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Single Target Spin Asymmetries and GPDs

Single Target Spin Asymmetries and GPDs. Jian-ping Chen, Jefferson Lab, Virginia, USA SSA Workshop, BNL, June 1-3, 2005 Nucleon structure and GPDs DVCS and Wide Angle Compton Scattering Target SSA with 2 g exchange to probe GPDs

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Single Target Spin Asymmetries and GPDs

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  1. Single Target Spin Asymmetries and GPDs Jian-ping Chen, Jefferson Lab, Virginia, USA SSA Workshop, BNL, June 1-3, 2005 • Nucleon structure and GPDs • DVCS and Wide Angle Compton Scattering • Target SSA with 2g exchange to probe GPDs • JLab E05-015: neutron SSA with vertically polarized 3He • Summary

  2. Nucleon Structure • Elastic scattering  nucleon has finite size Dirac Form Factor, F1(Q2) - charge distribution Pauli Form Factor, F2(Q2) – current distribution • DIS  parton distribution functions (PDFs) q(x) – quark longitudinal momentum distribution Dq(x) – quark longitudinal spin distribution quark flavors, g(x), … • Connection?

  3. X. Ji, D. Mueller, A. Radyushkin (1994-1997), … DIS: quark longitudinal momentum & helicity distributions Elastic: transverse charge & current densities Correlated distributions in transverse space - GPDs Beyond charge and quark distributions – Generalized Parton Distributions (GPDs) M. Burkardt, A. Belitsky (2000) …

  4. GPDs and ‘Handbag’ Diagram

  5. A Unified Description of Hadron Structure Elastic form factors Parton momentum distributions Real Compton scattering at high t GPDs Parton spin distributions Deeply Virtual Compton Scattering Quark angular Momentum

  6. Form factors (sum rules) ] [ 1 x DIS at =t=0 ò x = dx H ( x , , t ) F1 ( t )Dirac f.f. = - - H ( x , 0 , 0 ) q ( x ), q ( x ) ] [ 1 ò x = dx E ( x , , t ) F2 ( t )Pauli f.f. ~ = D D - ( x , 0 , 0 ) q ( x ), q ( x ) H 1 1 ~ ~ ò ò x = x = dx H ( x , , t ) G ( t ) , dx E ( x , , t ) G ( t ) , , A q P q - - 1 1 ~ ~ x H , E , H , E ( x , , t ) Quark angular momentum (Ji’s sum rule) [ ] 1 1 1 ò = - JG = x + x Jq xdx H q( x , , 0 ) E q( x , , 0 ) 2 2 - 1 X. Ji, Phy.Rev.Lett.78,610(1997) Link to DIS and Elastic Form Factors

  7. Quark distribution q(x) Accessed by beam/target spin asymmetry -q(-x) Accessed by cross sections t=0 Access GPDs DIS measures at x=0

  8. Program to access/determine GPD’s • Direct access: -Deep Inelastic Scattering (DIS) -Deep Virtual Compton Scattering (DVCS) -Deep Virtual Meson Production (DVMP) -Doubly Deep Virtual Compton Scattering (DDVCS) • Form Factors: Moments of GPDs: -Elastic Scattering -Wide Angle Compton Scattering -Single Target Spin Asymmetry through 2-g exchange

  9. SSA in DVCS to probe GPD

  10. DVCS BH FF GPDs plane gg*p f d4 e-’ g ~ |DVCS + BH|2 dQ2dxBdtd Qgg* ~|DVCS|2 + |BH|2 + BH*Im(DVCS) g* e- p ee’g* plane BH:given by elastic form factors DVCS: determined by GPDs DsLU~ BHIm(DVCS)sinf+ higher twist. Accessing GPDs through DVCS ep epg

  11. Ds 2s s+ - s- s+ + s- A = = x = xB/(2-xB) k = t/4M2 Separating GPDs through polarization Polarized beam, unpolarized target: ~ ~ DsLU~ sinf{F1H+ x(F1+F2)H+kF2E}df H, H, E Kinematically suppressed Unpolarized beam, longitudinal target: ~ ~ H, H DsUL~ sinf{F1H+x(F1+F2)(H+ … }df Unpolarized beam, transverse target: H, E DsUT~ sinf{k(F2H – F1E) + …..}df

  12. CLAS 4.3 GeV HERMES 27 GeV 0 -180 180 f(deg) First observation of DVCS/BH beam asymmetry 2001 e+p e+gX e-p e-pX CLASpreliminary 5.75 GeV <Q2> = 2.0GeV2 <x> = 0.3 <-t> = 0.3GeV2 Q2=1.5 GeV2 Q2=2.5 GeV2 f [rad] A(f) = asinf + bsin2f GPD analysis of CLAS/HERMES/HERAdata in LO/ NLOshows results consistent with handbag mechanism and lowest order pQCD A. Freund, PRD 68,096006 (2003), A. Belitsky, et al. (2003) b/a << 1 twist-3 << twist-2

