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Deeply Virtual Compton Scattering on Polarized Protons and Neutrons

Study of deeply virtual Compton scattering (DVCS) on longitudinally polarized protons and neutrons for GPDs. Investigates nucleon helicity with axial-vector and pseudoscalar parameters. Features the 'Handbag' factorization in the high-Q2 Bjorken regime.

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Deeply Virtual Compton Scattering on Polarized Protons and Neutrons

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  1. Deeply virtual Compton scattering on longitudinally polarized protons and neutrons at CLAS k’ q’ k N N’ GPDs Silvia Niccolai, IPN Orsay, for the CLAS Collaboration QNP2012, Palaiseau, April 19th 2012

  2. ~ e t g H, H, E, E (x,ξ,t) gL* (Q2) x+ξ x-ξ ~ ~ Axial-Vector:H (x,ξ,t) Pseudoscalar:E (x,ξ,t) ~ [ ] 1 1 1 ò = - JG = x + x q q q J xdx H ( x , , 0 ) E ( x , , 0 ) 2 2 - 1 Deeply Virtual Compton Scattering and GPDs e’ • Q2= - (e-e’)2 • xB = Q2/2Mn n=Ee-Ee’ • x+ξ, x-ξ longitudinal momentum fractions • t = (p-p’)2 • x  xB/(2-xB) 4 GPDs for each quark flavor p’ p conserve nucleon helicity « Handbag » factorization valid in the Bjorken regime: high Q2 ,  (fixed xB), t<<Q2 Vector:H (x,ξ,t) Tensor:E (x,ξ,t) flip nucleon helicity Quark angular momentum (Ji’s sum rule) «3D» quark/gluon image of the nucleon X. Ji, Phy.Rev.Lett.78,610(1997)

  3. Sensitivity to GPDs of DVCS spin observables g ~ Polarized beam, unpolarized target: Im{Hp, Hp, Ep} f e’ ~ DsLU~ sinfIm{F1H+ x(F1+F2)H -kF2E}df e leptonic plane N’ ~ ~ Unpolarized beam, longitudinal target: Polarized beam, longitudinal target: hadronic plane Re{Hp, Hp} Im{Hp, Hp} ~ ~ DsLL ~ (A+Bcosf)Re{F1H+x(F1+F2)(H+ xB/2E)…}df DsUL ~ sinfIm{F1H+x(F1+F2)(H+ xB/2E) –xkF2E+…}df Unpolarized beam, transverse target: Im{Hp, Ep} DsUT~ sinfIm{k(F2H – F1E) + …..}df x= xB/(2-xB) k=-t/4M2 ProtonNeutron ~ Im{Hn, Hn, En} ~ ~ Im{Hn, En, En} ~ Re{Hn, En, En} Im{Hn}

  4. Sensitivity to GPDs of DVCS spin observables g ~ Polarized beam, unpolarized target: Im{Hp, Hp, Ep} f e’ ~ DsLU~ sinfIm{F1H+ x(F1+F2)H -kF2E}df e leptonic plane N’ ~ ~ Unpolarized beam, longitudinal target: Polarized beam, longitudinal target: hadronic plane Re{Hp, Hp} Im{Hp, Hp} ~ ~ DsLL ~ (A+Bcosf)Re{F1H+x(F1+F2)(H+ xB/2E)…}df DsUL ~ sinfIm{F1H+x(F1+F2)(H+ xB/2E) –xkF2E+…}df Unpolarized beam, transverse target: Im{Hp, Ep} DsUT~ sinfIm{k(F2H – F1E) + …..}df x= xB/(2-xB) k=-t/4M2 ProtonNeutron ~ Im{Hn, Hn, En} ~ ~ Im{Hn, En, En} ~ Re{Hn, En, En} Im{Hn}

  5. What we have learned from the published CLAS asymmetries CLAS pDVCS TSAs eg1 (2000), not a DVCS- dedicated experiment CLAS pDVCS BSAs S. Chen et al, PRL 97, 072002 (2006) F.-X. Girod et al, PRL. 100 162002 (2008) Model-independent fit at fixed xB, t, Q2 of DVCS observables ImH has steeper t-slope than ImH: is axial charge more concentrated than the electromagnetic charge? ~ M. Guidal, Phys. Lett. B 689, 156-162 (2010)

