Solar neutrino problem
|Solar neutrino problem|
|Discrepancies in the measurements of actual solar neutrino types and what the Sun's interior models predict.|
|Former Standard Model|
|Neutrinos should have been massless according to the then-accepted theory; this means that the type of neutrino would be fixed when it was produced. The Sun should emit only electron neutrinos as they are produced by H–He fusion.|
|Only one third to one half of predicted number of electron neutrinos were detected; neutrino oscillation explains the difference but requires neutrinos to have mass.|
|Neutrinos have mass and so can change type.|
The solar neutrino problem was a major discrepancy between measurements of the numbers of neutrinos flowing through the Earth and theoretical models of the solar interior, lasting from the mid-1960s to about 2002. The discrepancy has since been resolved by new understanding of neutrino physics, requiring a modification of the Standard Model of particle physics – specifically, neutrino oscillation. Essentially, as neutrinos have mass, they can change from the type that had been expected to be produced in the Sun's interior into two types that would not be caught by the detectors in use at the time.
The Sun is a natural nuclear fusion reactor, powered by a proton–proton chain reaction which converts four hydrogen nuclei (protons) into helium, neutrinos, positrons and energy. The excess energy is released as gamma rays and as kinetic energy of the particles and as neutrinos — which travel from the Sun's core to Earth without any appreciable absorption by the Sun's outer layers.
As neutrino detectors became sensitive enough to measure the flow of neutrinos from the Sun, it became clear that the number detected was 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. This came to be known as the solar neutrino problem.
In the late 1960s, Ray Davis's 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 Cerenkov detectors confirmed the deficit, including the Sudbury Neutrino Observatory.
The expected number of solar neutrinos had been computed based on the standard solar model which Bahcall had helped to establish and which 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 that found the number of solar neutrinos was around a third of the number predicted by the standard solar model.
Proposed solutions 
Changes to the solar model 
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 energy neutrinos are produced by different nuclear reactions, whose rates have different dependence upon the temperature; 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 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.
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