Massive, blimplike experiment lowers weight limit on neutrino

Physicists have set a new limit on the mass of nature’s lightest particle of matter. The neutrino can weigh no more than 1.1 electron volts (eV)—less than one-500,000th the mass of an electron—say experimenters with the Karlsruhe Tritium Neutrino (KATRIN) experiment at the Karlsruhe Institute of Technology in Germany. Reported on 13 September at a meeting in Toyama, Japan, the new result halves the previous limit of 2 eV.

Physicists have tried to measure the neutrino’s mass for decades. However, the particle barely interacts with ordinary matter. So to deduce its mass, researchers study the radioactive “β decay” of tritium, in which a nucleus spits out an electron and a neutrino. By precisely measuring the maximum energy of the ejected electrons, physicists can infer the mass of the unobserved neutrinos. KATRIN (above) takes this classic approach to the ultimate limit, employing a 23-meter-long blimplike spectrometer to measure the electron from tritium with unprecedented precision.

The new limit is based on just 28 days’ worth of data. Ultimately, KATRIN experimenters hope to collect data for 1000 days and push the limit down by another factor of 10 to 0.2 eV—or show that the neutrino weighs more than that. Cosmological measurements already suggest the neutrino cannot weigh more than about 0.1 eV, but that estimate is based on several assumptions. So KATRIN physicists argue that their better, directly measured limit on neutrino mass is likely to make cosmology models more reliable.