Deeply Virtual Compton Scattering and Pseudoscalar Meson Electroproduction with CLAS
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Transcript of Deeply Virtual Compton Scattering and Pseudoscalar Meson Electroproduction with CLAS
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Deeply Virtual Compton Deeply Virtual Compton Scattering and Pseudoscalar Scattering and Pseudoscalar
Meson Electroproduction with Meson Electroproduction with CLASCLAS
Valery KubarovskyValery KubarovskyJefferson LabJefferson Lab
XII Workshop on High Energy Spin PhysicsSeptember 5, 2007, Dubna
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OutlineOutline
Physics MotivationPhysics Motivation DVCS results (CLAS/Jlab)DVCS results (CLAS/Jlab)
- Beam-spin asymmetry- Beam-spin asymmetry- Comparison with theoretical models- Comparison with theoretical models
electroproductionelectroproduction– Cross sectionCross section– Beam spin asymmetryBeam spin asymmetry– Cross section ratioCross section ratio
ConclusionConclusion
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Electron ScatteringElectron Scattering, a clean , a clean probe of the Proton probe of the Proton StructureStructure
Q2 = -(e-e’)2
ν = Ee – Ee’
xB = Q2/2M t = (p-p’)2
1/Q2 is the space-time resolution of the virtual
e’
p
eQ
p’
elastic
ve’
p
eQ
X
inclusive
ve’
p
eQ
p’
exclusive
v
Interaction described by:
Reveal different aspects of the proton’s internal structure
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Proton form factors, transverse charge & current densities
D. Mueller, X. Ji, A. Radyushkin, …1994 -1997M. Burkardt, A. Belitsky… Interpretation in impact parameter space
Structure functions,quark longitudinalmomentum & spin distributions
How are the proton’s charge densities related to its quark momentum distribution?
?
Correlated quark momentum and helicity distributions in transverse space - GPDs
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Basic Process – Handbag Mechanism
xB
2-xB
=GPDs depend on 3 variables, e.g. H(x, , t). They probe the quark structure at the amplitude level.
Deeply Virtual Compton Scattering (DVCS)
x – longitudinal quark momentum fraction
–t – Fourier conjugateto transverse impact parameter
– longitudinal momentum transfer
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Proton’s gravitational form factors
GPDs Quark angular momentum
Quark-quark correlations
3D Imaging of quark distributions
Universality of GPDs
Forces on quarks
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Universality of GPDs
Single Spin Asymmetries
Deeply Virtual Meson production
Real Compton Scattering
Elastic Form Factors
Parton Momentum Distributions
Deeply Virtual Compton Scattering
GPDs
∫H(x,t)dx
∫H(x,ξ,t)dxH(x=ξ,ξ,t)
∫H(x,t)x-1dx
∫H(x,ξ,t)dx
Experimental measurements of the exclusive processes is a challenge, requiring high luminosity to compensate for the small cross section and detectors capable of ensuring the exclusivity of the final state.
H(x,ξ=0,t=0)=q(x)H(x,ξ=0,t=0)=q(x)~
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A =
=
Measuring GPDs through polarization
Unpolarized beam, transverse target:
UT~ sin{k(F2H – F1E) + …. }d
Kinematically suppressed
H(t), E(t)
LU~ sin{F1H + ξ(F1+F2)H +kF2E}d~
Polarized beam, unpolarized target:
H(,t)
Kinematically suppressed
ξ ~ xB/(2-xB)
Unpolarized beam, longitudinal target:
UL~ sin{F1H+ξ(F1+F2)(H +ξ/(1+ξ)E) -.. }d~
Kinematically suppressed
H(,t)~
k = t/4M2 ~
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AUL=sin+sin2
Pioneering measurements in 2001Pioneering measurements in 2001
Evidence for twist-2 dominance in asymmetry.
DVCS first observed in non-dedicated experiments at HERA and JLab.
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First DVCS measurement with spin-aligned target
Unpolarized beam, longitudinally spin-aligned target:
UL~ sinIm{F1H+ξ(F1+F2)H +… }d~
= 0.252 ± 0.042 = -0.022 ± 0.045
Planned experiment in 2009 will improve accuracy dramatically.
CLAS preliminary
H=0~
H=0~
AUL sensitive to GPD H ~
fitmodelmodel (H=0)
~
S. Chen, et al., Phys. Rev. Lett 97, 072002 (2006)
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Deeply Virtual Meson Electroproduction
High Q 2 - Low -tComplement DVCS experiment.
Unique access to spin dependent GPDs
Low Q 2 - High -t New form factors related to 1/x moments of GPDs
High Q 2 - High -tRegion never accessed.
Q2
-t
p p’
L
(z)
x-
z
x+
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p p’
L
(z)
x-
z
x+
+ t.c.
