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Beam-Normal Single-Spin Asymmetry in Pb208 at Low Energy: Discrepancy Resolved or New Kinematic Puzzle?

A. Esser1, N. Kozyrev1, K. Aulenbacher1,2,3, S. Baunack1, M. Dehn1, A. Del Vincio1,4, L. Doria1, M. Hoek1, F. Keil1 et al.

F. Maas1,2,3, H. Merkel1,3, M. Mihovilovič5,6, U. Müller1, J. Pochodzalla1,2, B. S. Schlimme1, T. Shao1, S. Stengel1, M. Thiel1, L. Wilhelm1, and C. Sfienti1,2,3

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Phys. Rev. Lett. 135, 232502 – Published 2 December, 2025

DOI: https://doi.org/10.1103/fd61-xxk6

Abstract

A long-standing discrepancy between measured and predicted beam-normal single-spin asymmetries An in elastic electron scattering off Pb208 has challenged our understanding of two-photon exchange (TPE) in heavy nuclei. We report a new measurement at 570 MeV and Q2=0.04  GeV2/c2, yielding An=[−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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Phys. Rev. Lett. 135, 232502– Published 2 December, 2025
  • Received 26 August 2025
  • Revised 2 November 2025
  • Accepted 17 November 2025
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