ABSTRACT:
The ratio of the electric to magnetic proton form factors has
traditionally been determined using the Rosenbluth separation method,
in which the ratio is extracted from the angular dependence of the
cross section at fixed momentum transfer Q ^{ 2 }. Measurements at JLab
using the alternative, polarization transfer technique found a
dramatically different behavior of the ratio compared with the
Rosenbluth results. I discuss the resolution of this discrepancy
by considering the effects of twophoton exchange in elastic ep
scattering, taking particular account of the nucleon's finite size.
Contributions from excited nucleon intermediate states are also
considered, and estimates given of twophoton exchange corrections
to the form factors of the neutron and He3.
