THE KEV MAJORON AS A DARK-MATTER PARTICLE

We consider a very weakly interacting keV majoron as a dark matter particle (DMP), which provides both the critical density rho(cr) = 1.88 x 10(-29)h2 g/cm3 and the galactic scale M(gal) approximately m(Pl)3/m(J)2 approximately 10(12)M.(m(j)/1 keV)-2 for galaxy formation. The majoron couples to the leptons only through some new ''directly interacting particles'', called DIPs, and this provides the required smallness of its coupling constants. If the masses of these DIPs are greater than the scale V(s) characterizing the spontaneous violation of the global lepton symmetry they are absent at the corresponding phase transition (T approximately V(s)) and the majorons are produced during the phase transition, never being in thermal equilibrium during the history of the universe. In the alternative case M(DIP) < V(s) the majorons can be for a short period in thermal equilibrium. This scenario is not forbidden by nucleosynthesis and gives a reasonable growth factor for the density fluctuations compatible with the recent restrictions from the COBE experiment. It also provides as a possible signature the existence of an X-ray line at E(gamma) = m(J)/2, produced by the decay J --> gamma + gamma. A particle physics model which provides the required smallness of the majoron couplings is described. It realizes the possibility of the keV majoron as a DMP in a consistent way and may also lead to observable rates for flavour violating decays such as mu --> egamma and mu --> 3e, testable in the laboratory.