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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 - PowerPoint PPT Presentation
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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 exchange to probe GPDs
• JLab E05-015: neutron SSA with vertically polarized 3He
• Summary
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
q(x) – quark longitudinal spin distribution
quark flavors, g(x), …
• Connection?
Beyond charge and quark distributions – Generalized Parton Distributions (GPDs)
Elastic: transverse charge & current densities
DIS: quark longitudinalmomentum & helicity distributions
X. Ji, D. Mueller, A. Radyushkin (1994-1997), …
Correlated distributions in transverse space - GPDs
M. Burkardt, A. Belitsky (2000) …
GPDs and ‘Handbag’ Diagram
A Unified Description of Hadron Structure
Parton momentumdistributions
Elastic form factors
Real Comptonscattering at high t
Parton spin distributions
Deeply Virtual Compton Scattering
GPDs
Quark angular Momentum
Link to DIS and Elastic Form Factors
),,(~ ,~ , , txEHEH
JG = 1
1
)0,,q()0,,q(21
21 xE xHxdxJq
Quark angular momentum (Ji’s sum rule)
X. Ji, Phy.Rev.Lett.78,610(1997)
DIS at =t=0
)(),()0,0,(~)(),()0,0,(
xqxqxH
xqxqxH
Form factors (sum rules)
)(),,(~ , )(),,(~
) Dirac f.f.(),,(
,
1
1,
1
1
1
tGtxEdxtGtxHdx
tF1txHdx
qPqA
) Pauli f.f.(),,(1
tF2txEdx
Access GPDs
Accessed by cross sections
Accessed by beam/target spin asymmetry
t=0
Quark distribution q(x)
-q(-x)
DIS measures at =0
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- exchange
SSA in DVCS to probe GPD
Accessing GPDs through DVCS
d4dQ2dxBdtd ~ |DVCS + BH|2
BH : given by elastic form factorsDVCS: determined by GPDs
LU ~ BH Im(DVCS)sin + higher twist.
~ |DVCS|2 + |BH|2 + BH*Im(DVCS)
DVCS
BH
GPDs FF
e-’
p
e- *
plane
ee’* plane
*p
ep ep
Separating GPDs through polarization
LU~ sin{F1H + (F1+F2)H +kF2E}d~Polarized beam, unpolarized target:
Unpolarized beam, longitudinal target:
UL~ sin{F1H+(F1+F2)(H + … }d~
Unpolarized beam, transverse target:
UT~ sin{k(F2H – F1E) + …. }d
= xB/(2-xB)
k = t/4M2
H, H, E
Kinematically suppressed
H, H~
H, E
A =
=
~
First observation of DVCS/BH beam asymmetry
GPD analysis of CLAS/HERMES/HERA data in LO/ NLO shows results consistent with handbag mechanism and lowest order pQCDA. Freund, PRD 68,096006 (2003), A. Belitsky, et al. (2003)
sin + sin2
<< 1 twist-3 << twist-2
e+p e+X e-p e-pX
CLAS4.3 GeV
2001
0
HERMES27 GeV
-180 180(deg)
Q2=2.5 GeV2 Q2=1.5 GeV2
[rad]
CLAS preliminary
5.75 GeV
<Q2> = 2.0GeV2
<x> = 0.3<-t> = 0.3GeV2
e p ep
<Q2> = 2.0GeV2
<x> = 0.2<-t> = 0.25GeV2
CLAS preliminary
E=5.75 GeV
AUL
Longitudinally polarized target
AUL~sin{F1H+(F1+F2)H...}d~
DVCS/BH target asymmetry
Asymmetry observed at about the expected magnitude. Much higher statistics, and broad kinematical coverage are needed.
HERMES data on deuterium target
First Dedicated DVCS Experiments at JLab
Azimuthal and Q2 dependence of Im(DVCS) at fixed .Test Bjorken scaling.
=> Full reconstruction of all final state particles e, p, => High luminosity 1037
Data taking completed
, t, Q2 - dependence of Im(DVCS) in wide kinematics. Constrain GPD models.
PbWO4
Electromagneticcalorimeter
s.c.solenoid
CLAS
Currently taking data
Hall A (p and n)
LD2
Deeply Virtual Exclusive Processes - Kinematics Coverage of 12 GeV Upgrade
JLab Upgrade
unique to JLabHigh xB only reachablewith high luminosity H1, ZEUS
Wide Compton Scattering to probe GPD
Wide Angle Compton Scattering• WACS access GPD moments Compton Form Factors: JLab Hall A E99-114
nucl-ex/0410001),()(
1
1
2 txHx
dxetR q
qqV
),()(1
1
2 txEx
dxetR q
qqT
),(~
)(1
1
2 txHx
dxetR q
qqA
Data:
GPD:
V
ALL R
RK
V
T
LL
LS
R
R
K
K
Recoil polarization components:
04.0078.0114.0 LSK
04.0083.0678.0 LLK
02.010.0 LSK
15.057.0 LLK P. Kroll, hep-ph/0412169
Target SSA with 2 exchange to probe GPD
JLab E05-015: vertically polarized n (3He)
GPD moment with target SSA with 2effect JLab E05-015: Spokespersons: T. Averett, J.P. Chen, X. Jiang
Summary on target SSA with 2
• 2-exchange provides a new tool to probe nucleon dynamics
• Non-zero Ay is a clear signature of 2-exchange
• E05-015 goals:
Unambiguously establish a non-zero Ay
First experiment to use 2 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.
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 2• JLab E05-015: neutron one moment of GPD
constraints on E GPD.
Precision measurement of g2n
Higher twist effects:quark-gluon correlations
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)
g2 (x,Q2) g2WW (x,Q2) g 2(x,Q2)
g2WW(x,Q2 ) g1(x,Q2) g1(y,Q2)
x
1
dy
y
d2n(Q2 ) x2 2g1
n (x,Q2 ) 3g2n (x,Q2) dx
0
1
d2 2 B E / 3
Jefferson Lab Hall A Experiment E97-103Precision Measurement of the Neutron Spin Structure Function g2
n(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 g1
n data.
• Quantitative measurement of higher twist effects provides information on nucleon structure beyond simple parton model (e.g. quark-gluon correlations).
E97-103 Results: g2n vs. x
Improved precision of g2n by an order of magnitude
E97-103 results: g2n vs. Q2
• Measured g2n consistently higher than g2
ww
E97-103 results: g1n
• Agree with NLO fit to world data, evolved to our Q2
JLab E99-117 Precision Measurement of A1
n at Large xSpokespersons: J. P. Chen, Z. -E. Meziani, P. Souder, PhD Student: X. Zheng
• Precision A1n data 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
• A2n at high x, by-product, d2
n
A2n results
• By-product
• Precision better than the world best results
• Also g1n and g2
n results
• Improved d2n precision
by a factor of 2:
d2n=0.0062 ± 0.0028
• PRC 70, 065207 (2004)
Summary on g2n and d2
n 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