Abstract for JLAB Technical Note 01-046  

The need for Hall A HRS angle measurements on the level of 10-4 rad (or better) was evident from the time of construction of the facility. The present technique used for HRS angle measurement relies heavily on optical surveys whose geometrical accuracy is limited to 100-200 mu-m. The proposed technique will result in a geometrical accuracy on the level of 5-10 mu-m. The physics motivation for the construction of such a device is clear. A unique set of L/T (Rosenbluth) separation experiments has been proposed whose systematic uncertainties are dominated by the knowledge of the absolute HRS angle. For example, in an L/T separation performed on a proton target at Q2=5(GeV/c)2, the value of GpE/GpM changes by a full 20% for a change in recoil proton angle of 0.5 mrad. A precision study of the Coulomb sum rule also requires excellent understanding of the L/T separation systematics which depend on knowledge of the HRS angle. The precision of the proposed instrument completely solves this problem for all existing proposals, and even for investigations of two-photon effects in electron scattering through careful measurement of the angular dependence of the cross section.

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