• Factorization theorem states that in the limit Q2∞ exclusive electroproduction of mesons is described by hard rescattering amplitude, generalized parton distributions (GPDs), and the distribution amplitude (z) of the outgoing meson. • The prove applies only to the case when the virtual photon has longitudinal polarization• Q2∞ L~1/Q6 , T/L~1/Q2
• The full realization of this program is one of the major objectives of the 12 GeV upgrade
Collins,Frankfurt,Strikman -1997
High QHigh Q22 Low t Region Low t Region
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Pseudoscalar Mesons Pseudoscalar Mesons
In the case of pseudoscalar meson In the case of pseudoscalar meson production the amplitude involves the production the amplitude involves the axial axial vector-type GPDsvector-type GPDs
These GPDs are closely related to the These GPDs are closely related to the distributions of quark spin in the proton. The distributions of quark spin in the proton. The function reduces to the function reduces to the polarized polarized quark/antiquark densitiesquark/antiquark densities in the limit of zero in the limit of zero momentum transfermomentum transfer
The Fourier transform with respect to pThe Fourier transform with respect to pTT, the , the so-called so-called impact parameter distributionsimpact parameter distributions, , describes the transverse spatial distribution describes the transverse spatial distribution of quark spin in the proton.of quark spin in the proton.
H~
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Flavor Separation and Flavor Separation and Helicity-Dependent GPDsHelicity-Dependent GPDs
DVCS is the cleanest way of accessing GPDs. However, DVCS is the cleanest way of accessing GPDs. However, it is difficult to perform a it is difficult to perform a flavor separationflavor separation..
Vector and pseudoscalar meson production allows one Vector and pseudoscalar meson production allows one to separate flavor and isolate the to separate flavor and isolate the helicity-dependent helicity-dependent GPDsGPDs. .
EH~
,~
EH ,
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““Precocious Precocious Factorization”Factorization” Precocious factorization could be valid Precocious factorization could be valid
already at relatively low Qalready at relatively low Q22 especially especially for ratios of cross sections as a for ratios of cross sections as a function of xfunction of xBB
For example For example 00 and and ratio on the ratio on the protonproton
220
3
2
3
1
3
2
6
1:
3
1
3
2
2
1:
ududu
Eides,Frankfurt,Strikman -1997
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Cross Section Ratios Cross Section Ratios as a function of xas a function of xBB
)(
)(
epep
enen
)(
)(0
0
epep
enen
All data are available. 0 ratio from proton data will be released very soon
Eides,Frankfurt,Strikman -1997
)(
)(0
enen
enen
)(
)(0
epep
epep
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CLAS
JLab Site: The 6 GeV Electron Accelerator
3 independent beams with energies up to 6 GeV Dynamic range in beam current: 106
Electron polarization: 85%
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CCEBAF EBAF LLarge arge AAcceptance cceptance SSpectrometerpectrometer CLASCLAS
TOF counters
Drift chambers
Beam line and the target
Electromagnetic calorimeters
6 Superconducting coils
Cherenkov counters
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CLAS (forward carriage and side clamshells retracted)
Region 3 drift chamber
Panel 4 TOF
Panel 1 TOF
Panel 2 & 3TOF
Cerenkov & Forward angle EC
Large angle EC
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CLAS Lead Tungstate CLAS Lead Tungstate Electromagnetic CalorimeterElectromagnetic Calorimeter
/E~3%
424 crystals,18 RL, pointing geometry, APD readout
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Fit: ALU = sincos
Fully integrated asymmetry and one of 65 bins in Q2, x, t .
First results in wide First results in wide kinematicskinematics
A =
=
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t-dependence of leadingtwist term (sinΦ).
GPD model predictionsGPD model predictions
VGG parameterization reproduces –t > 0.5GeV2 behavior, and overshoots asymmetry at small t.
The latter could indicate that VGG misses some important contributions to the DVCS cross section.
VGG Model (Vanderhaeghen, Guichon, Guidal)
Fit: ALU = sincos
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Regge model Regge model predictionprediction
While there are good indications that Compton scattering does occur at the quark level a description of the process by Regge model (J-M Laget) is in fair agreement in some kinematic bins with our results.
