Gallium solar neutrino experiments

The three viewgraphs presented below are based upon results given in ``Gallium Solar Neutrino Experiments: Absorption Cross Sections, Neutrino Spectra, and Predicted Event Rates," Phys. Rev. C, 56, 3391 (1997), hep-ph/9710491.

Measured versus calculated 51Cr neutrino absorption by 71Ga

The GALLEX and the SAGE solar neutrino collaborations have separately measured in the laboratory the absorption cross sections for 51Cr neutrinos incident on 71Ga. These extraordinarily beautiful experiments use sources in the megacurie range to induce neutrino events in gallium solar neutrino detectors.

The results from both experiments are in excellent agreement with the calculations described in Phys. Rev. C, 56, 3391 (1997).

Since the 51Cr and the 7Be decays produce neutrinos of similar energies, the results summarized on this viewgraph show that the deficit of 7Be neutrinos that is observed in the GALLEX and SAGE experiments cannot be due to a failure of these experiments to detect the neutrinos.

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Predicted standard model gallium neutrino event rate versus year of publication.

The figure shows the event rates for all of the standard solar model calculations that my colleagues and I have published. The cross sections from the present paper have been used in all cases to convert the calculated neutrino fluxes to predicted capture rates. The estimated 1 sigma uncertainties reflect in all cases just the uncertainties in the cross sections that are evaluated in the present paper. For the 35 years over which we have been calculating standard solar model neutrino fluxes, the historically lowest value (fluxes published in 1969) corresponds to 109.5 SNU. This lowest-ever value is 5.6 sigma greater than the combined GALLEX and SAGE experimental result. If the points prior to 1992 are increased by 11 SNU to correct for diffusion (this was not done in the figure), then all of the standard model theoretical capture rates since 1968 through 1997 lie in the range 120 SNU to 141 SNU, i.e., (131 ± 11) SNU.

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Absorption cross sections for gallium as a function of energy.

The figure displays the best-estimate cross sections as well as the ± 3 sigma cross sections.

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The Zero Cross-Section Lower Limit

What is the minimum rate that could conceivably be measured in the gallium solar neutrino experiments if one only assumes that the sun currently supplies its luminosity by nuclear fusion reactions in its interior?

The answer to this question is 80 SNU. The argument is summarized on the viewgraph and is given in more detail in chapter 11 of my book on Neutrino Astrophysics. The limit is derived by artificially setting equal to zero in the solar model calculations the rates for all nuclear reactions that give rise to neutrinos from anything except the basic pp and pep neutrino-producing nuclear reactions. Since this limit assumes nothing more about the sun than that it is currently shining in its interior, I have called this viewgraph the ``Zero Cross-Section Lower Limit.''

The GALLEX and SAGE experiments may not even satisfy this extreme limit.

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71Ga: Experiment versus Theory

The GALLEX and SAGE measured results are compared with the predictions of the combined standard solar model and standard electroweak theory for the 71Ga experiments. The combined measured rate is more than 9 standard deviations from the predicted standard model rate.

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