Direct neutrino-mass measurement with sub-electronvolt sensitivity

Since the discovery of neutrino oscillations, we know that neutrinos have non-zero mass. However, the absolute neutrino-mass scale remains unknown. Here we report the upper limits on the effective electron antineutrino mass mν from the second physics run of the Karlsruhe Tritium Neutrino (KATRIN) experiment. In this experiment, mν is probed via a high-precision measurement of the tritium β-decay spectrum close to its endpoint. This method is independent of any cosmological model and does not rely on assumptions whether the neutrino is a Dirac or Majorana particle. By increasing the source activity and reducing the background with respect to the first physics campaign, we reached a sensitivity on mν of 0.7 eV at a 90% confidence level (CL). The best fit to the spectral data yields mν² = (0.26 ± 0.34) eV², resulting in an upper limit of mν < 0.9 eV at 90% CL. By combining this result with the first neutrino-mass campaign, we find an upper limit of mν < 0.8 eV at 90% CL.

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