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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.
Abstract:
A long-standing discrepancy between measured and predicted beam-normal single-spin asymmetries A_{n} in elastic electron scattering off ^{208}Pb has challenged our understanding of two-photon exchange (TPE) in heavy nuclei. We report a new measurement at 570 MeV and Q^{2}=0.04 GeV^{2}/c^{2}, yielding A_{n}=[-9.1±2.1(stat)±0.7(syst)] ppm. This nonzero value contrasts with previous results at higher energies and suggests a kinematic dependence of TPE effects not captured by current theory, prompting a reevaluation of earlier interpretations.
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