After decades of attempting to measure the mass of neutrinos, one of the most poorly understood common subatomic particles, physicists still don't have an answer. They do, however, have a maximum, and in the strange world of particle physics where units of energy measure mass, that is 0.8 electron volts for the electron neutrino. The figure is about 100,000 times less than the lightest other standard model fermion.

Neutrinos were first proposed because energy was missing after nuclear reactions, which physicists thought could only be accounted for if a particle was being created that we knew nothing about. Further work established these come in three types (electron neutrinos, muon neutrinos, and tau neutrinos, each of which has an antiparticle). For a long time one of science's hottest debates concerned whether neutrinos had mass or not. Eventually, it was resolved they do, but only in tiny amounts.

Defining just how tiny their mass is, has proven even harder to solve, but an answer of sorts has been provided in a new paper in Nature Physics. The work is the product of the Karlsruhe Tritium Neutrino Experiment (KATRIN) Collaboration.

Neutrinos are produced in a wide array of reactions but remain very hard to detect. Just 11 (and 8 antineutrinos) were detected when SN1987A became the closest supernova to Earth in centuries, an event that launched neutrino astronomy (and a lot more than 11 scientific papers). Yet it is thought neutrinos carry away the majority of the gravitational energy released in these enormous explosions. Even at a distance of 163,000 light-years, staggering numbers must have passed through our planet.

Rather than try to measure the neutrino mass by observing them directly the collaboration studied one of the simplest nuclear reactions, beta decay of tritium. Tritium has a half-life of 12.3 years and when it decays it emits an electron, and also releases a neutrino. By measuring how much energy the electrons had the authors could calculate what was missing with unprecedented accuracy. Once they had accounted for all other ways in which energy was released, or other sources might contaminate the measurements, neutrino energy was considered what was left.

Measuring everything else was not quick, however. “This laborious and intricate work was the only way to exclude a systematic bias of our result due to distorting processes,” authors Dr Magnus Schlösser of Karlsruher Institut Fur Technologie and Professor Susanne Mertens of Max Planck Institute for Physics said.

Out of all this, the authors calculate neutrinos' mass can be no more than 0.8 eV (1.3 x 10^-19). If you are puzzled as to why mass is being measured in units of energy, consider Einstein's most famous work, e=mc². Allowing for the minor matter of the speed of light squared, mass is energy and can be measured that way. If you insist on using units of mass, the maximum is 1.6 × 10^–36 kg.

Measuring neutrinos' mass has been a long-term project, but most efforts have either proven flawed or produce results dependent on the model of the universe used. Others have been less precise, with maximums above 1 eV. “The particle physics community is excited that the 1-eV barrier has been broken,” said Professor John Wilkerson of the University of North Carolina, who chairs the Board of KATRIN.

Inevitably, the researchers are not done, and will continue taking measurements until the end of 2024 in the hope of narrowing the possible range still further.

The work will shed light on the question of whether neutrinos make up some of the universe's mysterious dark matter, and if so how much.