The Karlsruhe Tritium Neutrino (KATRIN) experiment monitors the nuclear decay of the hydrogen isotope tritium.Credit: Michael Zacher

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

The first results1 from the Karlsruhe Tritium Neutrino (KATRIN) experiment in southwestern Germany reveal 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 it extremely challenging to take direct measurements of these particles.

KATRIN collected data over a few weeks of its initial run in April and May. The detector monitored the nuclear decay of a heavy isotope of hydrogen called tritium. During this process, a neutron turns into a proton and emits an electron and a neutrino. KATRIN cannot detect 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 of the unseen neutrinos, which in turn reveals their mass.

The team posted a preprint reporting the results, and has 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, and that they are only a first taste of the technology’s potential. “This is just showing to the community that KATRIN is up and running,” he says.

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