Two-Photon Exchange: Future Experimental Prospects

• Published in: Few-body systems Vol. 59; no. 6; pp. 1 - 5
• Main Author: Bernauer, Jan C.
• Format: Journal Article
• Language: English
• Published: Vienna Springer Vienna 01-11-2018; Springer Nature B.V; Springer
• Subjects: Atomic; Exchanging; Experiments; Form factors; Hadrons; Heavy Ions; Molecular; Momentum transfer; NSTAR 2017; Nuclear Physics; Optical and Plasma Physics; Particle and Nuclear Physics; Physics; Physics and Astronomy; Protons
• ISSN: 0177-7963; 1432-5411
• DOI: 10.1007/s00601-018-1439-y

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.