P. Achenbach

A beam of electrons, twisted like a corkscrew, slams into a proton. From the collision point, pairs of particles, a positive and a neutral pion, fly out. For the first time, physicists measured how often such pairs scatter at different angles when the electron's spin points left or right.

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).

A neutron confined in a nucleus behaves differently from a free one. Its gluon cloud, which holds quarks together, compresses like a spring under load. Previously, this could only be assumed, but now physicists have directly observed the effect for the first time. At Jefferson Lab, an electron beam struck liquid deuterium, knocking photons out of the nuclei.