In recent years a major focus in spin-dependent, deep-inelastic scattering has been the precision study of the moments of the spin structure functions, g_1 and g_2, for both the proton and the neutron. However, the detailed dependence of the functions g_1(x,Q^2) and g_2(x,Q^2) on the kinematical variables Bjorken x and momentum transfer Q^2 is poorly known in certain regions, i.e., for large and very small values of x for all Q^2 and for Q^2 values less than \sim 1.0 GeV^2 for all x. The detailed shapes of the structure functions in these regions are of special interest, as they permit insight into lingering questions concerning the nucleon's spin structure. Probing the x\to 1 region, e.g., potentially allows us to investigate the transition from a perturbative QCD regime to a non-perturbative, valence quark regime. In constrast, probing the low Q^2 behavior of the spin structure function g_2(x, Q^2), which is related to the nucleon's spin distribution transverse to the virtual photon direction, serves as an unique testing ground for the transition from incoherent to coherent, inclusive, lepton-parton scattering. The significance of quark-gluon correlations are expected to become more pronounced at lower values of Q^2 (< 1.0 GeV^2).
We have measured inclusive, longitudinal and transverse,
deep-inelastic scattering asymmetries in the ^3\vec
He(\vec e, e^\prime) reaction using the
highly polarized electron beam and the Hall A polarized
^3He target at Jefferson Lab. Neutron asymmetries for x
values between 0.33 and 0.61 were extracted which
allowed us to determine the flavor decomposed ratios \Delta
u/u and \Delta d/d. Moreover, a precision measurement of
g_2^n(Q^2) for Q^2 values down to 0.6 GeV^2 at x
\approx 0.2 was performed. The impact of these measurements
on our present understanding of the neutron spin structure
will be discussed.
[K6.002] Proton Knockout from 4He
Bodo Reitz (Jefferson Lab)
The (e,e'p) reaction on few-body nuclear targets is a
powerful tool to investigate specific aspects of the
nucleus. ^4He is an especially interesting target since it
has all the ingredients of a complex, heavy nucleus, while
as an A=4 system, microscopic calculations are still
feasible. Making use of the high luminosity electron beam at
Jefferson Lab and the high resolution spectrometers in Hall
A, high precision cross section measurements of (e,e'p)
reactions in kinematic regions previously unaccessible, are
now possible. I will focus my talk on a recent Jefferson Lab
Hall A experiment which has measured the ^4He(e,e'p)^3H
cross section at recoil momenta up to 500 MeV/c in various
kinematics. Many calculations predict a sharp minimum in the
spectral function for those recoil momenta and show that its
location is sensitive to the short range part of the
nucleon-nucleon potential. Measuring this cross section at
various kinematical settings for the same recoil momentum
additionally allows us to study reaction dynamics such as
final-state interactions and meson-exchange currents. I will
present preliminary results of this experiment.
[K6.003] NN Correlations Measured in 3He(e,e'pp)n
Lawrence Weinstein (Old Dominion University)
We now have reasonably good descriptions of average single nucleon properties in nuclei. The next step in understanding nuclear structure is to measure average nucleon pair properties in nuclei (ie: two nucleon correlations). We have recently performed the first large acceptance kinematically complete measurements of 1 to 4 GeV electron scattering from 3He with a 4pi magnetic spectrometer (the Jefferson Lab CLAS).
This talk will present 3He correlated momentum distributions
measured using two techniques: 1) the virtual photon is
absorbed by one nucleon and its correlated (high momentum)
partner also leaves the nucleus and 2) the virtual photon is
absorbed by the third nucleon in 3He and the residual
spectator correlated pair then flies apart. The measured pp
and pn pair relative momentum distributions extend up to 600
MeV/c.
[K6.004] Relativistic Effects in Three-Nucleon Systems
Alfred Stadler (University of Évora, and Centro de Física Nuclear, University of Lisbon, Portugal)
The Spectator or Gross formalism is a manifestly covariant framework based on field theory for the description of few-body systems. It consists of equations that effectively sum an infinite number of Feynman diagrams, with the characteristic feature that, in any included Feynman diagram, spectator particles are consistently placed on mass shell.
In recent years, the Spectator theory has been applied successfully to a variety of problems. In particular, realistic potentials of one-boson exchange type have been constructed yielding a very good description of the two-nucleon bound state and scattering data. Calculations of the deuteron electromagnetic form factors also achieved a very good agreement with the available data. The three-nucleon bound state was calculated using the two-nucleon Spectator amplitudes as dynamical input, and a binding energy close to the experimental value was obtained. We are now applying the Spectator theory to elastic and inelastic electron scattering from the three-nucleon bound state. I will review the relativistic effects found in the three-nucleon bound state and report on the status of the ongoing electron-scattering calculations.