Solar neutrino

(astrophysics) A neutrino produced in a nuclear reaction inside the sun.

Neutrinos produced in nuclear reactions inside the Sun. Neutrinos are produced as well in laboratory nuclear reactions. The first direct tests of how the Sun produces its luminosity (observed most conspicuously on Earth as sunlight) have been carried out by observing solar neutrinos. The results of these experiments confirm the theory of how the Sun shines and stars evolve. Moreover, the results show that neutrinos behave differently than predicted by the standard model of particle physics.

Many explanations have been advanced for the discrepancy between the observed and the predicted event rates in the solar neutrino experiments. These explanations can be divided into three general classes: (1) the standard solar model must be significantly modified; (2) something is seriously wrong with the experiments; (3) the standard model of how neutrinos behave must be significantly modified.

Precise measurements of the thousands of frequencies with which the Sun pulsates on its surface (with characteristic periods of the order of 5 minutes) have confirmed to an accuracy of 0.1% the predictions of the standard solar model for these pulsation frequencies. This agreement is convincing evidence that the standard solar model is an accurate description of the Sun. See also Helioseismology.

All of the solar neutrino experiments have been examined carefully by many different searchers. A variety of checks have been made to test whether there was a significant error or a large uncertainty in one of the experiments that might explain the difference between prediction and observation. No significant previously unknown errors or uncertainties have been found. Moreover, intense laboratory sources of neutrinos have been placed near the gallium neutrino detectors, and the expected number of events have been observed from these artificial sources. The consensus view among scientists in the field is that the solar neutrino experiments are yielding a valid but surprising result.

The only remaining possibility is that the theory of how the neutrino behaves must be changed. Indeed, in 2000, the results of a decisive experiment showed unequivocally that solar neutrinos change their type on their way from the center of the Sun. All of the results from solar neutrino experiments are consistent with the conclusion that the standard solar model predicts accurately the number of neutrinos of different energies that are emitted by the Sun but that some of the neutrinos change their type on the way from the center of the Sun to the detectors on Earth. See also Neutrino; Standard model; Sun.

Solar neutrinos (proton-proton chain) in the Standard Solar Model

Electron neutrinos are produced in the Sun as a product of nuclear fusion. By far the largest fraction of neutrinos passing through the Earth are Solar neutrinos.

The main contribution comes from the so-called proton-proton chain reaction. The net reaction is:

,

or in words:

4 protons + 2 electrons = Helium + 2 electron neutrinos.

The highest flux of solar neutrinos come directly from the proton-proton interaction, and have a low energy, up to 400 keV. There are also several other significant production mechanisms, with energies up to 18 MeV. ^[1]

The number of neutrinos can be predicted by the Standard Solar Model. The detected number of electron neutrinos was only 1/3 of the predicted number, and this was known as the solar neutrino problem. It led to the idea of neutrino oscillation and the fact that neutrinos can change flavour. This was confirmed when the total flux of solar neutrinos of all types was measured and it agreed with the earlier predictions of expected electron neutrino flux, as seen by Sudbury Neutrino Observatory, and thus confirmed that neutrinos have mass.

The energy spectrum of solar neutrinos is also predicted by solar models.^[2] It is essential to know this energy spectrum because different neutrino detection experiments are sensitive to different neutrino energy ranges. The Homestake Experiment used chlorine and was most sensitive to solar neutrinos produced by the decay of ⁷Be. The Sudbury Neutrino Observatory is most sensitive to solar neutrinos produced by ⁸Be. The detectors that use gallium are most sensitive to the solar neutrinos produced by the pp process.

See also

• Neutrino detection
• Solar neutrino problem

References

1. ^ A. Bellerive, Review of solar neutrino experiments. Int.J.Mod.Phys. A19 (2004) 1167-1179
2. ^ Solar Neutrino Viewgraphs

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