SNO

Background. For decades, experiments had counted only about a third of the solar neutrinos that models of the Sun predicted. This shortfall, the solar neutrino problem, had two possible explanations: either the models of the solar core were wrong, or neutrinos were changing on their way to Earth. SNO was designed to settle the question directly, two kilometres underground in an active nickel mine near Sudbury.

Objective. The experiment set out to measure both the electron-neutrino flux and the total flux of all three flavours arriving from the Sun. If the two disagreed in a specific way, flavour change would be proven without relying on any solar model.

Method. At its heart was a 1,000-tonne acrylic sphere of heavy water, in which the extra neutron makes possible reactions ordinary water cannot host. One reaction responded only to electron neutrinos, a second responded equally to all three flavours, and elastic scattering offered a third channel. The neutral-current reaction was the key, because it counted every flavour alike. Charged particles were tracked through their Cherenkov radiation.

Results. In 2001 and 2002 SNO found that solar electron neutrinos were indeed depleted, yet the total flux across all flavours matched the solar-model prediction almost exactly. The missing electron neutrinos had not vanished — they had transformed into muon and tau neutrinos en route. This was direct proof of neutrino oscillation and resolved the long-standing puzzle.

Significance. Together with Super-Kamiokande’s atmospheric result, SNO established that neutrinos oscillate and therefore have mass. Arthur B. McDonald shared the 2015 Nobel Prize for the work, and the cavern later hosted the SNO+ scintillator experiment.