A theoretical preprint says combined electron and positron measurements at laboratory energies of about 500 MeV or below could provide new information about the long-distance dynamics of protons and neutrons. It lays out three proposed areas: low-momentum-transfer elastic form factors, asymmetries sensitive to two-photon exchange, and virtual Compton-scattering measurements aimed at generalized polarizabilities.
The main idea is to reverse the charge of the incoming beam and compare the results. In elastic scattering, the preprint presents that comparison as a way to isolate and quantify two-photon-exchange, or TPE, corrections. Its form-factor discussion identifies TPE as the dominant theoretical uncertainty near the upper end of the accessible momentum-transfer range.
The findings are theoretical predictions and proposals, not observations from a completed experiment. The note reports no measured sample or data-based uncertainty interval. Its author recommends further simulations to assess the feasibility and likely precision of the proposed measurements as part of planning for a future JLab positron facility.
The charge of the beam is the key comparison
The first part of the program focuses on low-Q² elastic form factors. In the paper’s framework, these calculations are connected to questions about nucleon structure, including how the scattering cross section changes and how sensitive that change is to the proton radius. The proposed electron–positron comparison would give the analysis an additional charge-dependent contrast.
The calculations describe different beam-momentum and momentum-transfer dependence for TPE corrections in electron–proton and muon–proton scattering. They also describe TPE corrections as relatively larger, compared with radius-dependent cross-section variation, in electron–proton scattering than in muon–proton scattering, particularly at low beam momentum.
That is why the note treats opposite beam charges as useful. If the underlying form-factor contribution is viewed through both electron and positron scattering, the charge reversal is proposed as a way to quantify the TPE correction. The document does not report that this strategy has already been tested with data.
A small asymmetry could provide a sharper test
A second part of the note examines a target-normal single-spin asymmetry, written as A_N. The one-photon approximation gives zero for this quantity in the stated treatment. It appears through interference between one- and two-photon amplitudes and depends on the imaginary part of the TPE amplitude, making it a particularly direct TPE-sensitive observable in the proposed program.
The calculated A_N values are on the order of a few 10−3 and vary substantially with the scattering conditions. The note states that a pure TPE contribution should change sign when the beam changes from an electron to a positron: A_N(e+N) = −A_N(e−N). Positron measurements are proposed as a test of that charge-odd relation and of the effective-field-theory calculations behind it.
The note also discusses a beam-normal single-spin asymmetry, B_N, estimated at about 10−5 in the low-energy region under discussion. Positron measurements of that signal would be challenging because they require a transversely polarized positron beam.
Virtual Compton scattering offers another route
The third proposed area concerns virtual Compton scattering, or VCS, and its beam-charge asymmetry, or BCA. The preprint presents the VCS BCA as a way to access the real VCS amplitude by comparing the two beam charges.
The note says that information would complement unpolarized VCS measurements. It could help constrain subtraction constants in dispersion-relation calculations or a local γγNN coupling in an effective field theory, improving the extraction of generalized polarizabilities. No new BCA measurement is reported.
The broader framework combines chiral effective field theory with the 1/N_c expansion of quantum chromodynamics, or with a related small-scale expansion. It also treats complex analyticity, dispersion relations and lattice-QCD results as compatible tools or possible sources of dynamical input.
A research plan, not a finished measurement
The modeled targets are proton and neutron nucleons, and the calculations consider elastic, inclusive and virtual Compton-scattering configurations. Scattering on nuclei, including efforts to extract neutron structure from nuclear targets, is outside the stated scope.
Because the document is a theory note, it includes no recruited participants, experimental sample or statistical hypothesis test. Its predictions rely on effective-field-theory, dispersion-relation, form-factor and TPE calculations; the supplied analysis says that theoretical uncertainty is discussed but not fully quantified. Future simulations and direct measurements would be needed to establish achievable precision, test the proposed charge-reversal relation and determine how strongly BCA data could constrain generalized polarizabilities.
The document is arXiv version 1 in the hep-ph category, dated 20 August 2026. It reports support from the U.S. Department of Energy’s Office of Science, Office of Nuclear Physics, under Contract No. 89243126CSC000213.
Paper data and sources
Original title: Exploring chiral dynamics with low-energy electron- and positron-nucleon scattering
Authors: C. Weiss
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-20
DOI: Not available
Original paper · Full text