For a long time it has been the case that simple theoretical models of
the Sun have been unable to reproduce the correct neutrino fluxes measured by
the three kinds of detectors: the original chlorine detector in the US,
the Kamiokande scintillation detector in Japan, and the gallium
detectors of GALLEX (in Italy) and SAGE (in Russia). There are two
issues: the absolute values of the fluxes and the relative fluxes
measured by the three different methods.
Neutrinos come principally from three different reactions in the proton-proton chain of thermonuclear reactions that take place in the core of the Sun. Each have different energy. The most abundant are the neutrinos produced by the first reaction of the chain: the production of deuterium by the fusion of two protons. (This reaction takes place in two different parallel ways: one is direct fusion of two protons, emitting a positron and a very low-energy neutrino; the other is the almost simultaneous collision of two protons and an electron emitting a slightly-higher-yet-still-low-energy neutrino.) I call these neutrinos the pp neutrinos. They are hardly detected by chlorine and are not detected at all by Kamiokande. However, they are scientifically extremely important because, if our ideas about nuclear fusion are correct, all the reactions that eventually produce the heat that causes the Sun to shine start with the deuterium-producing reaction. (There is the possibility of a completely different set of reactions called the carbon-nitrogen-oxygen cycle, but we believe these reactions to be ineffectual. If they were not, the neutrino problem would be exacerbated, because even more neutrinos would be produced.) And we know how much the Sun shines. Therefore we can predict how many pp neutrinos are produced, almost irrespective of our model of the Sun) (we need to assume only that the Sun is essentially static, so that the rate at which it shines -- emits energy at its surface -- is the same as the rate at which thermonuclear energy is generated in the core). That is why GALLEX and SAGE were built -- to test basic nuclear physics. Unfortunately (or perhaps I should say interestingly), GALLEX and SAGE find a flux that is just equal to the prediction of the flux of pp neutrinos. But they predict that these detectors should also detect neutrinos from the other reactions, and therefore that they should record a higher value.
The other neutrinos come from the destruction of beryllium and from the destruction of boron, both produced as intermediate products of the reaction chains. These neutrinos have higher energy. Chlorine detects a mixture of the two, Kamiokande only those from boron. The interesting property of these reactions is that the first produces only little energy (which could be overestimated by the theoretical models) and the other produces no energy to speak of at all. Therefore their measurement depends more strongly on the theory of the structure of the Sun. However, helioseismology has confirmed that that theory is very nearly correct, at least provided one sticks to simple theoretical models. Recently a new bigger Kamiokande detector has come into operation, with results broadly similar to those of the original Kamiokande. The current situation is that all detectors measure neutrino figures that are well below theoretical expectation. Moreover, one cannot imagine how to adjust the solar models (by accepting that some other aspect of the theory is inaccurate) to bring them into line with all the experiments. I should point out that all the neutrinos produced by the Sun are so-called electron neutrinos, and so far these are all that the detectors detect. There are two other kinds: mu and tau.
The current situation, it seems to me, is either that (i) the models need to be more sophisticated and there are errors in the neutrino measurements such that the detected differences between theory and observation should not yet be taken too seriously, or (ii) neutrinos are not massless, as is generally believed, and as a consequence they can decay into other kinds of neutrinos that have not been detected by the current detectors, implying that the fluxes detected are different from those produced by the nuclear reactions; or, a combination of (i) and (ii).
I should say that most people working in the subject favor (ii). Indeed, there is a recent rumour that neutrinos have been observed experimentally to have mass, but we've heard rumors like that before, so we await confirmation. Moreover, Superkamiokande should be able to shed more light on the issues in due course, once sufficient data have been accumulated. There is also a big detector being built in Sudbury, Canada, which should be operational soon and which should easily detect any other kind of neutrino into which the electron neutrinos might transform.
My own opinion is that (i) needs also to be looked into, because there is evidence from helioseismology that the principles of the theory of the Sun are too naive. We must wait and see.
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