Nonthermal dark matter from cosmic strings

Abstract

Cosmic strings can be created in the early universe during symmetry-breaking phase transitions, such as might arise if the gauge structure of the standard model is extended by additional $U(1)$ factors at high energies. Cosmic strings presented in the early universe form a network of long horizon-length segments, as well as a population of closed string loops. The closed loops are unstable against decay, and can be a source of nonthermal particle production. In this work we compute the density of weakly-interacting massive particle dark matter formed by the decay of gauge theory cosmic string loops derived from a network of long strings in the scaling regime or under the influence of frictional forces. We find that for symmetry-breaking scales larger than ${10}^{10}\text{ }\text{ }\mathrm{GeV}$, this mechanism has the potential to account for the observed relic density of dark matter. For symmetry-breaking scales lower than this, the density of dark matter created by loop decays from a scaling string network lies below the observed value. In particular, the cosmic strings originating from a $U(1)$ gauge symmetry broken near the electroweak scale, that could lead to a massive ${Z}^{\ensuremath{'}}$ gauge boson observable at the LHC, produces a negligibly small dark matter relic density by this mechanism.