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Beam-Normal Single-Spin Asymmetry in ^{208}Pb at Low Energy: Discrepancy Resolved or New Kinematic Puzzle?
A Esser1, N Kozyrev1, K Aulenbacher1,2,3
1Johannes Gutenberg-Universität Mainz, Institut für Kernphysik, D-55099 Mainz, Germany.
Physical Review Letters
|December 19, 2025
Summary
A new measurement of beam-normal single-spin asymmetries in electron scattering off lead-208 nuclei reveals a nonzero value. This finding challenges current theories of two-photon exchange effects in heavy nuclei.
Area of Science:
- Nuclear Physics
- Particle Physics
- Quantum Electrodynamics
Background:
- A persistent discrepancy exists between measured and predicted beam-normal single-spin asymmetries (A_n) in elastic electron scattering off ^{208}Pb.
- This discrepancy challenges the understanding of two-photon exchange (TPE) contributions in heavy nuclei.
Purpose of the Study:
- To report a new measurement of A_n in elastic electron scattering off ^{208}Pb at 570 MeV and Q^2 = 0.04 GeV^2/c^2.
- To investigate the kinematic dependence of TPE effects.
Main Methods:
- Elastic electron scattering experiment.
- Measurement of beam-normal single-spin asymmetries (A_n).
Main Results:
- A nonzero value for A_n was measured: [-9.1 ± 2.1 (stat) ± 0.7 (syst)] ppm.
- The measured A_n at 570 MeV differs from previous results at higher energies.
Conclusions:
- The nonzero A_n suggests a kinematic dependence of TPE effects not currently explained by theory.
- The results necessitate a reevaluation of previous interpretations of TPE in heavy nuclei.
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