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Single Target Spin Asymmetries and GPDs

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DVCS and Wide Angle Compton Scattering. Target SSA with 2g ... PRC 70, 065207 (2004) Summary on g2n and d2n results. Precision measurement of g2n at low Q2 ... – PowerPoint PPT presentation

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Title: Single Target Spin Asymmetries and GPDs


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
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

Link to DIS and Elastic Form Factors
7
Access GPDs
DIS measures at x0
8
Program to access/determine GPDs
  • 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
Accessing GPDs through DVCS
ep epg
11
Separating GPDs through polarization
Polarized beam, unpolarized target


DsLU sinfF1H x(F1F2)H kF2Edf
H, H, E
Kinematically suppressed
Unpolarized beam, longitudinal target


H, H
DsUL sinfF1Hx(F1F2)(H … df
Unpolarized beam, transverse target
H, E
DsUT sinfk(F2H F1E) ….. df
12
First observation of DVCS/BH beam asymmetry
2001
ep egX
e-p e-pX
CLAS preliminary
5.75 GeV
ltQ2gt 2.0GeV2 ltxgt 0.3 lt-tgt 0.3GeV2
Q21.5 GeV2
Q22.5 GeV2
f rad
A(f) asinf bsin2f
GPD analysis of CLAS/HERMES/HERA data in LO/ NLO
shows results consistent with handbag mechanism
and lowest order pQCD A. Freund, PRD 68,096006
(2003), A. Belitsky, et al. (2003)
b/a ltlt 1 twist-3 ltlt twist-2
13
DVCS/BH target asymmetry
CLAS preliminary
AUL
E5.75 GeV
Longitudinally polarized target
ltQ2gt 2.0GeV2 ltxgt 0.2 lt-tgt 0.25GeV2
Asymmetry observed at about the expected
magnitude. Much higher statistics, and broad
kinematical coverage are needed.
HERMES data on deuterium target
14
First Dedicated DVCS Experiments at JLab
gt Full reconstruction of all final state
particles e, p, g gt 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
JLab Upgrade
16
CLAS12 - DVCS/BH Target Asymmetry
Selected kinematics

DssinfImF1Hx(F1F2)H...df
  • L 2x1035
  • T 1000 hrs
  • DQ2 1GeV2
  • Dx 0.05

17
Wide Compton Scattering to probe GPD
18
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
19
Target SSA with 2g exchange to probe GPD
JLab E05-015 vertically polarized n (3He)
20
GPD moment with target SSA with 2g effect
JLab E05-015 Spokespersons T.
Averett, J.P. Chen, X. Jiang
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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.

39
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.

40
Precision measurement of g2n
Higher twist effects quark-gluon correlations
41
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)

42
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 lt Q2 lt 1.34 GeV2, W gt 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).

43
E97-103 Results g2n vs. x
  • Improved precision of g2n by an order of
    magnitude

44
E97-103 results g2n vs. Q2
  • Measured g2n consistently higher than g2ww

45
E97-103 results g1n
  • Agree with NLO fit to world data, evolved to our
    Q2

46
JLab E99-117 Precision Measurement of A1n at
Large x Spokespersons J. P. Chen, Z. -E.
Meziani, P. Souder, PhD Student X. Zheng
  • Precision A1n data at high x
  • 2.7GeV2 lt Q2 lt 4.8 GeV2, W gt 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
  • A2n at high x, by-product, ? d2n

47
A2n results
  • By-product
  • Precision better than the world best results
  • Also g1n and g2n results
  • Improved d2n precision by a factor of 2
  • d2n0.0062 0.0028
  • PRC 70, 065207 (2004)

48
Summary on g2n and d2n results
  • Precision measurement of g2n at 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
  • A2n at high x, by-product
  • d2n a factor of 2 improvement, can
    compare with LQCD

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