Radiative neutrino decay in superstring models

The rate for the radiative decay of neutrinos is calculated in E6 superstring models with light colour triplet isosinglets h and ℎ. In contrast to the standard model, the rate is not suppressed either by the GIM mechanism or by helicity matching. Comparison with astrophysical data allows one to set an upper limit on the Yukawa couplings anti λQLanti h and λdcvch. Taking. Msub(h,anti h)=O(100 GeV) it is found that λe,μ, anti λe,μ<10-3-10-4, in the range 10-100 eV. From the cosmological requirement of not photo-ionizing light elements after nucleosynthesis, it follows that the ντ must lie in the same mass range and consequently λτ, anti λ<10-3. (orig./HSI)