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The effects of multi-photon-exchange and other higher-order QED correctionson elastic electron-proton scattering have been a subject of high experimentaland theoretical interest since the polarization transfer measurements of theproton electromagnetic form factor ratio $G_E^p/G_M^p$ at large momentumtransfer $Q^2$ conclusively established the strong decrease of this ratio with$Q^2$ for $Q^2 \gtrsim 1$ GeV$^2$. This result is incompatible with previousextractions of this quantity from cross section measurements using theRosenbluth Separation technique. Much experimental attention has been focusedon extracting the two-photon exchange (TPE) effect through the unpolarized$e^+p/e^-p$ cross section ratio, but polarization transfer in polarized elasticscattering can also reveal evidence of hard two-photon exchange. Furthermore,it has a different sensitivity to the generalized TPE form factors, meaningthat measurements provide new information that cannot be gleaned fromunpolarized scattering alone. Both $\epsilon$-dependence of polarizationtransfer at fixed $Q^2$, and deviations between electron-proton andpositron-proton scattering are key signatures of hard TPE. A polarized positronbeam at Jefferson Lab would present a unique opportunity to make the firstmeasurement of positron polarization transfer, and comparison withelectron-scattering data would place valuable constraints on hard TPE. Here, wepropose a measurement program in Hall A that combines the Super BigBiteSpectrometer for measuring recoil proton polarization, with a non-magneticcalorimetric detector for triggering on elastically scattered positrons. Thoughthe reduced beam current of the positron beam will restrict the kinematicreach, this measurement will have very small systematic uncertainties, makingit a clean probe of TPE.

Polarization transfer in $\vec{e}^+p \rightarrow e^+ \vec{p}$ scattering using the Super BigBite Spectrometer