A recent experiment in Hall A [4] measured GE/GM by polarization transfer and found that GE/GM decreases with increasing Q2 above Q2=1 GeV2, reaching a value of 0.6 at the highest Q2 measured (see figure 2). This polarization transfer measurement is less prone to systematic issues than the L-T measurements and quotes significantly smaller uncertainties. The new results are clearly inconsistent with a constant value of GE/GM=1, as favored by the global analysis of the L-T measurements. In fact, the Hall A results are consistent with only one of the four L-T measurements with data above Q2=2 GeV2. Additional data is being taken in Hall A that will extend this measurement up to Q2=5.6 GeV2. The new Hall A results not only gives us a different picture for the Q2-dependence of GE/GM, but also brings into question the SLAC L-T measurements. This impacts not only our understanding of the proton electric form factor, but may also impact other L-T measurements if there are a energy-dependent systematic uncertainties that have not been taken into account. An independent measurement of GE/GM in the region where we can achieve uncertainties comparable to or better than the Hall A measurement will be an important as a check on the value of GE/GM, and on the possibility of additional systematic uncertainties in the L-T or polarization transfer measurements. As the polarization transfer technique can be extended to large values of Q2, where the L-T separation becomes increasingly difficult, it is important to have a precise comparison of the two techniques in the region where both can extract GE/GM with high precision.
We propose to measure GE/GM for the proton in a region where the previous determinations differ well outside of experimental uncertainties by utilizing the L-T separation technique in a new way in which only ratios of cross sections are used. Because of this the results are independent of target thickness and beam intensity and quite insensitive to uncertainties in beam energy. Protons rather than the usual electrons will be counted which reduces the variation with scattering angle, reduces the size of the radiative corrections and has the added advantage that the proton energy depends only on Q2. Counting rates are high and a statistical accuracy of less than 0.3% can be achieved in less than a day of data taking at each point. We propose to take one-, two-, three-, and five- pass data with a fixed linac energy and in a total of 10 days determine GE/GM at Q2 = 1.45 GeV2 to 0.02, at 3.20 GeV2 to 0.05 and at 4.90 GeV2 to 0.10.