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Solar Neutrinos

The basic energy source in our sun is believed to be the pp cycle, in which four protons fuse to form , i.e., [7]. The dominant initial reactions are

 

The first of these results in the low energy pp neutrinos. Their number is the firmest prediction of the solar model because it is closely tied to the overall luminosity. However, they are very hard to detect because of their low energy. Most of the is from

 

However, approximately 15% is believed to be produced from the sequence

 

which yields neutrinos at two discrete energies, one of which is somewhat above the pp spectrum. Finally, a rare side reaction,

 

is associated with about 0.02% of the produced . This is insignificant energetically, but the resulting neutrino spectrum extends to much higher energy than the others, so they are easier to detect. The predicted spectrum [7] from the pp cycle and the rarer CNO cycle neutrinos are shown in Figure 1.

  
Figure: Predicted spectrum of solar neutrinos.

  
Table: Presently operating solar neutrino experiments.

There are currently four solar neutrino experiments, as shown in Table 1. The Kamiokande experiment [4] is a 1 KT water Cerenkov detector which measures the energy of the produced electrons. It is only sensitive to the highest energy neutrinos, but it is a real time experiment. It also yields some information on the direction of the incident neutrinos, which allowed Kamiokande to show that the neutrinos are really coming from the sun. Homestake [3] was the first solar neutrino experiment, and it has been running for 25 years. It consists of gallons of , and detects neutrinos via capture on the chlorine. It has a much lower energy threshold than Kamiokande, and is therefore sensitive to the higher line as well as the lower energy parts of the spectrum. However, its largest sensitivity is still to higher energies. In the last few years two gallium experiments, the SAGE experiment in the Baksan Neutrino observatory in the Caucasus mountains, and the GALLEX experiment in the Gran Sasso tunnel in Italy, have been running. They are sensitive to the low energy pp neutrinos, as well as to the higher energy neutrinos. The predicted contributions to the gallium and chlorine experiments in the standard solar model are shown in Table 2.

  
Table: Predicted rates in SNU ( atom) from the various flux components for the chlorine and gallium experiments, from [26]. The uncertainties are the total theoretical range, .

The results of the experiments are compared with the predictions of two standard solar models, that of Bahcall and Pinsonneault (BP) [26] and that of Turck-Chieze and Lopes (TCL) [27], in Table 3.

  
Table: Predictions of the BP and TCL standard solar models for the Kamiokande, Homestake, and Gallium experiments compared with the experimental rates. The Kamiokande flux is in units of , while the Homestake and gallium rates are in SNU. The experimental rates relative to the theoretical predictions are shown in the last two columns, where the first uncertainty is experimental and the second is theoretical. After 1986 the Homestake rate was slightly higher SNU, which corresponds to compared to BP and compared to TCL. All uncertainties are 1 .

It is seen that the predictions for the Kamiokande and Homestake experiments are between and of the BP expectations; the Kamiokande rate is still low compared to TCL, although somewhat closer; the Gallium rates are about 60% of the predictions. This deficit of neutrinos is shown in Figure 2, which also displays the typical neutrino energy to which each class of experiment is sensitive.

  
Figure: The experimental observations relative to the predictions of the BP and TCL standard solar models. The error bars on the points are experimental; the (1 ) theoretical uncertainties are displayed separately. Each experiment is sensitive to a range of neutrino energies. The values shown represent typical energies for each experiment.

The solar neutrino problem has two aspects. The older and less significant is that all of the experiments are below the SSM predictions. This was never a serious concern for the Kamiokande and Homestake experiments individually, which are mainly sensitive to the high energy neutrinos which are less reliably predicted. However, the predictions for the gallium experiments are harder to modify due to the constraint of the overall solar luminosity, and the statistics on the gallium experiments are starting to be good enough that the deficit observed there is hard to account for.

A second and more serious problem is that the Kamiokande rate indicates less suppression than the Homestake rate. The Homestake experiment has a lower energy threshold, and the lower observed rate suggests that there is more suppression in the middle of the spectrum (the line and the lower energy part of the spectrum) than at higher energies. This is very hard to account for by astrophysical or nuclear physics mechanisms: the is made from (eqn (5)), so any suppression of \ should be accompanied by at least as much suppression of . Furthermore, all known mechanisms for distorting the decay spectrum are negligible [35].

There are several generic explanations of the solar neutrino problem. In discussing astrophysical/nuclear solutions, one must distinguish between the uncertainties in the standard solar models, and nonstandard solar models with new physics ingredients. A second possibility is particle physics solutions, which invoke nonstandard neutrino properties. Of these I will concentrate on what I consider the simplest and most favored explanation, the Mikheyev-Smirnov-Wolfenstein (MSW) matter enhanced conversion of one neutrino flavor into another [15]. There are other possible explanations, such as the more complicated 3-flavor MSW [36], vacuum oscillations [37]--[42], neutrino decay [43], large magnetic moments [44], or violation of the equivalence principle [45]. Many of these are disfavored by the data and are, to my mind, less natural. The third possibility is that some or all of the experiments are wrong. However, this is becoming harder to accept, because the same difficulties follow from any two of the classes of experiments: one no longer has to believe all of the results to conclude that there is a problem.



next up previous
Next: Astrophysical Solutions Up: Solar Neutrinos (Erice 1994) Previous: Neutrino Mass




Mon Nov 27 19:39:39 EST 1995