Beam-Normal Single-Spin Asymmetry in $^{208}$Pb at low energy: discrepancy resolved or new kinematic puzzle?

A. Esser; N. Kozyrev; K. Aulenbacher; S. Baunack; M. Dehn; A. Del Vincio; L. Doria; M. Hoek; F. Keil; F. Maas; H. Merkel; M. Mihovilovic; U. Müller; J. Pochodzalla; B. S. Schlimme; T. Shao; S. Stengel; M. Thiel; L. Wilhelm; C. Sfienti

arXiv · 2508.18851

Beam-Normal Single-Spin Asymmetry in $^{208}$Pb at low energy: discrepancy resolved or new kinematic puzzle?

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Abstract

A longstanding 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$A_n = (-9.1 \pm 2.1~\text{(stat)} \pm 0.7~\text{(syst)})~\mathrm{ppm}$. 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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A. Esser, N. Kozyrev, K. Aulenbacher, S. Baunack, M. Dehn, A. Del Vincio, L. Doria, M. Hoek, F. Keil, F. Maas, H. Merkel, M. Mihovilovic, U. Müller, J. Pochodzalla, B. S. Schlimme, T. Shao, S. Stengel, M. Thiel, L. Wilhelm, C. Sfienti. 2025-08-26. Beam-Normal Single-Spin Asymmetry in $^{208}$Pb at low energy: discrepancy resolved or new kinematic puzzle?. https://arxiv.org/abs/2508.18851

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