  13. e p epg ~ AUL~sinf{F1H+x(F1+F2)H...}df DVCS/BH target asymmetry CLAS preliminary AUL E=5.75 GeV Longitudinally polarized target <Q2> = 2.0GeV2 <x> = 0.2 <-t> = 0.25GeV2 Asymmetry observed at about the expected magnitude. Much higher statistics, and broad kinematical coverage are needed. HERMES data on deuterium target

  14. Hall A (p and n) CLAS s.c. solenoid PbWO4 Electromagnetic calorimeter x, t, Q2 - dependence of Im(DVCS) in wide kinematics. Constrain GPD models. Currently taking data First Dedicated DVCS Experiments at JLab => Full reconstruction of all final state particles e, p,g => High luminosity 1037 LD2 Azimuthal and Q2 dependence of Im(DVCS) at fixed x. Test Bjorken scaling. Data taking completed

  15. Deeply Virtual Exclusive Processes - Kinematics Coverage of 12 GeV Upgrade unique to JLab High xB only reachable with high luminosity H1, ZEUS JLab Upgrade

  16. Wide Compton Scattering to probe GPD

  17. Wide Angle Compton Scattering • WACS access GPD moments Compton Form Factors: JLab Hall A E99-114 nucl-ex/0410001 Recoil polarization components: Data: GPD: P. Kroll, hep-ph/0412169

  18. Target SSA with 2g exchange to probe GPD JLab E05-015: vertically polarized n (3He)

  19. GPD moment with target SSA with 2g effect JLab E05-015: Spokespersons: T. Averett, J.P. Chen, X. Jiang

  20. Summary on target SSA with 2g • 2g-exchange provides a new tool to probe nucleon dynamics • Non-zero Ay is a clear signature of 2g-exchange • E05-015 goals: Unambiguously establish a non-zero Ay First experiment to use 2g Ay to study GPDs • Ayn sensitive to one GPD moment, cleaner interpretation Constraints on E GPD • Technically straight-forward measurement, no new equipment needed • ~ 1 month beam time to test GPD prediction for Ay at 15% level.

  21. Summary • GPD provides a unified framework • DVCS SSA direct access GDPs • Results from JLab, HERMES and other labs • Dedicated experiments and JLab upgrade • Wide Angle Compton Scatting access GPD moments • Recent results on KLL and KLS. • New way to measure GPD moments: STSA with 2g • JLab E05-015: neutron one moment of GPD constraints on E GPD.

  22. Precision measurement of g2n Higher twist effects: quark-gluon correlations

  23. Quark-Gluon Correlations • In simple partonic picture g2(x)=0 • Wandzura and Wilczek have shown that g2 can be written in two parts: • twist-2 contributions given by g1 • the other originating from quark-gluon correlations (twist-3)

  24. Jefferson Lab Hall A Experiment E97-103 Precision Measurement of the Neutron Spin Structure Function g2n(x,Q2): A Search for Higher Twist Effects T. Averett, W. Korsch (spokespersons) K. Kramer (Ph.D. student) • Precision g2n, 0.57 < Q2 < 1.34 GeV2,W > 2 GeV, at x ~ 0.2. • Direct comparison to twist-2 g2ww prediction using world g1n data. • Quantitative measurement of higher twist effects provides information on • nucleon structure beyond simple parton model (e.g. quark-gluon correlations).

  25. E97-103 Results: g2n vs. x Improved precision of g2n by an order of magnitude

  26. E97-103 results: g2n vs. Q2 • Measured g2n consistently higher than g2ww

  27. E97-103 results: g1n • Agree with NLO fit to world data, evolved to our Q2

  28. JLab E99-117Precision Measurement of A1n at Large xSpokespersons: J. P. Chen, Z. -E. Meziani, P. Souder, PhD Student: X. Zheng • Precision A1ndata at high x 2.7GeV2 < Q2 < 4.8 GeV2, W > 2 GeV • Extracting valence quark spin distributions • Test our fundamental understanding of valence quark picture • SU(6) symmetry • Valence quark models • pQCD (with HHC) predictions • Other models: Statistical Model, Chiral Soliton Model, PDF fits, …. • Crucial input for pQCD fit to PDF • A2nat high x, by-product,  d2n

  29. A2n results • By-product • Precision better than the world best results • Also g1n and g2n results • Improved d2n precision by a factor of 2: d2n=0.0062 ± 0.0028 • PRC 70, 065207 (2004)

  30. Summary on g2n and d2n results • Precision measurement of g2nat low Q2 • An order of magnitude improvement in precision • g2n consistently higher than g2 WW • Higher twist effects: quark-gluon correlations • Precision spin structure data at high x from JLab • Valence quark neutron spin structure • A1n at high x, an order of magnitude improvement: • A2nat high x, by-product • d2n: a factor of 2 improvement, can compare with LQCD

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