1. W. Marciano and A. Sanda, Phys.Lett. 67B, 303 (1977);.
2. S. Petkov, Yad.Fiz. 25,641 (1977) and 25,1336 (1977), [ Sov.J. Nucl.Phys, 25, 340 (1977) and 25,698 (1977)].
3. P.B. Pal and L. Wolfenstein, Phys.Rev. D25, 766 (1982) and references therein.
4. U. Amaldi et al., Phys.Rev. D36, 1385 (1987).
5. J. Maalampi, K. Mursula and M. Roos, Phys.Rev. Lett. 56, 1031 (1986).