single target spin asymmetries and gpds
DESCRIPTION
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 PresentationTRANSCRIPT
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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
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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?
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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) …
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GPDs and ‘Handbag’ Diagram
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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
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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
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Access GPDs
Accessed by cross sections
Accessed by beam/target spin asymmetry
t=0
Quark distribution q(x)
-q(-x)
DIS measures at =0
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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
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SSA in DVCS to probe GPD
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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
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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 =
=
~
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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
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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
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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
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Deeply Virtual Exclusive Processes - Kinematics Coverage of 12 GeV Upgrade
JLab Upgrade
unique to JLabHigh xB only reachablewith high luminosity H1, ZEUS
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Wide Compton Scattering to probe GPD
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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
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Target SSA with 2 exchange to probe GPD
JLab E05-015: vertically polarized n (3He)
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GPD moment with target SSA with 2effect JLab E05-015: Spokespersons: T. Averett, J.P. Chen, X. Jiang
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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.
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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.
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Precision measurement of g2n
Higher twist effects:quark-gluon correlations
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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
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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).
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E97-103 Results: g2n vs. x
Improved precision of g2n by an order of magnitude
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E97-103 results: g2n vs. Q2
• Measured g2n consistently higher than g2
ww
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E97-103 results: g1n
• Agree with NLO fit to world data, evolved to our Q2
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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
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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)
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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
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