Heavy-element mystery grows as lead defies decades of predictions

Recent experiments have deepened a longstanding mystery in nuclear physics involving lead, one of the heaviest stable elements. Traditionally, when electrons scatter off atomic nuclei, small spin-dependent effects caused by two-photon exchange processes are well predicted by theory and confirmed by experiments. However, earlier studies at Jefferson Lab found that this effect seemed to vanish entirely in lead, defying all theoretical expectations. A new high-precision experiment at Johannes Gutenberg University Mainz (JGU), using the Mainz Microtron (MAMI) accelerator, has confirmed that lead’s behavior is indeed anomalous but revealed an even more complex picture: the spin-dependent scattering effect does not disappear but instead varies dramatically with electron beam energy and scattering angle, showing a much larger and unexpected magnitude. This discovery highlights a significant gap in current nuclear theory, as no existing models can explain why lead’s spin-dependent scattering changes so abruptly with energy, unlike the smooth behavior observed in lighter nuclei. The findings underscore the need for new theoretical frameworks to understand electron interactions with