Solar neutrino problem
The solar neutrino problem concerned a major discrepancy between the number of neutrinos predicted by theoretical models of the solar interior and direct measurements of the number of neutrinos flowing through the Earth. The discrepancy was first observed in the mid-1960s and resolved around 2002.
According to the previously accepted Standard Model of particle physics, neutrinos were massless. This, in turn, implied that the type (flavor) of a neutrino would be invariable once it was produced. Of the three flavors of neutrinos, the Sun produces only electron neutrinos. As neutrino detectors became sensitive enough to measure the flow of neutrinos from the Sun, it became clear that the number detected was much lower than that predicted by models of the solar interior. In various experiments, the number of detected neutrinos was between one third and one half of the predicted number.
Evidence for neutrino oscillation—whereby a neutrino created with a specific flavor can be later measured to have a different flavor—arose first in 1998 and then more definitively in 2001. Neutrino oscillation implies that the neutrino has a non-zero mass. This new understanding required a modification to the Standard Model of particle physics. Neutrino oscillation enables the singular type of neutrino produced by the Sun to change into any of the two other types that would not have been caught by the detectors in use when the solar neutrino problem was first observed.
The Sun performs nuclear fusion via the proton–proton chain reaction, which converts four protons into alpha particles, neutrinos, positrons, and energy. This energy is released in the form of electromagnetic radiation, as gamma rays, as well as in the form of the kinetic energy of both the charged particles and the neutrinos. The neutrinos travel from the Sun's core to Earth without any appreciable absorption by the Sun's outer layers.
In the late 1960s, Ray Davis and John N. Bahcall's Homestake Experiment was the first to measure the flux of neutrinos from the Sun and detect a deficit. The experiment used a chlorine-based detector. Many subsequent radiochemical and water Cherenkov detectors confirmed the deficit, including the Kamioka Observatory and Sudbury Neutrino Observatory.
The expected number of solar neutrinos was computed using the standard solar model, which Bahcall had helped establish. The model gives a detailed account of the Sun's internal operation.
In 2002, Ray Davis and Masatoshi Koshiba won part of the Nobel Prize in Physics for experimental work which found the number of solar neutrinos to be around a third of the number predicted by the standard solar model.
In recognition of the firm evidence provided by the 1998 and 2001 experiments for neutrino oscillation, Takaaki Kajita from the Super-Kamiokande Observatory and Arthur McDonald from the Sudbury Neutrino Observatory were awarded the 2015 Nobel Prize for Physics.
Early attempts to explain the discrepancy proposed that the models of the Sun were wrong, i.e. the temperature and pressure in the interior of the Sun were substantially different from what was believed. For example, since neutrinos measure the amount of current nuclear fusion, it was suggested that the nuclear processes in the core of the Sun might have temporarily shut down. Since it takes thousands of years for heat energy to move from the core to the surface of the Sun, this would not immediately be apparent.
However, these solutions were rendered untenable by advances in both helioseismology, the study of how waves propagate through the Sun, and improved neutrino measurements.
Helioseismology observations made it possible to measure the interior temperatures of the Sun; these agreed with the standard solar models. (There are unresolved problems of the structure of what was found with helioseismology. Instead of the old "pot-on-the-stove" model of vertical convection, horizontal jet streams were found in the top layer of the convective zone. Small ones were found around each pole and larger ones extended to the equator. As might be expected, these had different speeds.)
Detailed observations of the neutrino spectrum from the more advanced neutrino observatories also produced results which no adjustment of the solar model could accommodate. In effect, overall lower neutrino flux (which the Homestake experiment results found) required a reduction in the solar core temperature. However, details in the energy spectrum of the neutrinos required a higher core temperature. This happens because different nuclear reactions, whose rates have different dependence upon the temperature, produce neutrinos with different amounts of energy; in order to match parts of the neutrino spectrum a higher temperature is needed. An exhaustive analysis of alternatives found that no combination of adjustments of the solar model was capable of producing the observed neutrino energy spectrum, and all adjustments that could be made to the model worsened some aspect of the discrepancies.
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 types (flavors) of neutrinos: electron neutrinos, muon neutrinos, and tau neutrinos. Electron neutrinos are the ones produced in the Sun and the ones detected by the above-mentioned experiments, in particular the chlorine-detector Homestake Mine experiment.
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, rendering them invisible to the detectors in the Homestake Mine and contemporary neutrino observatories.
The supernova 1987A indicated 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. If Kamiokande and IMB had high-precision timers to measure the travel time of the neutrino burst through the Earth, they could have more definitively established whether or not neutrinos had mass. 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. Since the detectors were not intended for supernova neutrino detection, 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. This proved that fewer neutrinos were detected coming through the Earth than could be detected coming directly above the detector. These observations only concerned muon neutrinos produced through the interaction of cosmic rays with the Earth's atmosphere. No tau neutrinos were observed at Super-Kamiokande.
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 electron-neutrinos from 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 with the earlier predictions from nuclear physics regarding fusion reactions inside the Sun.
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