Single-Spin Asymmetries at CLAS

1
Single-Spin Asymmetries at CLAS
H. Avakian, L.Elouadrhiri
(Jefferson Lab)
For the CLAS Collaboration
Conference on Intersections of Particle and
Nuclear Physics

• Transverse momentum of quarks and spin-azimuthal
  asymmetries
• Target single-spin asymmetries
• Beam single-spin asymmetries in
• semi-inclusive pion production
• exclusive pion production
• Summary

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Orbital Angular Momentum and Transverse momentum
of quarks
Parton Distribution Functions generalized to
contain information not only on longitudinal,
but also on the transverse distribution of
partons

• Generalized Parton Distributions (GPD) H, E ...
• Transverse-momentum dependent (TMD) parton
  distributions

Spin asymmetries azimuthal dependence
new class of DIS measurements Single-Spin
Asymmetries
SSA are sensitive to the orbital motion of
quarks, enable measurements of GPDs and
kT-dependent PDFs (TMDs)
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- Dependent Parton Distributions
f1, g1 studied for decades h1 and others
essentially unknown
Twist-2 - dependent PDFs by Mulders et
al.
opens access to mixed hadron-parton and
parton-hadron transitions
Gauge invariant definition of TMDs discussed by
Collins and Belitsky,Ji Yuan Nucl.Phys.
B656 165, 2003
TMDs appear in sinf and sin2f SSA In polarized
semi-inclusive DIS
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Polarized Semi-Inclusive DIS
Cross section defined by scale variables x,y,z
Hadron-Parton/ Parton-Hadron transitions
described by distribution and fragmentation
functions.
Two fundamental QCD mecha- nisms (Collins and
Sivers) identified, to generate SSA
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Contributions to s in ep?epX

• s for longitudinally polarized leptons
  scattering off unpolarized protons

Different structure functions can be extracted
as azimuthal moments of the total cross section.
More spin-dependent azimuthal moments arise for
polarized targets
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The CLAS Detector
Scattering of 5.7 GeV polarized electrons off
polarized NH3 and unpolarized hydrogen
n
X

• 8M p in DIS kinematics,
• Q2gt1 GeV2, W2gt4 GeV2,
• ylt0.85, zgt0.5
• beam polarization 73
• target polarization 72 (f0.2)

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Longitudinally Pol. Target SSA for p
Target SSA CLAS (4.3 GeV, and 5.7 GeV )
consistent with HERMES (27.5 GeV)
predictions for Sivers effect from BHS
Phys.Lett.B53099,2002 Sivers only interpretation
require large AUT
AUL z-dependence also consistent both in
magnitude and sign with predictions based on
Collins mechanism
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Longitudinally Pol Target SSA for p
P1 0.01100.0024 P2-0.00650.0025
D1 unpol. FF
H1
SSA x-dependence CLAS (5.7 GeV) is consistent
with predictions for Collins effect from
Efremov et al. hep-ph/0208124
-
- Collins FF

• sin2f moment (Kotzinian-Mulders asymmetry)
• is a clean source of Collins SSA
• x 4 more CLAS data available for analysis

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Unpolarized target x,z factorization studies at
5.7GeV
The z distribution for different ranges of
Bjorken x
ep ? e p X ( Ee 5.7 GeV, MX gt 1.1)
No significant variation observed in z
dependence for different x ranges.
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Factorization studies in CFR at CLAS
The separation of CFR with 0.5ltzlt0.8 is not
changing significantly with beam energy.
DIS Q2gt1 W2gt4 ylt0.85
No significant variation observed in z dependence
of ALU for different x ranges
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ALU from CLAS 4.3 and 5.7GeV data
Low MX in average correspond to large z
5.7GeV
No significant dependence on beam energy
4.3 GeV
No sign flip in ALU in the exclusive limit
(unlike HERMES target SSA)
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Beam SSA sinf Moment

• ALU

Beam SSA ALU from CLAS at 4.3GeV and HERMES
(SPIN-2002) at 27.5 GeV
Beam SSA measurements for different beam energies
are consistent.
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ALU x-dependence CLAS 4.3 vs. 5.7 GeV
0.5ltzlt0.8
5.7GeV
4.3 GeV
Photon Sivers Effect Afanasev Carlson
(preliminary)
CLAS preliminary
Beam SSA analyzed also in terms of Collins effect
by Efremov et al.
4.3 GeV and 5.7 GeV data consistent
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Photon Sivers Effect
Target SSA (Brodsky et al.)
Essential constituents
Light-cone spin wave function of a nucleon
Beam SSA (Afanasev Carlson)
Final state interaction from gluon exchange
Beam SSA has the same source and provides access
to essential parameters with less ambiguities
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Exclusive pions CLAS 4.3 vs. 5.7GeV
W2gt4 GeV2,Q2gt1 GeV2
Sign flip at z 0.5
4.3 GeV
-t lt 0.5GeV2
5.7 GeV
At zgt0.5 struck quark in pion
LUND-MC
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Beam SSA ALU x,t-dependence
t(PN-PN)2 lt0.5GeV2 ,W2gt5GeV2, Q2gt2.5GeV2
(ltQ2gt3.0GeV2gt)
CLAS PRELIMINARY
CLAS PRELIMINARY
-
Beam SSA for exclusive pions may provide access
to GPDs
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Summary

• Beam spin asymmetries observed for the first
  time in exclusive p electroproduction in hard
  scattering kinematics.
• No significant dependence on x observed for z
  dependencies of beam SSA.
• Kinematic dependencies of beam and target SSA
  measured for different beam energies are
  consistent, and can be described by a variety of
  theoretical models.
• Combined analysis of SSA for polarized beam and
  target needed to separate contributions from
  different mechanisms and extract corresponding
  distribution functions.

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LUND-MC Studies
NH3 dilution factor (f0.21) from data is
consistent with LUND and simple quark counting
(dotted line).
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Beam SSA
In similar to polarized target case kinematics,
no sign of significant sin2f term in the data