The neutrino remains one of the least understood of the fundamental particles, and yet it impacts on some of the toughest questions in cosmology and particle physics. It is a particle with no electrical charge and almost no mass, and, it hardly ever interacts with normal matter, which calls for experiments of exquisite accuracy and delicacy, as well as long observing or experimental runs. A typical neutrino physics experiment takes years to perform. Results from SNO, and from related experiments in Japan, show that the standard model of particle physics is now in serious trouble.

SNO is a heavy-water Cerenkov detector located at the Creighton Mine, close to Sudbury, Ontario. The 100-member team (SNO Collaboration) is drawn from Canada, the U.K., and the United States, and is directed by Art McDonald (Queen’s University, Kingston, Ontario). The detector consists of 1,000 tons of ultra-pure heavy water enclosed in a 12-m acrylic plastic vessel, which in turn is surrounded by ultra-pure ordinary water in a giant 22-m by 34-m cavity. Outside the acrylic vessel a sphere containing 9,456 light sensors or photomultiplier tubes detects tiny flashes of light emitted as neutrinos are stopped or scattered in the heavy water. The detector is optimized for solar neutrinos.

The SNO experiment looks at the fate of electron neutrinos which are produced inside the Sun by the beta-decay of ⁸B -> ⁸Be + positron + electron neutrino. In the classic solar neutrino problem, the measured flux of electron neutrinos is only 35% to 65% of the predicted flux. Today the solution to this deficit problem is given by a mechanism (called MSW, after its authors) whereby the electron neutrinos self-interact on the journey from the Sun to Earth, undergoing flavor oscillations which mix in muon and tau neutrinos to the flux, at the expense of electron neutrinos. The MSW oscillation mechanism is enhanced in the presence of a large mass, such as the Sun.

All experimental set-ups prior to SNO were primarily capable of detecting just electron neutrinos, and therefore they were cheated out of the higher energy neutrinos. The uniqueness of SNO is that it can detect all three flavors of neutrino, thanks to a remarkable property of heavy water: all three neutrino flavors are capable of combining with deuterium to produce two protons and an electron. These charged-current and neutral-current reactions are the guts of SNO physics.

Hot Paper #3 provided the first strong indication that the solar neutrino flux has a non-electron flavor component, and this paper gave the first determination of the flux of ⁸B neutrinos from the Sun. All this agreed perfectly with predictions of the electron neutrino flux expected from the standard model of stellar energy generation, thus ending the 30-year solar neutrino problem. However, the results in #3 rely on a comparison of SNO results with the Super-Kamiokande experiment in Japan, so they are not truly independent. A scientific conclusion relying on two experiments, as well as the theory connecting them, is not really robust enough for loud claims of "new physics," which had to wait for #2, published nine months later.

In #2, the SNO Collaboration considers data from 240 days of live operation. The neutrinos reaching SNO are detectable through three distinct interactions: charged-current or neutral-current interactions with deuterium, and electron scattering. The charged-current reaction is exclusive to electron neutrinos whereas all three flavors participate in the neutral-current reaction and electron scattering. In #2 the neutral-current data are analyzed to produce a value for the flux of non-electron neutrinos, so this is effectively the first direct evidence for oscillations in solar neutrinos as well as the first direct measurement of the neutrino rate from <^>8B nuclear reactions.

The final paper, newcomer #6, looks at the day-night asymmetry in the neutrino energy spectrum. The night detections are of neutrinos that have travelled the extra path length through the earth to reach SNO. All three reactions show significant day-night asymmetries, with the charged current reaction (electron neutrinos only) enhanced at night, and the other two reactions reduced. A global fit of SNO’s spectra with data from other experiments strongly favors a solution of MSW oscillations enhanced by passage through the mass of the earth.