~ sinfIm{F1H+x(F1+F2)(H+.. } DsLL~ cosfRe{F1H+x(F1+F2)(H+.. } ~ Kinematically suppressed Target Spin Asymmetry: t- Dependence Unpolarized beam, longitudinal target: Eg1dvcs provides order of magnitude more data compared to published eg1 data(5 CLAS days), Measurements with polarized target will constrain the polarized GPDs and combined with beam SSA measurements would allow precision measurement of unpolarized GPDs. H.Avakian, Paris March 8

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

  8. g MC vs Data • Region where BH totally dominates (small t, small photon qLAB) • Negligible DVCS x-section, small p0 contamination • Rapidly changing prefactors, mainly small f, hard to detect photons • Large angles • Uniform coverage in angle f, photon measurement less challenging • DVCS x-section non negligible introduce some model dependence) • p0 dominates the single photon sample (in particular at low Q2 ) • Kinematic distributions in x,Q2,t consistent with the CLAS data H.Avakian, Paris March 8

  9. f-dependent amplitude 5.7 GeV • Strong dependence on kinematics of prefactor f-dependence, at t≈tcol,P1(f)→0 • Radiative corrections may be significant H.Avakian, Paris March 8

  10. Radiative corrections I. Akushevich true x z1/2m defined from minimum photon energy cut, x1/2-defined shifted kinematics H.Avakian, Paris March 8

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

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

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

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

  15. All single photons DVCS data DVCS MC DVCS identification cuts epg(DVCS) epg(p0) • Angular cut on difference between calculated and measured photons used to identify DVCS events H.Avakian, Paris March 8

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

  17. Nuclear background epg NH3 epg epp0 Carbon eg1-dvcs Dilution for qgX<1 degree f=0.87 Angular cut cleans up also the nuclear background JLab DPWG, May 19

  18. DVCS: p0 –background ep → epp0 • Use epgg(p0) to estimate the contribution of p0in the epX, epgsample. • contamination by π0 photons • π0SSA. ~70000 exclusive p0s

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

  20. DVCS x-sections from e1dvcs F.X. Girod F.X. Girod Hyon-Suk Jo Alex Kubarovski CLAS PRELIMINARY Radiative corrections and p0 contamination accounted, waiting for cross check H.Avakian, Paris March 8

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

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

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

  25. Radiative corrections H.Avakian, Paris March 8

  26. H.Avakian, Paris March 8

  27. BH cosf moment BH cosf moment can generate ~3% sin2f in the ALU H.Avakian, Paris March 8

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

  29. All single photons DVCS data DVCS MC g MC vs Data • Exclusive photon production simulated using a realistic MC • Kinematic distributions in x,Q2,t consistent with the CLAS data H.Avakian, Paris March 8

  30. f-dependent amplitude 5.7 GeV BH f=0 f=45 f=90 DVCS x=0.25 Strong dependence on kinematics of prefactor f-dependence, at y=ycol P1(f)=0 Fraction of pure DVCS increases with t and f H.Avakian Deep Processes Meeting March 3 JLab

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

  32. Deeply Virtual Compton Scattering ep→e’p’g Interference responsible for SSA, contain the same lepton propagator P1(f) as BH GPD combinations accessible as azimuthal moments of the total cross section. Way to access to GPDs H.Avakian, Paris March 8

  33. GPDs from ep->e’p’g Requirements for precision (<10%) measurements of GPDs from DVCS SSA: • Define the procedure to extract GPDs from ALU • effect of finite bins ~10% • Define background corrections • pion contamination ~10% • radiative background VGG-99 p0 dominates the single photon sample at low Q2 in the kinematics where BH is small H.Avakian, Paris March 8

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