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

We propose a mechanism of nonthermal production of a neutralino cold dark matter particle, χ, from the decay of cosmic strings which form from the spontaneous breaking of a U(1) gauge symmetry, such as UB-L(1), in an extension of the minimal supersymmetric standard model (MSSM). By explicit calculation, we point out that with a symmetry breaking scale η of around 108 GeV, the decay of cosmic strings can give rise to Ωχ ≅ 1. This gives a new constraint on supersymmetric models. For example, the dark matter produced from strings will over close the universe if η is near the electroweak symmetry breaking scale. To be consistent with Ωχ ≤ 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 UB-L(1) symmetry, the requirement of Ωχ ≤ 1 puts an upper bound on the neutrino mass of about 30eV provided neutrino masses are generated by the see-saw mechanisms. (author)