The most enigmatic of elementary particles, neutrinos were postulated in 1930, but were not observed until a quarter of a century later. It took another forty years to determine that they are not massless.

Neutrinos are a ubiquitous if imperceptible part of our environment. Neutrinos created in the Big Bang together with the cosmic background radiation pervade the entire Universe. The Sun is a poweful source of MeV neutrinos. Neutrinos in the GeV range are created when cosmic rays strike the atmosphere, 15 kilometers or so above the Earth's surface. Every nuclear reactor emits antineutrinos copiously. High-energy neutrinos are regularly produced at accelerators through particle decay and carefully fashioned magnetic fields can focus produced unstable charged particles to create neutrino beams.

Traditionally, efforts were made to set upper limits on the masses of the neutrinos associated with the electron, muon, and tau lepton. As explained in Chapter 6, if the electron neutrino were sufficiently massive the electron spectrum in tritium beta decay would be distorted near the end point. This prompted many painstaking measurements over the past thirty years. The expression for the spectrum actually depends on the square of the neutrino mass and the best fits can return unphysical, negative values for this.