The KATRIN experiment studies tritium, a rare form of hydrogen that decays radioactively, emitting an electron and an antimatter mirror image of the neutrino, an antineutrino. Measuring the energies of the electrons can reveal the masses of the antineutrinos that flitted away. That’s because mass and energy are two sides of the same coin; a more massive neutrino would mean less energy could go to the electron in the decay