Standard Solar Model
The Standard Solar Model is the best current physical model of our
sun. Very generally, in the Standard Solar Model the sun is a ball of mostly
hydrogen plasma which is held together through self
gravitation. At the core of the sun the temperature and density are large enough that hydrogen
nuclei may be converted to
helium through several different processes. The
conversion of hydrogen to helium releases a large amount of
energy, and also results in the production of two
electrons and two
electron neutrinos.
Encyclopedia
The
Standard Solar Model is the best current physical model of our
sun. Very generally, in the Standard Solar Model the sun is a ball of mostly
hydrogen plasma which is held together through self
gravitation. At the core of the sun the temperature and density are large enough that hydrogen
nuclei may be converted to
helium through several different processes. The
conversion of hydrogen to helium releases a large amount of
energy, and also results in the production of two
electrons and two
electron neutrinos. The energy continually produced in the core keeps the sun in equilibrium, neither
exploding nor
collapsing further. As the ratio of hydrogen to helium in the core
changes, the core temperature and density also change, and this affects the size and luminosity of the sun. Like the
Standard Model of
particle physics and the standard cosmology the SSM changes over
time in response to relevant new theoretical or experiment discoveries.
Neutrino production
Hydrogen is fused into helium through several different interactions in the sun. The vast majority of neutrinos are produced through the
pp chain, a process in which four protons are combined to produce two
protons, two
neutrons, two electrons, and two electron neutrinos. Neutrinos are also produced by the
CNO cycle, but that process is considerably less important in our sun than in other stars.
Most of the neutrinos produced in the sun come from the first step of the pp chain but their energy is so low they are very difficult to detect. A rare side branch of the pp chain produces the "
boron-8" neutrinos with a maximum energy of roughly 15MeV, and these are the easiest neutrinos to observe. A very rare interaction in the pp chain produces the "hep" neutrinos, the highest energy neutrinos produced in any detectable quantity by our sun. The hep neutrinos are predicted to have a maximum energy of about 18MeV.
All of the interactions described above produce neutrinos with a
spectrum of energies. The inverse beta decay of Be7 produces neutrinos at either roughly 0.9 or 0.4MeV.utrino detection
The weakness of the neutrino's coupling with other particles means that most neutrinos produced in the core of the sun can pass all the way through the sun without being absorbed. It is possible, therefore, to observe the core of the sun directly by detecting these neutrinos.
History
The first experiment to successfully detect solar neutrinos was Ray Davis's chlorine experiment, in which neutrinos were detected by observing the conversion of chlorine nuclei to argon in a large tank of perchloroethylene. The experiment found about 1/3 as many neutrinos as were predicted by the Standard Solar Model of the time, and this problem became known as the solar neutrino problem.
While it is now known that the chlorine experiment detected neutrinos, some physicists at the time were suspicious of the experiment, mainly because they didn't trust such radiochemical techniques. Unambiguous detection of solar neutrinos was provided by the Kamiokande-II experiment, a water Cerenkov detector with a low enough energy threshold to detect neutrinos through neutrino-electron elastic scattering. In the elastic scattering interaction the electrons strongly point in the direction that the neutrino was travelling, away from the sun. This ability to "point back" at the sun was the first conclusive evidence that the sun is powered by nuclear interactions in the core. While the neutrinos observed in Kamiokande-II were clearly from the sun, the rate of neutrino interactions was again suppressed. Even worse, the Kamiokande-II experiment measured about 1/2 the predicted flux, rather than the chlorine experiment's 1/3.
The solution to the solar neutrino problem was finally experimentally determined by the Sudbury Neutrino Observatory. The radiochemical experiments were only sensitive to electron neutrinos, and the signal in the water Cerenkov experiments was dominated by the electron neutrino signal. The SNO experiment, by contrast, had sensitivity to all three neutrino flavours. By simultaneously measuring the electron neutrino and total neutrino fluxes the experiment demonstrated that the suppression was due to the MSW effect, the conversion of electron neutrinos from their pure flavour state into the second neutrino mass eigenstate as they passed through a resonance due to the changing density of the sun. The resonance is energy dependent, and "turns on" near 2MeV.ep neutrinos
The highest energy neutrinos have not yet been observed due to their small flux compared to the boron-8 neutrinos, so thus far only limits have been placed on the flux. No experiment yet has had enough sensitivity to observe the flux predicted by the SSM.
Future experiments
While radiochemical experiments have in some sense observed the pp and Be7 neutrinos they have measured only integral fluxes. The "holy grail" of solar neutrino experiments would detect the Be7 neutrinos with a detector that is sensitive to the individual neutrino energies. This experiment would test the MSW hypothesis by searching for the turn-on of the MSW effect. Some exotic models are still capable of explaining the solar neutrino deficit, so the observation of the MSW turn on would, in effect, finally solve the solar neutrino problem.
Core temperature prediction
The flux of boron-8 neutrinos is highly sensitive to the temperature of the core of the sun, . For this reason, a precise measurement of the boron-8 neutrino flux can be used in the framework of the Standard Solar Model as a measurement of the temperature of the core of the sun. This estimate was performed by Fiorentini and Ricci after the first SNO results were published, and they obtained a temperature of .
See also
References
External links
