Solar neutrino problem
|Solar neutrino problem|
|Measurements of the neutrinos vs. solar's interior models|
|Neutrino is massless; fixed ratio between the number of neutrinos and the number of photons in the cosmic microwave background|
|Only detected between 1/3 and 1/2 of predicted number; neutrino oscillation|
|Neutrinos with mass change type; Detection of multiple neutrino types|
The solar neutrino problem was a major discrepancy between measurements of the 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. Essentially, if neutrinos do have mass, then they can change from the type that had been expected to be produced in the sun's interior into a type that would not be caught by the detectors in use at the time.
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. As one researcher put it, "we must trust the measurements, because even if they are wrong by three standard deviations, theory is incorrect.
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 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. There are unresolved problems of the structure of what was found with helioseismology. Instead of the old pot on the stove 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 you might expect, they had different velocities.
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.
The television show Law & Order had an episode that used the solar detection problem and the physics involved as a plot device.
Experimental evidence for neutrino mass
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 the small number detected versus the convective overturn model of supernovae. However, the data was insufficient to draw any conclusions with certainty.
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. Actually all that 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. 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.
- Solar neutrino data
- Nobel Prize in 2002, "partly for the detection of cosmic neutrinos"
- Solving the Mystery of the Missing Neutrinos
- Raymond Davis Jr.'s logbook