Measurement of the Proton Elastic Form Factor Ratio at Low Q2

We propose to study the proton elastic form factor ratio µGE/GM in the range of Q2 =
0.01 – 0.7 GeV2
. Our goal is to vastly improve the knowledge of the ratio at low Q2
, which,
in combination with separate cross section data, will also allow significant improvements
in knowledge of the individual form factors. In this low Q2
range, substantial deviations of
the ratio from unity have been observed, and data, along with many fits and calculations,
continue to suggest that structures might be present in the individual form factors, and
in the ratio. Beyond the intrinsic interest in the structure of the nucleon, improved form
factor measurements also have implications for deeply virtual Compton scattering, for
determinations of the proton Zemach radius, and for parity violation experiments. The
experiment has two parts which necessarily must run separately. We request 14 days in
Hall A with 80% polarized beam to make recoil polarimetry measurements with the FPP
for the higher Q2 data (0.25 GeV2 < Q2 < 0.7 GeV2
). We request 11 days in Hall A with
80% polarized beam to make polarized beam – polarized target asymmetry measurements,
with the UVa NH3 target and the septum (0.015 GeV2 < Q2 < 0.4 GeV2
). The total
time request is for 25 days in Hall A with 80% polarized beam. The recoil polarimetry
measurements were conditionally approved by PAC31, here we propose an additional set
of measurements (an incremental increase of 11 days) which will extend the approved
range of measurements a factor of 20 lower in Q2
.