The revival of two old ways to measure the electron-neutrino mass

About

Three decades ago, the measurement of the electron neutrino mass(es) in atomic electron capture (EC) decays was scrutinized. For certain isotopes an atomic resonance enormously enhances the expected EC rates. The favoured EC technique --based on calorimeters as opposed to spectrometers-- has the advantage of eliminating all the "atomic and molecular" data-analysis problems besieging the traditional (spectrometric) Tritium beta-decay approach. Yet, after an initial surge of measurements, the EC method did not seem to be competitive with Tritium decay. But lately there has been stunning progress on micro-calorimeters and on the measurement of atomic mass differences. Meanwhile, the Tritium-decay neutrino-mass limits have improved by a factor of 15, and the difficulty of the experiments by the cube of that figure. Can the "calorimetric" EC theory cope with this increased challenge? I shall answer this question affirmatively. I shall also broach the ongoing beta-decay and EC experiments and report on progress in neutrino-less double EC, the second way cited in the title, relevant for neutrinos with Majorana masses..

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