In the last edition of “Science Notebook” (September 27), we reported an upper limit on the neutrino mass obtained indirectly by modelling mass distribution in the universe. An international team of scientists has now announced a new limit on neutrino mass through direct measurement.
On September 13, scientists from the Karlsruhe Tritium Neutrino (KATRIN) experiment reported that the estimated range for the rest mass of the neutrino is no larger than 1.1 electron volt (eV). These first results cut the mass range for the neutrino by half by lowering the earlier upper limit from 2 eV. The lower limit for the neutrino mass, 0.02 eV, was set by previous experiments by other groups. The astrophysical modelling reported recently placed the range between 0.086 eV and 0.26 eV.
Neutrinos are mysterious particles that have stirred up physics, cosmology and astrophysics. According to the Standard Model of particle physics, neutrinos should have no mass. By 2001, two detectors, Super-Kamiokande and the Sudbury Neutrino Observatory, demonstrated that they actually have a nonzero mass, a breakthrough that won the 2015 Nobel Prize. So neutrinos have mass, but how much?
The measurement method at KATRIN involves measuring how an electron-neutrino pair, which is emitted in the radioactive decay of gaseous tritium (a radioactive isotope of hydrogen), shares the 18,560 eV of total available energy. Scientists can measure electrons and try to calculate neutrino properties based on measurements made on the electron. Most of the electron-neutrino pairs emitted by tritium share the total energy equally. But in rare cases, the electron takes nearly all the energy, leaving only a tiny amount for the neutrino, which must include its rest mass. If KATRIN can measure the electron’s energy accurately, the neutrino’s energy, and therefore its mass, can be calculated. If the neutrino is massless, there is no lower limit to the energy the neutrino can carry, so the electron energy spectrum should extend all the way to the 18.56 keV limit. If the neutrino has mass, then it must carry away at least the amount of energy equivalent to its rest mass by E=mc2, and the electron spectrum will have a different shape close to the total energy limit.