An experiment in Germany has made the best measurement yet of the maximum mass of neutrinos — light subatomic particles that are so devilishly difficult to measure that physicists have only been able to estimate the upper limit of their mass.

The first results ¹ from the Karlsruhe Tritium Neutrino (KATRIN) experiment in southwestern Germany find that neutrinos weigh at most 1.1 electronvolts (eV). This measurement is a two-fold improvement over previous upper-bound measurements of 2 eV. Guido Drexlin, co-spokesperson for the KATRIN collaboration, presented the results on 13 September at a conference in Toyama, Japan.

Neutrinos are among the most abundant particles in the Universe. They are also the lightest of all the known subatomic particles that have mass — weighing around 500,000 times less than an electron. But they tend to cross matter undetected, which makes direct measurements extremely challenging . Physicists have yet to make a direct measurement of a neutrino’s mass.

KATRIN collected data over a few weeks of its initial run in April and May. The detector monitors the nuclear decay of the heavy isotope of hydrogen called tritium, in which a neutron turns into a proton and emits an electron and a neutrino in the process. KATRIN cannot catch the neutrinos directly. Instead it measures the range of energies of the electrons that shoot around inside a 23-metre-long, blimp-shaped chamber, which is the largest ultra-high-vacuum system in the world. This measurement reveals the range of energies that the unseen neutrinos would have, which in turn reveals the particles’ mass.

The team posted a preprint reporting the results on the arXiv repository, and submitted it to a journal for publication. Drexlin says that the results were a vindication of 18 years of work since KATRIN was first designed, but also that they are only a first taste of its potential. “This is just showing to the community that KATRIN is up and running,” he says.

Over the next five years, Drexlin’s collaboration plans continuous improvements in sensitivity that could enable KATRIN to make an actual measurement of a neutrino’s mass — or narrow the range of their estimate as far as the machine’s sensitivity will allow. Cosmological observations suggest that neutrinos could be 0.1 eV or lighter.