Non-thermal production of neutralino cold dark matter from cosmic string decays

We propose a mechanism of nonthermal production of a neutralino cold darkmatter particle, $\chi$, from the decay of cosmic strings which form from thespontaneous breaking of a U(1) gauge symmetry, such as $U_{B-L}(1)$, in anextension of the minimal supersymmetric standard model (MSSM). By explicitcalculation, we point out that with a symmetry breaking scale $\eta$ of around$10^8$ GeV, the decay of cosmic strings can give rise to $\Omega_\chi \simeq1$. This gives a new constraint on supersymmetric models. For example, the darkmatter produced from strings will overclose the universe if $\eta$ is near theelectroweak symmetry breaking scale. To be consistent with $\Omega_\chi \leq1$, the mass of the new U(1) gauge boson must be much larger than the Fermiscale which makes it unobservable in upcoming accelerator experiments. In asupersymmetric 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.Comment: 4 pages, uses Revtex, 1 figur