A new experiment slashes the maximum possible mass of tiny neutrinos

The maximum possible mass of a barely there particle has just gotten smaller.

Subatomic particles called neutrinos are extremely lightweight. Now, scientists with the KATRIN experiment in Karlsruhe, Germany, have shrunk the potential mass range for these runts of the particle litter. Neutrinos must have a mass of 1.1 electron volts or less, the researchers report in a paper posted September 13 at arXiv.org and in a talk on the same day at the Topics in Astroparticle and Underground Physics conference in Toyama, Japan.

That new number — about half of the previous ceiling on neutrino mass — means that it would take more than 460,000 neutrinos to reach the mass of an electron, “and possibly a lot more,” says physicist Diana Parno of Carnegie Mellon University in Pittsburgh.

Parno and colleagues studied decays of tritium, a radioactive variety of hydrogen (SN: 9/19/18). In each such decay, a type of neutrino known as an electron antineutrino is emitted, along with an electron. By measuring the electrons’ energy, the scientists tested how much of the decays’ energy went into the mass of the neutrinos.

To make matters more complicated, neutrinos typically don’t have a well-defined mass. Due to the intricacies of quantum mechanics, the particles are made up of three different mass states at once. What KATRIN measures is an “effective mass,” a combination of those three masses.

Even tighter constraints on neutrino masses have been made using studies of how matter clumped together in the early universe. But those mass limits apply only if physicists’ understanding of the rules of the cosmos is correct (SN: 3/12/08). KATRIN’s new estimate requires no such assumption.