Does the Sun Appear Brighter at Night in Neutrinos?

Abstract

We calculate accurately the number of solar neutrino events expected as a function of solar zenith angle, with and without neutrino oscillations, for detectors at the locations of Super-Kamiokande, SNO, and the Gran Sasso National Laboratory. Using different earth models to estimate geophysical uncertainties, and different solar models to estimate solar uncertainties, we evaluate distortions predicted by the MSW effect in the zenith angle distributions of solar neutrino events. The distortions are caused by oscillations and by $ν-e$ interactions in the earth that regenerate $ν_e$ from $ν_μ$ or $ν_τ$. We show that the first two moments of the zenith-angle distribution are more sensitive to the small mixing angle MSW solution than the conventionally studied day-night asymmetry. We present iso-sigma contours that illustrate the potential of Super-Kamiokande, SNO, BOREXINO, ICARUS and HERON/HELLAZ for detecting the earth regeneration effect at their actual locations (and at the equator). MSW solutions favored by the four pioneering solar neutrino experiments predict characteristic distortions for Super-Kamiokande, SNO, BOREXINO, and ICARUS that range from being unmeasurably small to $> 5σ$ (stat) after only a few years of observations.