Measurements of the electromagnetic form factor of the Proton at JLab
Vina Punjabi, Charles Perdrisat, Jefferson Lab Hall A Collaboration, GEp-III Collaboration
Abstract
The ratio of the proton's elastic electromagnetic form factors
G
Ep
/
G
Mp
was obtained by measuring
P
t
and
P
ℓ
, the transverse and longitudinal recoil proton polarization, respectively. For the elastic reaction
e
⃗
p→e
p
⃗
,
G
Ep
/
G
Mp
is proportional to
P
t
/
P
ℓ
. The simultaneous measurement of
P
t
and
P
ℓ
in a polarimeter reduces systematic uncertainties. The results for the ratio
G
Ep
/
G
Mp
measured in Hall A so far show a systematic decrease with increasing
Q
2
, indicating for the first time a definite difference in the distribution of charge and magnetization in the proton. Together these experiments cover the Q
2
- range of 0.5 to 5.6 GeV
2
. A new experiment is currently being prepared, to extend the Q
2
-range to 9 GeV
2
in Hall C.