Vacuum oscillations and excess of high energy solar neutrino events observed in Superkamiokande

Abstract

The excess of solar-neutrino events above 13 MeV that has been recently observed by Superkamiokande can be explained by the vacuum oscillation solution to the Solar Neutrino Problem (SNP). If the boron neutrino flux is 20% smaller than the standard solar model (SSM) prediction and the chlorine signal is assumed 30% (or 3.4σ) higher than the measured one, there exists a vacuum oscillation solution to SNP that reproduces both the observed spectrum of the recoil electrons, including the high energy distortion, and the other measured neutrino rates. The most distinct signature of this solution is a semi-annual seasonal variation of the 7 Be neutrino flux with maximal amplitude. While the temporal series of the GALLEX and Homestake signals suggest that such a seasonal variation could be present, future detectors (BOREXINO, LENS and probably GNO) will be able to test it.Superkamiokande [1][2][3] has recently observed an excess of solar-neutrino events at electron energies higher than 13 MeV. This excess cannot be interpreted as a distortion of the boron neutrino spectrum due to neutrino oscillations [1][2][3][4], if one restricts oneself to those oscillation solutions that explain the observed gallium, chlorine and water-cerenkov neutrino rates.It is tempting to think that this excess is the result of low statistics or small systematic errors at the end of the boron neutrino spectrum. For example, because of very steep end of the electron spectrum, even small systematic error in electron energy (e.g., due to calibration) could enhance the number of events in the highest energy bins. One should 1 wait for future Superkamiokande data, where such possible systematic effects will be further elaborated. The data from the SNO detector, which will come in the operation soon, e.g., see Ref.[5], can shed light on this excess.Another possible explanation of this excess [6,7] is that the Hep neutrino flux might be significantly larger (about a factor 10-20) than the SSM prediction. The Hep flux depends on solar properties, such as the 3 He abundance and the temperature, and on S 13 , the zeroenergy astrophysical S-factor of the p + 3 He → 4 He + e + + ν reaction. Both SSM based [7] and model-independent [8] approaches give a robust prediction for the ratio Φ ν (Hep)/S 13 . Therefore, this scenario implies a cross-section larger by a factor 10-20 than the present calculations (for reviews see [7,9]). Such a large correction to the calculation does not seem likely, though it is not excluded. A large Hep neutrino flux remains a possible explanation of the excess. The signature of Hep neutrinos, the presence of electrons above the maximum boron neutrino energy, can be tested by the SNO experiment.The Superkamiokande collaboration noticed [1] that vacuum oscillations with large ∆m 2 explain the observed high-energy excess. However, the same Ref.[1] emphasizes that those oscillation parameters that reproduce the excess do not solve the Solar Neutrino Problem (SNP), i.e., they do not explain the global rates observed by the fou...

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