Physicists have taken a step towards nailing down the mass of the neutrino, perhaps the most mysterious of all elementary particles.

The team at the Karlsruhe Tritium Neutrino (KATRIN) experiment in Germany reports that neutrinos have a maximum mass of 0.8 electron volts. Researchers have long had indirect evidence that the particles should be lighter than 1 eV, but this is the first time that this has been shown in a direct measurement. The results were reported on 14 February in Nature Physics.

The previous upper limit of 1.1 eV was reported by KATRIN in 2019. The experiment has so far been able to put only an upper bound on the mass. But researchers say that it might be able to make a definite measurement once it finishes collecting data in 2024, and is the only experiment in the world capable of doing this.

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