Axion radiation from strings

Abstract

This paper revisits the problem of the string decay contribution to the axion cosmological energy density. We show that this contribution is proportional to the average relative increase when axion strings decay of a certain quantity N ax which we define. We carry out numerical simulations of the evolution and decay of circular and non-circular string loops, of bent strings with ends held fixed, and of vortex-antivortex pairs in two dimensions. In the case of string loops and of vortex-antivortex pairs, N ax decreases by approximately 20%. In the case of bent strings, N ax remains constant or increases slightly. Our results imply that the string decay contribution to the axion energy density is of the same order of magnitude as the well-understood contribution from vacuum realignment.

Axion dark matter from topological defects

“…[18,[40][41][42][43], it was claimed thatω a (t) is comparable to the horizon scale at the time t. However, authors of Refs. [44][45][46] suggested that the spectrum of radiated axions becomes hard because of a turbulent decay process, and thatω a (t) can become larger than the value of the order of the horizon scale. Later, the evolution of global strings in the expanding universe was investigated based on the field theoretic simulations in Refs.…”

Experimental Constraints on the Axion Dark Matter Halo Density

“…Conversely, if the axion mass is less than some value, they would have been overproduced in the early universe. This lower mass limit has been calculated for various axion production mechanisms under different earlyuniverse scenarios, e.g., "vacuum realignment" (Preskill, Wise, & Wilczek 1983;Abbott & Sikivie 1983;Dine & Fischler 1983), "string decay" (Battye & Shellard 1994;Yamaguchi, Kawasaki, & Yokoyama 1999;Hagmann, Chang, & Sikivie 2001), and "wall decay" (Chang, Hagmann, & Sikivie 1999). The vacuum misalignment mechanism provides a lower mass limit of ∼10 Ϫ6 eV, which we adopt for our search strategy; the other mechanisms produce a value that is in fairly close agreement.…”

Unifying inflation and dark matter with the Peccei-Quinn field: Observable axions and observable tensors

“…In the case where equation (20) is satisfied and PQ symmetry is restored after reheating the cosmic strings that are formed when it breaks again cannot be inflated away. The decay of these cosmic strings can then produce axions and contribute to the relic density [44,[49][50][51] with , (22) where the prefactor reflects various theoretical uncertainties regarding string decay and the QCD phase transition (see [45] for more details). This introduces a conservative upper bound on f a in order not to over produce DM of…”