APS April Meeting 2017
Volume 62, Number 1
Saturday–Tuesday, January 28–31, 2017;
Washington, DC
Session X16: DNP Prize Session
10:45 AM–12:33 PM,
Tuesday, January 31, 2017
Room: Washington 3
Sponsoring
Unit:
DNP
Chair: Gordon Cates, University of Virginia
Abstract ID: BAPS.2017.APR.X16.2
Abstract: X16.00002 : Bonner Prize: The Elastic Form Factors of the Nucleon
11:21 AM–11:57 AM
Preview Abstract
Abstract
Author:
Charles F Perdrisat
(Perdrisat)
A series of experiments initiated in 1998 at the then new Continuous
Electron Beam Accelerator, or CEBAF in Newport News Virginia, resulted in
unexpected results, changing significantly our understanding of the structure
of the proton. These experiments used a relatively new technique to obtain
the ratio of the two form factors of the proton, namely polarization. An
intense beam of highly polarized electrons with energy up to 6 GeV was made
to interact elastically with un-polarized protons in a hydrogen target. The
polarization of the recoiling protons, with energies up to 5 GeV, was
measured from a second interaction in a polarimeter consisting of blocs of
graphite or CH2 and tracking wire chambers. The scattered electrons were
detected in an electromagnetic lead-glass calorimeter, to select elastically
scattered events. After a short introduction describing the path which
brought me from the University of Geneva to the College of William and Mary
in 1966, I will introduce the subject of elastic electron scattering,
describe some of the apparatus required for such experiments, and show the
results which were unexpected at the time. These results demonstrated
unequivocally that the two form factors required to describe elastic ep
scattering, electric G$_{\mathrm{E}}$ and magnetic G$_{\mathrm{M}}$ in the
Born approximation, had a drastically different dependence upon the four-momentum squared q$^{\mathrm{2}}= $ q$^{\mathrm{2}}-\omega ^{\mathrm{2}}$ with q the momentum, and $\omega $ the energy transferred
in the reaction. The finding, in flagrant disagreement with the data available
at the time, which had been obtained dominantly from cross section
measurements of the type first used by Nobel Prize R. Hofstadter 60 years
ago, have led to a reexamination of the information provided by form factors
on the structure of the nucleon, in particular its quark-gluon content. The
conclusion will then be a brief outline of several theoretical
considerations to put the results in a proper perspective.
To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2017.APR.X16.2