Light twisted in one direction hits a proton. The particle can absorb the photon or deflect it — depending on how its own spin, a tiny internal top, is oriented. In the experiment at the MAMI accelerator in Mainz, a beam of polarized photons struck targets of polarized protons and deuterons (nuclei of heavy hydrogen). The Crystal Ball/TAPS detector caught everything flying out after the collisions, covering nearly the full solid angle—97%.
The measurements were taken in small energy steps from 200 to 1400 MeV. This revealed fine structure in the response: how absorption probability changes with photon energy. Such detailed data were previously unavailable. Comparison with theoretical calculations showed good agreement, and the difference between deuteron and proton yielded information about the free neutron, which is unstable on its own and lives only about 15 minutes.
The bottom line: the GDH sum rule was confirmed for the proton, neutron, and deuteron. This result is not just a checkmark in a textbook. It is a precise benchmark for calibrating models that describe how quarks and gluons assemble into nucleons and how these nucleons behave inside the atomic nucleus.
A free neutron lives on average about 15 minutes, but inside a deuteron it is stable, and its properties were extracted by subtracting the proton's contribution.
In Robert Wilson's novel Spin, polarized light is used to communicate with an alien intelligence — here it serves as a probe for the tiniest vortices of matter.