Closing the gap in the neutrino mass.

Each second, more than 100 billion neutrinos—electrically neutral elementary particles of the Standard Model of particle physics—that originate from the Sun's core pass through a human body. Neutrinos are described by "flavor," each with distinct properties: electron, muon, and tau neutrino. Oscillations between these flavor states as the particle travels through space indicates that neutrinos have a distinct mass. However, neutrinos are the only fundamental particle whose mass is still unknown. On page 180 of this issue, the KATRIN Collaboration (1) report the new results obtained with the Karlsruhe Tritium Neutrino (KATRIN) experiment that narrows down the allowed range of the neutrino mass. This latest upper limit of 0.45 eV at 90% confidence level implies that the mass of a neutrino is less than 1 millionth the mass of an electron. Determining this fundamental quantity can further develop the Standard Model of particle physics and help better understand the Universe. [ABSTRACT FROM AUTHOR]