Solar neutrino problem
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, if neutrinos have mass, then 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.
Encyclopedia
| Solar neutrino problem |
| Measurements of the neutrinos vs. solar's interior models |
| Former Standard Model |
| Neutrino is massless; fixed ratio between the number of neutrinos and the number of photons in the cosmic microwave background |
| Observation |
| Only detected between ? and ½ of predicted number; neutrino oscillation |
| Resolutions |
| Neutrinos with mass change type; Detection of multiple neutrino types |
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, if neutrinos have mass, then 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.
Introduction
The
sun is a natural
nuclear fusion reactor, mainly fusing four
hydrogen nuclei to
helium,
neutrinos and energy through a
Proton-proton chain reaction. 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.
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 nuclear burning in the Sun predicted. In various experiments, the number of detected neutrinos was between ? and ½ of the predicted number. This came to be known as the
solar neutrino problem.
Measurements
The first experiment to detect the effects of neutrino oscillations was Ray Davis's Homestake Experiment, in which he observed a deficit in the flux of
solar neutrinos using a chlorine-based detector. Many subsequent radiochemical and water
Cerenkov detectors confirmed the deficit, including the
Sudbury Neutrino Observatory.
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 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
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; these agreed with the
standard solar models.
Changes to the Standard Model
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 ,
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.
Resolution
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 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 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.
See also
References
External links
