Overview of Experimental
Program
(updated September 23, 2004)
1. Structure of the Nucleon
a. EG1
experiment (E93-009, E91-023, E93-036)
In Hall B at
Jefferson Lab we use polarized electrons incident on polarized NH3
and ND3 targets to study the spin structure of the nucleon in and
above the resonance region. We are
interested in the inclusive double polarization asymmetry, which allows
us to extract the well known spin structure function
g1(x,Q2) and its integral in the resonance
region. In addition, the large
acceptance of the Cebaf Large Acceptance Spectrometer (CLAS) in Hall B enables us to measure
multi-particle final states. We are
therefore investigating the double polarization asymmetry for exclusive
pion production from the nucleon in order to probe the spin structure of nucleon
resonances.
b. BoNuS Experiment at
Jefferson Lab
We are building a novel slow proton recoil detector
(a "Radial Time Projection Chamber") using Gas Electron Multipliers (GEMs) to
detect slow spectator protons in the reaction d(e,e'p_S)X. By tagging the
scattered electron with a low-momentum (down to 70 MeV/c) proton moving
backwards relative to the momentum transfer, we can ensure that the reaction
took place on a nearly on-shell neutron in deuterium. This experiment (E03-012)
will allow us to extract the free neutron structure function at high x and in
the resonance region, for the first time undisturbed by Fermi motion and
off-shell effects.
2. The Electric Form Factor of the Proton (E04-116)
Different
measurements of the charge form factor of the proton (which is the fourrier
transform of the charge distribution) differ by up to a factor of 3. By studying
the ratio of positron and electron scattering off of the proton, we can
investigate two-photon effects, which may be responsible for this discrepancy.
To do this, making a clean positron beam will be a significant technical
challenge.
3. Real and Virtual Compton Scattering
The new theoretical formalism developed by Radyushkin and Ji, known as Generalized Parton
Distributions (GPDs), has opened up exciting new experimental
opportunities at 6 GeV:
a. Real Compton Scattering in Hall A (E99-114)
We have measured real Compton scattering from
the proton at large angles as a function of the center of mass energy
s. The calculation of this
cross section for QCD in the asymptotic limit is quite different from the GPD
prediction by Radyushkin.
b. Deeply Virtual Compton Scattering in
Hall A (E00-110
and E03-106)
Virtual Compton scattering (VCS) refers to
the reaction e + p ŕ e + p + g
and has been under study by our group for several years. In experiment E93-050 we
investigated the generalized polarizabilities of the proton and the behavior of
the cross section as a function of s in the resonance region. Now we are extending these measurements
to higher energy to measure the VCS reaction in the deep inelastic regime and at
high Q2 (DVCS) where GPD predictions are
expected to be valid (E00-110). We also have a newly approved experiment
to measure deeply virtual Compton scattering on the neutron (E03-106).
c. DVCS in Hall B (E01-113)
The DVCS reaction will also be
studied in Hall B. Analysis of data
taken at 4.2 GeV with CLAS showed a strong DVCS signal and has already been
published (PRL 87, 182002 (2001)). A dedicated experiment has been
approved (E01-113) to investigate the validity of GPD predictions at 6
GeV.
4. The Deuteron
a.
D(e,e˘p)n in Hall A (E01-020)
This experiment will be the first truly
systematic study of the high momentum part of the deuteron wave function which
arises from the short-range part of the interaction. In order to extract
information
about the short-range interaction a systematic study of this
reaction will be undertaken. Only JLab provides the right combination of
attributes in order to carry out this work, so this study will be the
first
of its kind. A large number of different kinematics will be explored so that the
reaction details (i.e., the nature of the interaction of the electrons
with the nucleus, including effects from excitation of the nucleons, currents
consisting of virtual particles exchanged between the nucleons, relativity,
etc.) can be disentangled from details of the deuteron structure.
This experiment builds on our experience with previous measurements on the
deuteron (E94-004).
b. D(e,e˘ps)X in Hall B (E94-102)
We
are studying the high momentum components of the deuteron wave function by
detecting spectator protons at backward angles in CLAS. In this way we can “tag” high momentum
(short distance) NN pairs over a wide kinematic range. We are also using these data to investigate
exlcusive p- production on deuterium which
is sensitive to the neutron resonance structure.
5. Polarization
Transfer
Experiments on 2H (E89-028) and
4He (E93-049 and E03-104) use the 
reaction
whereby the polarization of knocked-out protons is measured after polarized
electrons from the accelerator scatter from the nucleus. The proton polarization
reflects the preference for their spins to point in a given direction and this
preference is dictated by details of the scattering process. Further, the
polarizations are expected to be sensitive to the distributions of charge and
magnetization of the protons within the nucleus. Differences from their
free space values will be signaled by polarizations which differ from those
obtained for free protons as measured for a hydrogen target. This information
provides vital input for building models of nucleons and nuclei based on their
quark and gluonic constituents.
6. Heavier Nuclei
a.
Coincidence Reaction Studies
with CLAS (E2)
Due to the strong interaction and
short distances between nucleons in nuclei, there is a significant probability
for nucleon wave functions to overlap, resulting in short range nucleon-nucleon
correlations (SRC) in nuclei. These correlations are responsible for the high
momentum components in nuclei. In addition, because the nucleons overlap, the
local density can be several times larger than the average nuclear density,
providing an extreme environment for the study of nucleons. By measuring all
three nucleons knocked out of 3He, we have made the first measurement
of correlated two-nucleon momentum distributions. We are currently extending
those studies to higher momenta.
b.
16O(e,e˘p) in Hall A (E00-102)
Our previous experiment on 16O in
Hall A (E89-003) yielded unexpected indications of diffractive
structure at high missing momenta in ALT˘, the longitudinal-transverse asymmetry, and
surprising success of the relativistic distorted wave impulse approximation in
describing the reaction up to a missing momentum of 350 MeV/c. In this new experiment, therefore, we
will study the cross section and ALT˘ at higher missing momentum in order to
determine
·
the limits of validity of the
single-particle model of valence proton knockout
·
the effects of relativity and
spinor distortion on valence proton knockout
·
the bound state wave function
and spectroscopic factors for valence knockout
