Direct Probes of Neutrino Mass

For 90 years the shape of beta spectra has been our clearest window on the most mysterious property of the neutrinos, their mass. The discovery of neutrino oscillations gave proof that neutrinos have mass, in explicit contradiction to the minimal standard model. But how much mass? Oscillations cannot give a number for the mass, other than that the sum of the three masses must be at least 0.05 eV. Precision cosmology is increasingly sensitive to the sum, and as difficulties with the ΛCDM model become more apparent, a laboratory determination is ever more valuable. Sterile neutrinos may exist and can be found experimentally if they mix with the active ones. KATRIN, the first new laboratory experiment on the beta spectrum of tritium in more than 20 years, has now shown the mass to be no greater than 0.8 eV and continues toward its sensitivity limit. Calorimetric experiments on ¹⁶³Ho will soon probe the eV scale. A different approach called “Project 8” has passed proof-of-concept tests with a scheme that may have unprecedented sensitivity.