The Solar neutrino problem reference article from the English Wikipedia on 24-Jul-2004
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Solar neutrino problem

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The solar neutrino problem was a major discrepancy between measurements of the neutrinos flowing through the Earth and theoretical models of the solar's 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.

The sun is a natural nuclear fusion reactor, fusing hydrogen to helium. Our current understanding of physics is quite clear about what happens: four hydrogen nuclei (protons), with and without the help of catalysts, are transformed into helium, neutrinos, and energy. The energy is released as gamma rays and as kinetic energy of the particles, including the neutrinos — which travel from the sun's core to Earth without any appreciable absorption by the sun's outer layers.

History of the Problem

As neutrino detectors became accurate enough to measure the flow of neutrinos from the sun, it became clear that researchers weren't getting as many of them as the models of solar combustion predicted. In various experiments, the number of detected neutrinos was between 1/3 and 1/2 of the predicted number. Therefore either the current models of the sun were wrong or the models of neutrino behavior were wrong. This came to be known as the solar neutrino problem.

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 helioseismology, the study of how waves propagate through the sun. Based on such observations it became possible to measure the interior temperatures of the sun and these agreed with the standard solar models.

Currently, the solar neutrino problem is believed to have resulted from an inadequate 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), muon-neutrinos, and tau-neutrinos. In the 1970s, it was widely believed that neutrinos were massless and their types were invariant. However, theoreticians in the 1980's realized 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 escaped detection.

Experimental evidence for neutrino mass

The first 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. More direct evidence came in 2002 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. 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.

In 2002 Raymond Davis Jr 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 Model.

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