Neutrinos have no mass

What actually happened

The evidence arrived through a puzzle. Detectors kept finding only about a third of the electron neutrinos the Sun should be producing, a discrepancy known as the solar neutrino problem, and for thirty years the assumption was that either the detectors or the solar models were off.

Neither was. In 1998 the Super-Kamiokande experiment in Japan showed that neutrinos change flavor in flight, and the Sudbury Neutrino Observatory in Canada confirmed in 2001 that the missing solar neutrinos had transformed into other types rather than disappearing. Oscillation requires mass, because a massless particle traveling at light speed experiences no proper time and cannot change. Takaaki Kajita and Arthur McDonald shared the 2015 Nobel Prize.

This is one of the few confirmed cracks in the Standard Model, which had assumed massless neutrinos as a structural feature. We still do not know their actual masses, only that they are tiny and nonzero, nor whether a neutrino is its own antiparticle.