Axions from strings: the attractive solution

By Marco Gorghetto, Edward Hardy, G. Villadoro. Journal of High Energy Physics, 2018

266 citations recorded by the source corpus.

A bstractWe study the system of axion strings that forms in the early Universe if the Peccei-Quinn symmetry is restored after inflation. Using numerical simulations, we establish the existence of an asymptotic solution to which the system is attracted independently of the initial conditions. We study in detail the properties of this solution, including the average number of strings per Hubble patch, the distribution of loops and long strings, the way that different types of radiation are emitted, and the shape of the spectrum of axions produced. We find clear evidence of logarithmic violations of the scaling properties of the attractor solution. We also find that, while most of the axions are emitted with momenta of order Hubble, most of the axion energy density is contained in axions with energy of order the string core scale, at least in the parameter range available in the simulation. While such a spectrum would lead to a negligible number density of relic axions from strings when extrapolated to the physical parameter region, we show that the presence of small logarithmic corrections to the spectrum shape could completely alter such a conclusion. A detailed understanding of the evolution of the axion spectrum is therefore crucial for a reliable estimate of the relic axion abundance from strings.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/3663fa80d0f63c8baca0982958e261a7ffea1508

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Citations in the corpus

The landscape of QCD axion models (2020, background): “[200] find an axion spectrum peaked at the energy of the order of the string core scale, which would correspond to a negligible number density of relic axions from strings.”
Early seeds of axion miniclusters (2018, methodology): “However, we note that [65] and [64] disagree on the mean radiated axion energy when they simulate “standard” strings.”
Beyond the Standard Models with cosmic strings (2019, background): “generate the correct Dark Matter abundance from Goldstone production. However, the value of the energy of the emitted Goldstone, needed to compute the axion relic abundance, is still matter of debate [84]. Particularly, the precise dependence of the emitted mean energy with the infrared cut-o H 1 is”
More axions from strings (2020, background): “Although the spectrum is mostly UV dominated in the range of parameters that can be reached by present simulations [7], we find clear evidence of a non-trivial running of the spectral index, which is more compatible with an IR dominated spectrum once extrapolated to the physical parameters.”
Physics potential of the International Axion Observatory (IAXO) (2019, background): “14) corresponds to K ≈ 13–54, while it was claimed in [115] that its value can differ by many orders of magnitude according to different interpretations of simulation results.”