Neutrinos pinned below 0.45 eV; KATRIN halves the particle’s mass ceiling

The Karlsruhe Tritium Neutrino (KATRIN) experiment has lowered the upper limit on a neutrino’s mass to 0.45 electron-volts, roughly half its own 2022 record and more than a million-fold lighter than an electron. The result, published April 11 in Science (DOI: 10.1126/science.adq9592), tightens the screws on cosmological models that rely on neutrino mass to explain dark-matter signatures.

Highlighted in Nature, the research drew from 259 days of tritium-decay data and approximately 36 million electrons, pushing the direct, model-independent upper limit on the electron-neutrino mass down to 0.45 eV (90% C.L.), an almost two-fold gain on its own 2022 bound of 0.8 eV. The result rests on a high-resolution MAC-E filter that tracks the endpoint of the β-electron spectrum from gaseous molecular tritium. At 0.45 eV, the heaviest a neutrino could be is 1.1 × 10⁶ times lighter than an electron’s 511 keV rest-mass energy. Such tight constraints narrow the parameter space for cosmological models that invoke neutrino mass to reconcile structure-formation data with the cosmic microwave background.

KATRIN’s milestone crowns three decades of steady progress in laboratory β-spectroscopy: Mainz (2005) capped the mass at 2.3 eV, Troitsk (2013) at 2.05 eV, KATRIN’s first physics run (2019) at 1.1 eV, and its 2022 campaign at 0.8 eV. Each step has tightened the limit by roughly a factor of two, and the current value represents a five-fold improvement since KATRIN turned on. With roughly 1,000 days of total data still to analyse, the team projects sensitivity down to 0.3 eV, and perhaps 0.2 eV, before successor concepts such as KATRIN++ take over. That trajectory keeps laboratory searches competitive with cosmological fits and neutrinoless-double-beta studies, sustaining pressure on theories that aim to explain how neutrinos acquire mass at all.