Electrons usually scatter from atomic nuclei in ways that we can predict extremely well. But lead has long been an exception. Normally, flipping the spin of the incoming electrons should produce a small but measurable change in the scattering, and for most nuclei this prediction agrees beautifully with experiment. Only lead refused to behave as expected, leaving a long-standing experimental inconsistency that no existing calculation could fully clarify.

In a new measurement performed at the Mainz Microtron MAMI, researchers of the CRC studied the same process at a different beam energy and scattering angle. This time, the effect is clearly not zero. Instead of resolving the earlier mystery, the result shows that the observable changes strongly with energy in a way not captured by current theoretical descriptions.

The work, now published in Physical Review Letters, deepens this long-standing puzzle and highlights the need for new theoretical approaches and further precision measurements—an area central to the CRC’s scientific program.