Vacuum oscillations and the distorted solar neutrino spectrum observed by Superkamiokande

The excess of solar-neutrino events above 13 MeV that has been recently observed by Superkamiokande can be explained by vacuum oscillations (VO). If the boron neutrino flux is 20% smaller than the standard solar model (SSM) prediction and the chlorine signal is assumed 30% (or 3.5σ) higher than the measured one, there exists a VO solution that reproduces both the observed boron neutrino spectrum, including the high energy distortion, and the other measured neutrino rates. This solution is already testable by the predicted anomalous seasonal variation of the gallium signal. Its most distinct signature , a large anomalous seasonal variation of 7 Be neutrino flux, can be easily observed by the future detectors, BOREXINO and LENS. Superkamiokande [1] has recently observed an excess of solar-neutrino events at energy higher than 13 MeV. This excess cannot be interpreted as a distortion of the boron neutrino spectrum due to neutrino oscillations [1,2], if one restricts oneself to the standard oscillation solutions that explain the observed gallium, chlorine and water-cerenkov neutrino rates. It is tempting to think that this excess is a result of low statistics at the end of the boron neutrino spectrum. On the other hand, all systematic errors are reduced at higher energies: the background decreases, the detection efficiency increases, and the recoil electron direction is better determined. The data from the SNO detector, which is in the operation now (e.g., see [3]), can shed light on this excess. Another possible explanation [4,5] is that the hep neutrino flux might be significantly larger (about a factor 20–30) than the SSM prediction. The hep flux depends on solar 1 properties, such as the 3 He abundance and the temperature, and on S 13 , the zero-energy astrophysical S-factor of the p + 3 He → 4 He + e + + ν reaction. Both SSM based [5] and model-independent [6] approaches give a robust prediction for ratio Φ ν (hep)/S 13. Therefore, this scenario implies a cross-section larger by a factor 20–30 than the present calculations (for reviews see [5,7]). Such a huge mistake in the calculation does not seem likely, though the required large cross-section does not contradict to the " first principle physics " [5]. In this letter we propose another explanation of the observed excess based on the distortion of spectrum by vacuum oscillations. Combined with the SSM, VO can explain the observed rates of all three solar …