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

Cosmic strings form in many extensions of the MSSM, in particular in models with an extra U(1) gauge symmetry. We propose a new production mechanism for neutralino cold dark matter in such models. We show that if the strings forming scale is less than about 10(8) GeV, the dark matter produced from strings can overclose the universe. 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 supersymmetric models with an extra UB-L(1) gauge symmetry, the requirement of Omega less than or equal to 1 puts an upper bound on the neutrino mass of about 30 eV provided neutrino masses are generated by the see-saw mechanism.