Non-Thermal Dark Matter from Cosmic Strings

TL;DR

This paper investigates how decay of cosmic string loops from early universe phase transitions could produce non-thermal dark matter, potentially explaining observed relic densities for high symmetry-breaking scales.

Contribution

It provides a detailed calculation of WIMP dark matter density from cosmic string loop decay across different symmetry-breaking scales, highlighting conditions for matching observed relic density.

Findings

For symmetry breaking scales >10^10 GeV, the mechanism can account for observed dark matter density.

For scales below 10^10 GeV, the produced dark matter density is insufficient.

Electroweak scale strings produce negligible dark matter via this process.

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 present 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 non-thermal particle production. In this work we compute the density of WIMP 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 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' gauge boson observable at the LHC, produce a negligibly small dark matter relic density by this mechanism.