THECONTINUOUSELECTRONBEAMACCELERATORFACILITY: CEBAF at the Jefferson Laboratory

The ratio of the proton's elastic electromagnetic form factors was obtained by measuring the transverse and longitudinal polarizations of recoiling protons from the elastic scattering of polarized electrons with unpolarized protons. The ratio of the electric to magnetic form factor is proportional to the ratio of the transverse to longitudinal recoil polarizations. The ratio was measured over a range of four-momentum transfer squared between 0.5 and 3.5 GeV-squared. Simultaneous measurement of transverse and longitudinal polarizations in a polarimeter provides good control of the systematic uncertainty. The results for the ratio of the proton's electric to magnetic form factors show a systematic decrease with increasing four momentum squared, indicating for the first time a marked difference in the spatial distribution of charge and magnetization currents in the proton.