The neutrino could be the weirdest subatomic particle; though abundant, it requires some of the most sensitive detectors to observe. Scientists have been working for decades to figure out whether neutrinos have mass and if so, what that mass is. The Karlsruhe Tritium Neutrino (KATRIN) experiment in Germany has now revealed its first result constraining the maximum limit of that mass. //

The KATRIN experiment begins with 25 grams of a kind of radioactive hydrogen gas, called tritium, stored in a 30-foot container held at cryogenic temperatures—cold enough such that even neon gas is a liquid. These tritium atoms undergo a kind of radioactive decay called beta decay, where one of their neutrons turns into a proton, spitting out an electron and an electron-antineutrino in the process (which would have the same mass as the electron neutrino). These decay products go into a house-sized detector called a spectrometer that measures the energy of the electrons. The electron and neutrino each carry away some of the energy of the reaction, but how much they take away can vary. Scientists must look at the spectrum of all the different electron energies, focusing particularly on the electrons that have taken away the maximum energy, whose neutrinos would in turn have taken away the minimum energy. Analysis of the shape of the resulting graphs reveals the maximum mass of any of the neutrino mass states. //

The mere fact that oscillation exists sets a lowest possible average mass of the three mass states, less than 0.1 electron volts (eV). After a month of operating and 18 years of planning and construction, KATRIN has now predicted an upper limit of any of the three mass states at 1.1 eV, where an electron weighs around 500,000 eV and a proton weighs nearly a billion.

KATRIN scientists announced the results at the 2019 Topics in Astroparticle and Underground Physics conference in Toyama, Japan, last Friday.