Masses of isotopes of hydrogen and helium

The atomic masses of the proton, deuteron, triton, helion and alpha-particle are considered to be fundamental constants. More specifically, high precision measurements are motivated by recent rovibrational spectroscopy of hydrogen molecular ions, the tritium-beta-decay neutrino mass experiments KATRIN and PROJECT-8, and measurements of the g-factors of ³He⁺ and ⁴He⁺. Combining atomic masses with QED theory for hydrogen molecular ion rovibrational transitions and helium ion g-factors enables searches for physics beyond the Standard Model and new values for electron atomic mass. The measurements are carried out using pairs of ions, in some cases molecular ions, in a cryogenic Penning trap. Significant discrepancies with some earlier precise measurements have been found.