The Effect of the Mass of the Electron and Neutrino in a Reaction

Tritium is an isotope of hydrogen. One of those neutrons turns into a proton and then emits an electron and a neutrino. And so we can't see the neutrino directly, but we can measure the electron energy very, very precisely. So it's sort of like a way to measure the neutrinos mass by seeing how much energy slurps out from this reaction.