References - Bogoliubov Laboratory of Theoretical Physics - JINR

SPIN PHYSICS WITH CLAS
Y. Prok 12,†
(1) Christopher Newport University
(2) Thomas Jefferson National Accelerator Facility
† E-mail: yprok@jlab.org
Abstract
Inelastic scattering using polarized nucleon targets and polarized charged lepton
beams allows the extraction of double and single spin asymmetries that provide
information about the helicity structure of the nucleon. A program designed to
study such processes at low and intermediate Q 2 for the proton and deuteron has
been pursued by the CLAS Collaboration at Jefferson Lab since 1998. Our inclusive
data with high statistical precision and extensive kinematic coverage allow us to
better constrain the polarized parton distributions and to accurately determine
various moments of spin structure function g1 as a function of Q 2 . The latest
results are shown, illustrating our contribution to the world data, with comparisons
of the data with NLO global fits, phenomenological models, chiral perturbation
theory and the GDH and Bjorken sum rules. The semi-inclusive measurements of
single and double spin asymmetries for charged and neutral pions are also shown,
indicating the importance of the orbital motion of quarks in understanding of the
spin structure of the nucleon.
1 Spin Structure Functions
One fundamental goal of Nuclear Physics is the description of the structure and properties
of hadrons, and especially nucleons, in terms of the underlying degrees of freedom,
namely quarks and the color forces between them. Much progress has been made over
the last decades towards this goal, both experimentally (e.g., through structure function
and form factor measurements) and theoretically (effective theories like the quark model,
chiral perturbation theory as well as complete solutions of QCD on the lattice). At the
same time, there are many important questions that require further investigation, such as:
What is the quark structure of nucleons in the valence region, in particular in the limit of
large momentum fraction carried by a single quark, x → 1? How can we describe the transition
from hadronic degrees of freedom to quark degrees of freedom for the nucleon? How
can we describe the nucleon in three dimensions and what are the correlations between
transverse momentum and spin? How does quark orbital angular momentum contribute
to the spin of the nucleon? A program designed to study these questions, and utilizing the
CLAS detector, 6 GeV polarized electron beam, and longitudinally polarized solid ammonia
targets (NH3 and ND3) has been pursued by the CLAS Collaboration at Jefferson
Lab since 1998. This program entails both inclusive measurements of inelastic electron
scattering as well as coincident detection of leading hadrons (pions etc.) produced in
such events. Due to the large acceptance of CLAS, a large kinematical region is accessed
simultaneously. Both the scattered electrons and leading hadrons from the hadronization
of the struck quark are detected, allowing us to gain information on its flavor.
292