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The proton elastic form factor ratio can be measured either via Rosenbluthseparation in an unpolarized beam and target experiment, or via the use ofpolarization degrees of freedom. However, data produced by these two approachesshow a discrepancy, increasing with $Q^2$. The proposed explanation of thisdiscrepancy---two-photon exchange---has been tested recently by threeexperiments. The results support the existence of a small two-photon exchangeeffect but cannot establish that theoretical treatment at the measured momentumtransfers are valid. At larger momentum transfers, theory remains untested.This paper investigates the possibilities of measurements at DESY and JeffersonLab to measure the effect at larger momentum transfers.

Positron Beams and Two-Photon Exchange: The Key to Precision Form Factors