The old KATRIN bound on the lightest neutrino mass was 0.8 eV. Now it is down to 0.45 eV. This pushes the limit on the sum of the three neutrino masses to 1.41 eV in a normal hierarchy and 1.46 eV in an inverted hierarchy. After a full run of data collection, KATRIN is expected to lower than bound to 0.2 eV.
The fact that neutrinos carry a non-vanishing rest mass is evidence of physics beyond the Standard Model of elementary particles. Their absolute mass bears important relevance from particle physics to cosmology. In this work, we report on the search for the effective electron antineutrino mass with the KATRIN experiment. KATRIN performs precision spectroscopy of the tritium β-decay close to the kinematic endpoint. Based on the first five neutrino-mass measurement campaigns, we derive a best-fit value of m^2(ν)=−0.14+0.13−0.15 eV^2, resulting in an upper limit of mν < 0.45 eV at 90 % confidence level. With six times the statistics of previous data sets, amounting to 36 million electrons collected in 259 measurement days, a substantial reduction of the background level and improved systematic uncertainties, this result tightens KATRIN's previous bound by a factor of almost two.
M. Aker, et al., "Direct neutrino-mass measurement based on 259 days of KATRIN data" arXiv:2406.13516 (June 19, 2024).
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