The subtle secrets of exotic helium

Researchers in the Atomic Spectroscopy and Collisions using Cold Antiprotons (ASACUSA) collaboration have observed hyperfine structure in an exotic form of Helium. Hyperfine structure corresponds to the small shifts in the electron energy levels caused by the magnetic interaction between the electrons and the nucleus. The work also provides a precise test of charge-parity-time (CPT) symmetry. The measured transition frequencies were in excellent agreement with recent theoretical calculations, which predicted quadruplet hyperfine states caused by the magnetic moment states.