Another Look at “Just-So” Oscillations

We take another look at “Just-So” solar neutrino oscillations and characterize them by the energy at which the distance-varying angle is π/2. The SuperKamiokande spectrum can be fit with an energy in the range of 9-12 MeV. In this case, the pp neutrinos are reduced to half the standard solar model prediction and Be neutrinos must make up a significant part of the gallium signal, in contrast to the small-angle MSW prediction. Furthermore, the Be neutrinos could show a large seasonal variation, which might be detectable in more precise gallium data. PACS numbers: 26.65.+t, 14.60.Pq, 96.40.Tv Typeset using REVTEX 1 According to recent results from SuperKamiokande [1], there is an excess of events at the high-energy end of the solar neutrino spectrum relative to the standard solar model (SSM) [2] and the small-angle MSW solution of the solar neutrino problem [3,4]. In particular, the last three points above 12 MeV appear to rise more rapidly than a simple comparison with the SSM and MSW would indicate. This excess suggests that solar neutrinos may be undergoing “Just-So” in vacuo oscillations with large mixing [5] instead of matter-enhanced MSW oscillations with small mixing, and that the relevant ∆m is of order 10 eV rather than 10 eV. The key point about “Just-So” oscillations [5] is that the neutrino oscillation length is sufficiently close to the distance between the Sun and the Earth so that the oscillation probability for the higher energy B solar neutrinos is in the region of slow oscillations rather than the rapid oscillations at the lower end of the solar neutrino spectrum. This means that instead of taking on the average value of one-half, the survival probability of B neutrinos depends upon their energy and can run the gamut from zero to one. To mark the division between these two regions of probability, we characterize the oscillations by the energy at which the distance dependent angle χL Eν = 1.27 ∆m Eν LSun (1) is equal to π/2; LSun is the Earth-Sun distance. At this energy, Eπ/2, the oscillation probability P (νe → νx; Eν) = sin 2 2θ sin (