2009 APS April Meeting
Volume 54, Number 4
Saturday–Tuesday, May 2–5, 2009;
Denver, Colorado
Session L4: Nucleon Microscopy
3:30 PM–5:18 PM,
Sunday, May 3, 2009
Room: Plaza F
Sponsoring
Unit:
DNP
Chair: Kees De Jager, Thomas Jefferson National National Accelerator Facility
Abstract ID: BAPS.2009.APR.L4.3
Abstract: L4.00003 : The Proton in the Nuclear Medium*
4:42 PM–5:18 PM
Preview Abstract
Abstract
Author:
Simona Malace
(University of South Carolina)
Whether the nucleon changes its fundamental properties while
embedded in
nuclear medium has been a long-standing question in nuclear physics.
Nucleons are composite objects of quarks and gluons, the degrees of
freedom of the
Quantum Chromodynamics (QCD) Lagrangian. Yet, because of the highly
non-perturbative nature of the phenomena in the confinement region,
there are no available calculations for nuclei within the QCD
framework.
Conventionally, nuclei are effectively and well described as
clusters of
nucleons held together by a strong, long-range force mediated by
meson
exchange. In the effective nuclear field theory, the interaction
of an
electromagnetic probe with a nucleon inside the nucleus is
described by taking
into account the presence of the nuclear medium. Conventional
nuclear
medium effects such as the nucleon being off-shell,
meson-exchange currents,
isobar configurations, and final-state interactions need to be
incorporated
as corrections to the impulse approximation picture.
In this context, the question arises whether in the nuclear
current operator
free or nuclear medium modified form factors should be used.
What are the effects of the nuclear medium on the sub-nucleon
structure?
Polarization transfer in quasi-elastic nucleon knockout is sensitive
to the properties of the nucleon in the nuclear medium. In our
recently completed experiment E03-104 at Jefferson Lab in Hall A
we measured the
proton recoil polarization in the $^4$He($\vec e,e^\prime \vec
p\,$)$^3$H
reaction at a $Q^2$ of 0.8 (GeV/$c$)$^2$ and 1.3 (GeV/$c$)$^2$ with
unprecedented precision. These data complement earlier data
between 0.4
and 2.6 (GeV/$c$)$^2$ from both Mainz and Jefferson Lab. The
measured ratio
of polarization-transfer coefficients differs from a fully
relativistic
calculation, and is well described by either the inclusion of a
medium
modification of the proton form factors predicted by a quark-meson
coupling model or strong charge-exchange final-state interactions.
The measured induced polarizations agree well with the fully
relativistic
calculation and indicate that these strong final-state
interactions may
not be applicable.
*Work supported in part by NSF PHY-0555604
To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2009.APR.L4.3