Absolute Neutrino Mass Bound Tightened

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 that bound to 0.2 eV. This would push the limit on the sum of the three neutrino masses to 0.66 eV in a normal hierarchy and 0.71 eV in an inverted hierarchy. 

This is about six times less tight a bound on the neutrino masses than the cosmology based neutrino mass bounds, which approach 0.12 eV or less for the sum of the three neutrino masses, but this is a much less model dependent limit than cosmology based limit.

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).

The best fit value for the electron neutrino mass is slightly below zero, when in reality, it can't have a value of less than zero. So, the probability density is heavily concentrated around a value indistinguishable from zero (a bit more than 74% in a Gaussian probability distribution which clearly isn't the true appropriate probability distribution).

So, while the limits of the experiment probably can't rule out of mass of more than 0.2 eV at a 90% confidence interval with a full run of KATRIN experiment, the best fit value after the full run is still likely to be indistinguishable or almost indistinguishable from zero. 

This would imply a best fit value for the sum of the three neutrino masses of about 0.06 eV in a normal hierarchy and 0.11 eV in an inverted hierarchy. And, almost every available observational dataset favors a normal hierarchy over an inverted hierarchy, although not truly decisively.