Non-thermal Production of Neutralino Cold Dark Matter from Cosmic String Decays

Abstract

We propose a mechanism of nonthermal production of a neutralino cold dark matter particle, $\chi$, from the decay of cosmic strings which form from the spontaneous breaking of a U(1) gauge symmetry, such as $U_{B-L}(1)$, in an extension of the minimal supersymmetric standard model (MSSM). By explicit calculation, we point out that with a symmetry breaking scale $\eta$ of around $10^8$ GeV, the decay of cosmic strings can give rise to $\Omega_\chi \simeq 1$. This gives a new constraint on supersymmetric models. For example, the dark matter produced from strings will overclose the universe if $\eta$ is near the electroweak symmetry breaking scale. To be consistent with $\Omega_\chi \leq 1$, the mass of the new U(1) gauge boson must be much larger than the Fermi scale which makes it unobservable in upcoming accelerator experiments. In a supersymmetric model with an extra $U_{B-L}(1)$ symmetry, the requirement of $\Omega_\chi \leq 1$ puts an upper bound on the neutrino mass of about $30 eV$ provided neutrino masses are generated by the see-saw mechanism.