End of the fourth neutrino species? New KATRIN data severely limits the possibility of sterile neutrinos

The current results of the KATRIN experiment at the Karlsruhe Institute of Technology specify the known limits for hypothetical sterile neutrinos and significantly reduce the parameter space for a fourth type of neutrino. The study is based on the evaluation of around 36 million electron spectra recorded during 259 days of measurements. The analysis was published in Nature. According to the research groups involved, no evidence of an additional neutrino flavor could be identified. The new data thus tightens the existing limits for models that assume such a particle type.

Neutrinos occur in three known flavors, which can transform into each other during propagation. Over the past decades, various experiments have registered deviations in measurement data that could not be fully explained by the established three-flavor model. These included deviations in reactor neutrino experiments and gallium experiments. In contrast, other facilities such as the IceCube detector and the STEREO experiment reported consistent results without recognizable anomalies. The question of a possible fourth, non-interacting neutrino species therefore remained open.

The KATRIN experiment was designed for an extremely precise determination of the neutrino mass. The basis for this is the high-resolution measurement of the electrons produced during tritium beta decay. The mass of the associated neutrinos can be indirectly deduced from the energy spectrum. However, the same measurement method makes it possible to search for characteristic spectral deviations that could indicate sterile neutrinos. An additional spectral kink and a slight deformation of the overall spectrum would be expected. Despite high precision and low background noise, none of the analyzed spectra showed such a signature.

The measurement data yielded exclusion limits for sterile neutrinos in the range of approximately one electron volt to several hundred electron volts. Several of the anomalies discussed in other experiments lay precisely in this range. The new results contradict these observations and therefore considerably reduce the parameter space for models with light sterile neutrinos. The researchers involved speak of a significantly reduced probability for the existence of a fourth neutrino species in this mass range.

KATRIN’s data collection will continue until 2025 and is expected to include over 220 million electron spectra in the relevant energy range by then. A significant increase in statistical significance is expected. From 2026, an additional detector is planned that will extend the sensitivity to larger neutrino masses. Such heavier hypothetical neutrinos in the kiloelectronvolt range are discussed in various cosmological models as candidates for dark matter. The extended measurements could therefore provide additional constraints or new evidence.

Conclusion

The current results of the KATRIN experiment severely restrict the parameter space for sterile neutrinos. Models with light sterile neutrinos lose plausibility, while future measurement phases will become relevant above all for the range of higher hypothetical neutrino masses. The question of a fourth type of neutrino remains scientifically open, but appears much less likely at the present time.