The Proton has a Different Size in Different Nuclei

The electron, which is mostly impervious to the nuclear forces, can penetrate deep inside a nucleus. Therefore, scattering high energy electrons from a nucleus is an excellent way of exploring the electric and magnetic properties of the nucleus as a whole and of its constituent protons and neutrons, especially when the electron transfers some of its spin to a proton in a telltale way. For example, recent results from such an experiment, conducted at the Jefferson Lab, gave evidence that the proton is not necessarily spherical. Now a new experiment at Jlab, comparing electrons scattering from single protons (a hydrogen nucleus) with electron scattering from helium nuclei, suggests that each nucleus "kneads" its protons in a different way (see figure). The kneading allows the constituent quarks inside the proton to spread out a bit at time, perhaps into a peanut shape, even though its average shape is round. (Strauch et al., Physical Review Letters, upcoming article)