  6. The eg1-dvcs experiment at CLAS • Data taken from February to September 2009 • Beam energies = 4.735, 5.764, 5.892, 5.967 GeV • Beam polarizaton ~ 85% • CLAS+IC to detect forward photons • Target: longitudinally polarized via DNP (5 Tesla, 1 Kelvin, 140 Ghz microwaves) NH3 (~80%) and ND3(~30%) – Luminosity ~ 5∙1034 cm-2 s-1 • Target polarization monitored by NMR • ~75 fb-1 on NH3 (parts A, B), ~25 fb-1 on ND3 (part C) Polarized ammonia Carbon Empty cell C.D. Keith et al., NIM A 501 (2003) 327

  7. pDVCS (ep→e’p’g): particle ID Db (DC/TOF) for positive tracks Energy deposited in EC for negative tracks p+ protons deuterons Electron ID cuts: Charge: -1 0.2 < E/p < 0.4 (energy deposited in EC) Ein > 0.06 (energy deposited in inner EC) p > 0.8 Nphe(CC)>2 Geometrical matching between EC, SC, CC z vertex cut EC, DC fiducial cuts Proton ID cuts: Charge > 0 z vertex cut ¦Δβ¦ < 0.035 DC fiducial cuts EC photons not yet included in the analysis (<10% events) IC photon ID cuts: E >2.5 GeV Geometrical fiducial cuts

  8. pDVCS: channel selection & coverage Kinematical and exclusivity cuts to select DVCS events: Cone angle: before/after cuts • Eγ>2.5 GeV Q2>1 GeV2 W>2 GeV • Cone Angle (angle between detected and predicted γ) • MM2 epX • Missing Energy • Coplanarity (angle between (γ*,p) and (γ,p) planes) • MM2 epgX • Missing Transverse Momentum (in reaction frame)

  9. pDVCS- Sanity check: Beam Spin Asymmetry eg1-dvcs Integrated over all kinematics, only IC photons included Only eg1-dvcs part B data (~2/3) No p0 background subtraction yet F.-X. Girod et al, PRL. 100 162002 (2008) Beam polarization: ~ 83 %

  10. Erin Seder, UConn Gary Smith, Glasgow pDVCS: Target Spin Asymmetry Preliminary <xB>~0.3 <Q2>=2.3 (GeV/c2)2 Dilution factor: f~ 0.76 Target polarization: PT=-85%, +90% Only IC photons included only eg1-dvcs part B data No p0 background subtraction yet

  11. Gary Smith, Glasgow pDVCS: Double (Beam-Target) Spin Asymmetry p0 vs xB p0 vs -t First bin in -t ALL THESE RESULTS ARE VERY PRELIMINARY! Dilution factor: f~ 0.76 Target polarization: PT=-85%, +90% Beam Polarizarion: PB= 83% p1 vs -t p2 vs -t Only IC photons included only eg1-dvcs part B data No p0 background subtraction yet

  12. H2 n NH3 ND3 ND3 DVCS on different targets DariaSokhan, IPNO Calculate DVCS on a “free” neutron Free proton F.-X. Girod et al, PRL. 100 (2008) 162002 Free proton in nuclear medium Quasi-free neutron in deuterium and in heavier nuclear medium Quasi-free protonin deuterium and in heavier nuclear medium

  13. Sanity check: ALU – proton in NH3/ND3 DariaSokhan, IPNO Raw beam-spin asymmetries No p0 background subtraction Good agreement between the two analyses

  14. nDVCS in ND3 – channel selection B < 0.95 (EC timing) pX Standard PID cuts for electron and photon Exclusivity cuts:

  15. nDVCS ALU beam-spin asymmetry from ND3 Very preliminary DariaSokhan, IPNO Projections for 90 days of running with CLAS12 <xB>~0.3 <Q2>=2.3 (GeV/c2)2 Integrated over all kinematics No p0 subtraction yet Statistics very low, but ALU≠0! AUL analysis also underway More data will be taken with CLAS12 at 11 GeV, on liquid deuterium target

  16. Summary and outlook • Combining various DVCS spin observables for proton and neutron targets is necessary to provide constraints for model-independent extractions of Compton Form Factors (→GPDs) • The eg1-dvcs experiment combined the CLAS-DVCS setup (CLAS+IC) with polarized • hydrogen and deuterium targets • Preliminary results for TSA for pDVCS are in good agreement with existing data, and the statistics with respect to previous CLAS data has been improved by more than a factor 5 • Preliminary results for double-spin asymmetries show dominance of the constant term • Very preliminary results for nDVCS (very low statistics) hint to non-zero beam-spin asymmetries • A lot of work (mainly on background subtraction) still needs done • Much more data for both pDVCS and nDVCS on a wider phase space will come from CLAS12 Thanks again to Erin Seder, Gary Smith, Daria Sokhan

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