Structure Formation with a Decaying Mev Tau Neutrino and a Kev Majoron

We consider the scenario of large scale structure formation with tau neutrino with mass in the MeV range and lifetime of order of years which decays into a massive majoron with KeV mass. The latter are the present-day cold dark matter particles. In contrast to the usual collisionless dark matter, the majorons have a relatively strong self-interaction and the picture of structure formation is rather different from the standard. The decay $J \ra \gamma \gamma$ leads to the existence of an X-ray line at $E_\gamma = m_J/2$ which could be detectable. Electron and muon neutrinos are expected to be very light, as required in order to account for the solar neutrino deficit through \ne to \nm oscillations. Supersymmetry with spontaneously broken R parity provides a natural particle physics model for our scenario.