Light, lighter, neutrinos: Putting a ghost on a scale

The KATRIN experiment has established a new, unsurpassed upper limit of 8⋅10^–37 (equivalent to an energy of 0.45 eV) for the neutrino mass.  It was determined directly and independently of any model using electron spectroscopy from tritium beta decay. The data-taking phase was completed in October 2025 after a total of 1000 measurement days. Analysis of this data is expected to yield a further 1.5-fold increase in sensitivity. In 2026, the commissioning of the new TRISTAN detector within the KATRIN beamline will extend the experiment towards the search for sterile neutrinos, a candidate for dark matter.