New result with the A1 spectrometers at the MAMI accelerator reveals unexpected behaviour in heavy nuclei and opens a path toward precision tests at the future MESA facility

Why does lead behave so differently from every other atomic nucleus when struck by electrons? A team of physicists at Johannes Gutenberg University Mainz (JGU) has taken an important step toward answering this question, only to find that the mystery is even deeper than previously thought. The findings were published in the prestigious scientific journal Physical Review Letters.

Electrons usually scatter from atomic nuclei in ways that can be predicted with remarkable accuracy. One well-tested feature is that flipping the spin of the incoming electrons should slightly change the scattering pattern, an effect driven by the exchange of two “virtual photons” between the electron and the nucleus. For most nuclei, theory predicts exactly how large this tiny effect should be, and decades of experiments have confirmed those predictions. Lead, however, has always stood out. Earlier measurements performed at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility showed that, for lead, this spin-dependent effect seemed to vanish entirely, a result that no existing theory could explain.

The experiment at the Mainz Microtron

In a new experiment conducted with the high-resolution A1 spectrometers at the Mainz Microtron (MAMI), the JGU team measured the same process at a different beam energy and scattering angle. This time, the effect was clearly present and surprisingly large. Instead of resolving the earlier anomaly, the new measurement intensifies it: the behaviour of the lead nucleus changes drastically with energy in a way that current theory does not capture.

“This result confirms that the puzzle is real,” says Professor Dr. Concettina Sfienti, who heads the project. “It means there is unexplored physics in how electrons probe heavy nuclei, and we need new theoretical ideas to understand it.”

The work was carried out in the Collaborative Research Center (CRC) 1660 “Hadrons and Nuclei as Discovery Tools”, funded by the German Research Foundation (DFG). A core mission of CRC 1660 is to use precision experiments to uncover subtle effects in nuclear structure that could open new windows into the Standard Model of particle physics. The unexpected behaviour of lead is now emerging as one of the CRC’s most intriguing cases, a striking example of how high-precision measurements can reveal gaps even in well-established theory.

Significant implications for future experiments at MESA

The findings also carry strong implications for the future P2 experiment at the new MESA accelerator, currently being built on the Mainz campus as part of the PRISMA⁺⁺ Cluster of Excellence. At MESA, researchers will measure extremely small effects in electron scattering to test the Standard Model with unprecedented accuracy. Understanding the role of two-photon exchange in heavy nuclei – such as the surprising behaviour now seen in lead – is essential for achieving the precision needed at P2. “With this new result from MAMI, we gain a much clearer sense of what needs to be understood before we push to the next level of precision at MESA,” Sfienti explains. “What we measure today directly shapes the roadmap for the high-precision physics of tomorrow.”