https://www.scientificamerican.com/article/new-measurement-aims-to-solve-neutrino-mystery/

Miniscule, invisible neutrino particles are ubiquitous but nearly impossible to catch. They fly though the Earth—and us—every moment, by the billions. They are the smallest known matter particles in the universe and were predicted to be massless. Strangely, though, they are not. The reason why is still a mystery, and scientists do not yet know how massive they are. Now a new experiment aiming to measure their mass directly has found they cannot weigh more than one electron volt (eV)—that is one 500,000th the mass of the electron, the next-lightest particle.

On September 13 the long-awaited results came from the Karlsruhe Tritium Neutrino (KATRIN) experiment, which recently began operating at the Karlsruhe Institute of Technology in Germany. The new measurement, based on just one month of data, cuts the possible upper limit of the neutrino mass in half, compared with the best previous estimate from a direct-measurement experiment. Neutrinos come in three types, called “flavors,” and the limit applies to the average of the three respective masses. From other particle physics experiments, scientists also know the average mass cannot be less than 0.02 eV. “The neutrino is now boxed in,” says Hamish Robertson, a KATRIN team member and a professor emeritus of physics at the University of Washington. “The exciting part is that we showed, in only a month, that we’ve already improved on the world knowledge that existed before.”