The full significance of this dual description remains to be investigated
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DVMP: Kinematic DVMP: Kinematic CoverageCoverage
4 dimensional grid in Q4 dimensional grid in Q22, x, xBB, t, and , t, and
00 ,epep
,epep
t
Q2 xB
W
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KinematicsKinematics 4 dimensional grid4 dimensional grid
BinsBins FromFrom ToTo UnitsUnits
77 1.01.0 4.64.6 GeVGeV22
77 0.10.1 0.580.58
66 0.090.09 2.02.0 GeVGeV22
1212 00 360360 degreesdegrees
22 )'( eeQ
m
QxB 2
2
2)'( ppt
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Remarks on the Remarks on the following slidesfollowing slides CLAS data CLAS data All data are All data are preliminarypreliminary No radiative correction were appliedNo radiative correction were applied Cross sections are in arbitrary units Cross sections are in arbitrary units No No LL//TT separation separation 12 GeV: 12 GeV: Rosenbluth L/T separation
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DistributionDistribution
)cos)1(22cos(2
1),,,( 2
dt
d
dt
d
dt
d
dt
dtxQ
dtd
d LTTTLT
0* pp
Fit of the -distribution gives us three structure functions
dt
ddt
ddt
d
dt
d
LT
TT
LT
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dd/d/d 0* pp
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TTLLas a function as a function of tof t
0* epp
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LTLTas a function of tas a function of t
0* epp
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TTTT as a function t as a function t
0* epp
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TTL L ) ) TTTT LTLT as a function of tas a function of t
t GeV2
Q2=2.3xB=0.4
cos)1(22cos)(),,,( 2LTTTLTtxQ
dtd
d
0* pp
Non-zero Non-zero TTTT andand LTLT imply imply
that both transverse that both transverse
and longitudinal amplitudesand longitudinal amplitudes
participate in the processparticipate in the process
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TTL L ) ) TTTT LTLT in Regge Model (J-M in Regge Model (J-M Laget)Laget)
• The dashed lines correspond to the /b1 Regge poles and elastic rescattering
• The full lines include also charge pion nucleon and Delta intermediate states.
• Regge model qualitatively describes Regge model qualitatively describes the experimental datathe experimental data
/b1 elastic rescat. charge pion
0* pp
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dd/dt/dt 0* pp
tQxB Bedt
d ),( 2
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t-Slope Parameter as a t-Slope Parameter as a Function of xFunction of xBB and Q and Q22
xB
B(xB ,Q2)
•B(xB , Q2) is almost independent of Q2
•B(xB) is decreasing with increasing xB
0* pp
tQxB Bedt
d ),( 2
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t-dependencet-dependence
xB
B(xB )
1.1'
)/1ln('2)(
),(
)/1ln('22'
')(
xxB
exdt
d
xxtxf
txt
ttq q
txBedt
d )(
This is not fit of data. This is GPD predictionswith Regge inspired t-dependence xt
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Impact Parameter Impact Parameter Dependent PDFsDependent PDFs Fourier transform of GPDFourier transform of GPD
For impact parameter dependent For impact parameter dependent parton distributions the perp width parton distributions the perp width should go to zero for xshould go to zero for x11
In momentum space, this implies that t-In momentum space, this implies that t-slope should decrease with increasing slope should decrease with increasing x, what we observe experimentallyx, what we observe experimentally
),0,(~
)2(
1),,( 22
2
xHedbbxIPD biyx
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Impact Parameter ProfileImpact Parameter Profileof axial current of axial current distributiondistribution
||91.022)2(
1)0,,(
xedbbxIPD biyx
X
xb
The curve is whatwe obtained from experimental data
The size of the The size of the proton decreases proton decreases with increasing xwith increasing x
From data fit
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00 and and Beam Spin Beam Spin AsymmetryAsymmetry
)sin)1(2
cos)1(22cos(2
1),,,(
'
2
dt
dh
dt
d
dt
d
dt
d
dt
dtxQ
dtd
d
TL
LTTTLT
h is the beam helicity
sin44
44
VV
dd
ddA
Any observation of a non-zero BSA would be indicative of an L’T interference.If L dominatesLTand L’T go to zero
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00 Beam Spin Asymmetry Beam Spin AsymmetryA()XB=0.25Q2=1.95 GeV2
t=-0.29 GeV2
Balck curve – A=sinRed curve – Regge model
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A=sinas a function of t The red curves The red curves
correspond to the correspond to the Regge model Regge model (JML)(JML)
BSA are BSA are systematically of systematically of the order of 0.03-the order of 0.03-0.09 over wide 0.09 over wide kinematical range kinematical range in xin xBB and Q and Q22
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Beam Spin Beam Spin AsymmetryAsymmetry
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Conclusion
Beam-spin asymmetries were extracted in the valence quark region, as a function of all variables describing the reaction. Present GPD parameterization describe reasonably well the main features of our data. The measured kinematic dependencies will put stringent constraisins on any DVCS model.
Cross sections and beam-spin asymmetries for the 0
and exclusive electroproduction in a very wide kinematic range will be released soon
These data will help us to understand better the transition from soft to hard mechanisms
New experiments at 6 GeV with polarized and unpolarized target are coming
CLAS12 will continue the GPD study with broader kinematics at 12 GeV machine.
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Questions to theoryQuestions to theory What will our data tell us?What will our data tell us?
What does t-slope B(QWhat does t-slope B(Q22,x,xBB) tell us ?) tell us ? What can we learn from the QWhat can we learn from the Q22 evolution of evolution of
cross section?cross section? Can Can LTLT and and TTTT help us to constrain R= help us to constrain R=LL//T T
?? Can we constrain the GPDs within some
approximations and corrections which have to be made due to non-asymptotic kinematics?
How big are the corrections? How close are we to asymptote?
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Q: What will come out Q: What will come out from our marriage?from our marriage?
p p’
zx+
L
(z)
x-
+ = ?
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THE ENDTHE END