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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.

1 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. As one researcher put it, "we must trust the measurements, because even if they are wrong by three standard deviations, theory is incorrect. To give you an idea of how large three standard deviations of error is, if a graduate student made such a bad measurement, it would cause him to be dismissed."

The problem was troubling because it meant that either General Relativity was incorrect, models of stellar evolution were incorrect, or the "standard model of physics" was incorrect. Since each of these models had proven remarkably accurate, the choice was an unpalatable one.

Early attempts to explain the discrepancy proposed that the models of the sun were wrong, i.e. the temperatureTemperature is the physical property of a system which underlies the common notions of "hot" and "cold"; the material with the higher temperature is said to be hotter. General description The formal properties of temperature are studied in thermodynamics. and pressurePressure (symbol: p is a measure of force per unit area. where p is the pressure F is the force A is the area Often F is taken to be the of the magnitude of the mean vector force normal to the surface of area A upon which it exerts; the "surface" not nece 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 helioseismologyHelioseismology is the study of how pressure waves propagate in the Sun. These waves are measured by the doppler shift of visible solar material. Changes in the propagation of pressure waves through the Sun reveal inner structures and allows astrophysicis, 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 1980s 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.

As a note on popular culture, the television show Law and Order had an episode that used the solar detection problem and the physics involved as a plot device.