Probing the Neutron Skin Puzzle in \(²⁰⁸Pb\): Precision Polarized Electron Scattering at Mainz

The determination of the neutron skin thickness of \(²⁰⁸Pb\) from the PREX-II measurement of the parity-violating asymmetry \(APV\) has generated considerable discussion within the nuclear physics community, revealing notable discrepancies with previous experimental results and theoretical predictions. PREX also reported a beam-normal single-spin asymmetry \(A_n\) for \(²⁰⁸Pb\), a key background to \(APV\), that deviates from contemporary calculations — a discrepancy now known as the PREX anomaly. We present a new measurement of \(A_n\) in elastic electron scattering from natural lead performed at MAMI and discuss its implications for solving the PREX anomaly. We also outline the concept of the forthcoming Mainz Radius EXperiment (MREX) at MESA, designed to deliver an independent measurement of \(APV\) in \(²⁰⁸Pb\) with twice the precision of PREX-II. Abstract Published by the Jagiellonian University 2026 authors