Abstract
The elastic form factors of the nucleon characterize the distributions of charge and magnetization in momentum space and are important input for calculations of strong interaction phenomena and nuclear structure. The dramatic discrepancy in the observed ratio of elastic proton form factors between the Rosenbluth separation and polarization transfer methods has invoked numerous theoretical and experimental investigations. The previously neglected effect from two-photon exchange has become the favored explana- tion for the discrepancy. While the effect can not be calculated from first principles, it can be verified experimentally in several ways, most stringently by comparing the positron- proton and electron-proton elastic cross sections. The OLYMPUS experiment at DESY has been carried out to quantify the effect of two-photon exchange using intense stored positron and electron beams along with an internal unpolarized hydrogen target and a large acceptance detector to measure the ratio of the positron-proton and electron-proton elastic scattering cross sections. The status of proton form factor measurements and of the experimental efforts to verify the effect of two-photon exchange is presented, with some emphasis on the OLYMPUS experiment.
Highlights
Elastic electromagnetic form factors of the proton and neutron are fundamental quantities characterizing the distributions of charge and magnetization of the nucleon
While the imaginary parts give rise to small single-spin asymmetries, which can be measured with transversely polarized electron beam [21], transversely polarized target, or induced transverse recoil polarization, only the real parts of the two-photon exchange amplitude are relevant for proton form factor extractions
The real parts of the two-photon exchange amplitudes can affect the linearity of the ε-dependence of the Rosenbluth cross sections [22], introduce a ε- dependence of the form factor ratio [23], or generate an e+ p/e− p cross section asymmetry [24,25,26]
Summary
Elastic electromagnetic form factors of the proton and neutron are fundamental quantities characterizing the distributions of charge and magnetization of the nucleon. Right: Proton electric to magnetic form factor ratio from Rosenbluth-separated cross sections (black symbols) [2] and from double polarization experiments (colored symbols) [6, 7].
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