References - Bogoliubov Laboratory of Theoretical Physics - JINR

1.3 Flavor Decomposition of the Helicity Structure
If we want to understand the three-dimensional structure of the nucleon, we have to go
beyond inclusive measurements that are only sensitive to the longitudinal momentum
fraction x carried by the quarks. The large acceptance of CLAS allows us to collect data
on semi-inclusive (SIDIS) reactions simultaneously. In these reactions, a second particle,
typically a meson, is detected along with the scattered lepton. By making use of the
additional information given by the identification of this meson, one can learn more about
the polarized partons inside the nucleon than from DIS alone. The asymmetry measured
by DIS experiments is sensitive to combinations of quark and anti-quark polarized parton
distribution functions (Δq+Δ¯q), as well as (via NLO analysis) the gluon PDF ΔG. SIDIS
experiments exploit the statistical correlation between the flavor of the struck quark and
the type of hadron produced to extract information on quark and antiquark PDFs of all
flavors separately. Combined NLO analysis of DIS and SIDIS data can therefore give a
more detailed picture of the contribution of all quark flavors and both valence and sea
quarks to the total nucleon helicity. Beyond the determination of the polarized PDFs,
SIDIS data can also yield a plethora of new insights into the internal structure of the
nucleon as well as the dynamics of quark fragmentation. For instance, looking at the
z- andpT -dependence of the various meson asymmetries (both double spin asymmetries
and single spin target or beam asymmetries), one can learn about the intrinsic transverse
momentum of quarks and their orbital angular momentum.
2 Measurements and Data Analysis
A1 and g1 were extracted from measurements of the double spin asymmetry A� in inclusive
ep scattering:
g1 = F1
1+γ2 [A�/D +(γ − η)A2], (4)
where F1 is the unpolarized structure function, A2 is the virtual photon asymmetry, and
γ, D and η are kinematic factors. F1 and A2 are calculated using a parametrization of
the world data, and A� is measured. The spin asymmetry for ep scattering is given by:
A� = N− − N+
N− + N+
CN
fPbPtfRC
+ ARC, (5)
where N−(N+) is the number of scattered electrons normalized to the incident charge
with negative (positive) beam helicity, f is the dilution factor needed to correct for the
electrons scattering off the unpolarized background, fRC and ARC correct for radiative
effects, and CN is the correction factor associated with polarized 15 N nuclei in the target.
A� was measured by scattering polarized electrons off polarized nucleons using a cryogenic
solid polarized target and CLAS in Hall B. The raw asymmetries were corrected for the
beam charge asymmetry, the dilution factor and radiative effects. Since the elastic peak is
within the acceptance range, the product of beam and target polarization was determined
from the known ep elastic asymmetry.
The longitudinally polarized electrons were produced by a strained GaAs electron
source with a typical beam polarization of ∼ 70%. Two solid polarized targets were used:
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