The KeV Majoron as Dark Matter Particle

Abstract

We consider a very weakly interacting KeV majoron as dark matter particle (DMP), which provides both the critical density $ρ_{cr} = 1.88 \times 10^{-29} h^{2}$ $g/cm^{3}$ and the galactic scale $M_{gal}$ $\sim m^{3}_{Pl}/m^{2}_{J} \sim 10^{12} M_{\odot} (m_{J}/1 KeV)^{-2}$ for galaxy formation. The majoron couples to leptons only through some new &#34;directly interacting particles&#34;, called DIPS, and this provides the required smallness of the 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 ($T \sim V_s$) 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 COBE. A possible signature is an X-ray line at $E_γ= \frac{m_J}{2}$, produced by the decay $J \to γ+ γ$. A model is described which realizes the possibility of the KeV majoron as DMP and may also lead to observable rates for decays such as $μ\ra e γ$ and $μ\ra 3e$.