Single-Spin Asymmetries at CLAS

1
Single-Spin Asymmetries at CLAS
H. Avakian (JLab)
APS Meeting APR03

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

APS Meeting APR03
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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
Complementary sets of non-perturbative functions
sensitive to different aspects of transverse
distributions

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

TMD distributions contain direct information
about the quark orbital motion
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Spin-Azimuthal Asymmetries
Spin asymmetries azimuthal dependence
new class of DIS measurements Spin-Azimuthal
Asymmetries
Significant progress made recently in studies of
Single-Spin Azimuthal Asymmetries (SSA) with
longitudinally polarized target (HERMES),
transversely polarized target (SMC), and
polarized beam (CLAS).

• SSA are sensitive to the orbital momentum of
  quarks, enable measurements of
• GPDs and kT-dependend PDFs (TMDs)
• provide a window to the physics of partonic final
  and initial state interactions
• model calculations indicate that SSA are not
  affected significantly by a wide
• range of corrections.
• Good agreement in SSAs measured in a wide energy
  range in electroproduction
• and pp scattering.

SSAs appropriate observable at JLAB beam
energies and Q2
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Novel Distributions
Transversity probes relativistic nature of
quarks, does not mix with gluons. First moment -
tensor charge

0.56/-0.09 (Q22)
Sivers function describes unpolarized quarks in
transversely polarized nucleon. A non-zero T-odd
f1T-, requires final state interactions
interference between different helicity states
(Brodsky et al., Collins, Ji et al. 2002)
Collins function describes fragmentation of
transversely polarized quarks into unpolarized
hadrons. Physics mechanisms to generate non-zero
T-odd H1- by Collins 1993, Bacchetta et al. 2002
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Polarized Semi-Inclusive DIS
Cross section defined by scale variables x,y,z
Hadron-Parton transition by distribution
function f1u(x) probability to find a u-quark
with a momentum fraction x
Parton-Hadron transition by fragmentation
function Dp (z) probability for a u-quark to
produce a p with a momentum fraction z
1u
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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.
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Contributions to s in Polarized SIDIS
?e, SL, ST electron and proton long. and trans.
pol. sum over quarks and
anti-quarks.
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x,z Factorization
The z distribution for different ranges of
Bjorken x
No significant variation observed in z dependence
for different x ranges.
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Long. Pol Target SSA for p
AUL are consistent both in magnitude and sign
with predictions based on Collins mechanism
Target SSA CLAS (4.3 GeV) is consistent with
HERMES (27.5 GeV) Curves for Sivers effect from
BHS-2002.
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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 from CLAS 4.3 and 5.7GeV data
Low MX in average correspond to large z
5.7GeV using 2 methods
4.3 GeV
Unlike AUL (target SSA) no sign flip in ALU in
the exclusive limit
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ALU x-dependence CLAS 4.3 vs 5.7 GeV
0.5ltzlt0.8
5.7GeV (e16)
4.3 GeV (e1c)
Photon Sivers Effect Afanasev Carlson
(preliminary)
CLAS preliminary
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 provide access
to essential parameters with less ambiguities
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First Extraction of e(x) from CLAS Data
SSA analyzed in terms of the fragmentation effect
Efremov et al.
x-dependence of CLAS beam SSA (ALU)

z-dependence of HERMES target SSA (AUL )
2gT-hL
First glimpse of Twist-3 e(x)
Jaffe,Ji 1992
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z-dependence e1c vs e16
Low z contaminated by target fragmentation.
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Exclusive pions CLAS 4.3 vs 5.7GeV
Sign flip at z 0.5
At zlt0.5 struck quark in neutron
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Exclusive pions ALU x,t-dependence
Extracted at zgt0.6
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Summary

• Significant SSAs observed in semi-inclusive and
  exclusive pion production in DIS region
• Beam SSA from p electroproduction in DIS at 4
  and 6 GeV are consistent with each other, HERMES
  data at 27.5GeV and predictions based on partonic
  picture.
• No significant x-dependence observed in
  z-distributions of positive pions in a chosen
  kinematic range at energies accessible currently
  at JLab (4-6 GeV).