Sofia Quaglioni

The antiproton is the proton's twin, only with a negative charge. When such a twin approaches a nucleus, it doesn't just bounce off: a whole dance of attraction and repulsion unfolds between them.

An ordinary atomic nucleus is like a tight ball: protons and neutrons are packed closely together. But some light nuclei, such as helium-6, are different. Two neutrons seem to step aside and form a dilute cloud around the nucleus — called a halo.