Researchers associated with the KArlsruhe TRItium Neutrino (KATRIN) investigation have achieved the most precise determination to date of the upper bound for the neutrino’s mass, establishing it at 0.45 electron volts (eV) – a magnitude less than one-millionth of an electron’s mass.
Interior view of the KATRIN main spectrometer. Image credit: M. Zacher / KATRIN Collaboration.
Neutrinos represent the most ubiquitous subatomic particles in the cosmos, existing in three distinct forms or “flavors”: the electron neutrino, the muon neutrino, and the tau neutrino.
These flavors exhibit a phenomenon known as oscillation, whereby a solitary neutrino can transmute into any of these types as it propagates through space. This behavior offers substantial empirical support for the notion that neutrinos possess mass, a finding that diverges from the Standard Model’s initial premise of massless neutrinos.
Nevertheless, their precise mass remains an enduring enigma within the realm of particle physics.
In a recently published scientific paper, scientists constituting the KATRIN Collaboration have unveiled the outcomes derived from the first five measurement cycles of the KATRIN experiment.
The experiment’s methodology, as elucidated by the researchers, involves “determining the neutrino’s mass by scrutinizing the beta decay process of tritium.”
It was further elaborated that “during this decay event, a neutron undergoes a transformation into a proton, simultaneously releasing an electron and an electron antineutrino – the latter being the antiparticle counterpart to the neutrino.”
“By meticulously analyzing the distribution of total energy released during the decay between the emitted electron and the electron antineutrino, the mass of the neutrino can be accurately inferred.”
Spanning a period of 259 days between the years 2019 and 2021, the KATRIN physicists meticulously measured the energy profiles of roughly 36 million electrons, constituting a dataset six times more extensive than those from prior experimental runs.
These cumulative findings have established the most rigorous laboratory-derived upper limit for the effective electron neutrino mass, constraining it to less than 0.45 eV at a 90% confidence interval.
This significant outcome signifies the third successive enhancement in refining the neutrino mass limit, demonstrating a twofold improvement over the preceding benchmark limit.
Dr. Kathrin Valerius, who serves as one of the two co-spokespersons for the KATRIN experiment and is also a physicist at the Karlsruhe Institute of Technology, stated, “For this particular result, we have processed data from five measurement campaigns, accumulating approximately 250 days of data collection between 2019 and 2021 – representing roughly a quarter of the total data anticipated from the entirety of the KATRIN project.”
Dr. Susanne Mertens, a physicist affiliated with the Max Planck Institute for Nuclear Physics and the Technical University Munich, remarked, “With the progression of each campaign, we have consistently gained novel insights and have further refined the operational parameters of the experimental setup.”
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Max Aker et al. (KATRIN Collaboration). 2025. Direct neutrino-mass measurement based on 259 days of KATRIN data. Science 388 (6743): 180-185; doi: 10.1126/science.adq9592
