Solar Neutrino Problem - Resolution

Resolution

The solar neutrino problem was resolved with an improved understanding of the properties of neutrinos. According to the Standard Model of particle physics, there are three different kinds of neutrinos:

• electron neutrinos (which are the ones produced in the Sun and the ones detected by the above-mentioned experiments, in particular the chlorine-detector Homestake Mine experiment),
• muon neutrinos, and
• tau neutrinos.

Through the 1970s, it was widely believed that neutrinos were massless and their types were invariant. However, in 1968 Pontecorvo proposed that if neutrinos had mass, then they could change from one type to another. Thus, the "missing" solar neutrinos could be electron neutrinos which changed into other types along the way to Earth and therefore were not seen by the detectors in the Homestake Mine and contemporary neutrino observatories.

The supernova 1987A produced an indication that neutrinos might have mass, because of the difference in time of arrival of the neutrinos detected at Kamiokande and IMB. However, because very few neutrino events were detected it was difficult to draw any conclusions with certainty. In addition, whether neutrinos have mass or not could have been more definitively established had Kamiokande and IMB both had high precision timers which would have recorded how long it took the neutrino burst to travel through the Earth. If neutrinos were massless, they would travel at the speed of light; if they had mass, they would travel at velocities slightly less than that of light. Because the detectors were not intended for supernova neutrino detection, however, this was not done.

The first strong evidence for neutrino oscillation came in 1998 from the Super-Kamiokande collaboration in Japan. It produced observations consistent with muon-neutrinos (produced in the upper atmosphere by cosmic rays) changing into tau-neutrinos. What was proved was that fewer neutrinos were detected coming through the Earth than could be detected coming directly above the detector. Not only that, their observations only concerned muon neutrinos coming from the interaction of cosmic rays with the Earth's atmosphere. No tau neutrinos were observed at Super-Kamiokande.

The convincing evidence for solar neutrino oscillation came in 2001 from the Sudbury Neutrino Observatory (SNO) in Canada. It detected all types of neutrinos coming from the Sun, and was able to distinguish between electron-neutrinos and the other two flavors (but could not distinguish the muon and tau flavours), by uniquely using heavy water as the detection medium. After extensive statistical analysis, it was found that about 35% of the arriving solar neutrinos are electron-neutrinos, with the others being muon- or tau-neutrinos. The total number of detected neutrinos agrees quite well with the earlier predictions from nuclear physics, based on the fusion reactions inside the Sun.