INFN FE-08-93 June 1993 MATTER INDUCED NEUTRINO DECAY: NEW CANDIDATE FOR THE SOLUTION TO THE SOLAR NEUTRINO PROBLEM

Effects of the dense matter can induce the neutrino decay even if neutrinos are stable in vacuum. This is due to coherent interactions with matter which lead to energy level-splitting between the neutrino and antineutrino states and thereby provide available phase space for the majoron emission. We show that the matter induced decay can be a plausible candidate for the explanation of solar neutrino deficit, provided that there exist new flavour-changing weak range interactions of neutrino with matter constituents, and constant of τ -neutrino coupling to the majoron is sufficiently large. This mechanism naturally implies the hierarchy between the ratios Z = measured signal SSM expectation for Ga-Ge, νee scattering and Cl-Ar experiments ZGA > ZK > ZCl. The important feature of the matter induced decay is the prediction of solar antineutrino flux with substantially degraded energy spectrum compared to solar neutrinos. This scenario can be unambiguously tested by future solar neutrino detectors BOREXINO/BOREX and SNO. Talk presented at 5 Int. Workshop on ”Neutrino Telescopes”, Venice, March 2-4, 1993. To appear on the Proceedings. E-mail: 39967::berezhiani, [email protected], [email protected] E-mail: 39967::rossi, [email protected] 1. Solar Neutrino Problem (SNP). The discrepancy between the neutrino flux predicted by the Standard Solar Model (SSM) [1, 2] and the observed flux, is the main open issue in neutrino physics. The ratio Z of the observed signal to that is expected from the SSM is different for different solar neutrino experiments. Namely, the Homestake Cl-Ar experiment gives [3] ZCl = 0.28± 0.04 (1) The results of Kamiokande II and Kamiokande III are [4]: ZKII = 0.46± 0.05± 0.06, ZKIII = 0.56± 0.07± 0.06 (2) The combined result of both implies ZK = 0.49 ± 0.05 ± 0.06. Finally, the results of the Ga-Ge radiochemical experiments GALLEX [6] and SAGE [7] are Z Ga = 0.63± 0.14± 0.06, Z Ga = 0.44± 0.15± 0.11 (3) with the combined result ZGa = 0.54±0.11 (all the above data are given with 1σ error due to experiment and do not include ”3σ” theoretical uncertainties of the SSM [2]). Focusing on the central values and neglecting experimental uncertainties, the following hierarchy of the data is obeyed: