Abstract
The proton elastic form factor ratio is accessible in unpolarized Rosenbluth-type experiments as well as experiments which make use of polarization degrees of freedom. The extracted values show a distinct discrepancy, growing with \(Q^2\). Three recent experiments tested the proposed explanation, two-photon exchange, by measuring the positron–proton to electron–proton cross section ratio. In the results, a hard two-photon exchange effect at the couple-of-percent level is visible, significantly different from theoretical calculation. Theory at larger momentum transfer remains untested. This paper discusses the possibilities for future measurements at larger momentum transfer.
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This work was supported by the Office of Nuclear Physics of the U.S. Department of Energy, Grant No. DE-FG02-94ER40818.
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This article belongs to the Topical Collection “NSTAR 2017—The International Workshop on the Physics of Excited Nucleons”.
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Bernauer, J.C. Two-Photon Exchange: Future Experimental Prospects. Few-Body Syst 59, 116 (2018). https://doi.org/10.1007/s00601-018-1439-y
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DOI: https://doi.org/10.1007/s00601-018-1439-y