Axion string signatures: a cosmological plasma collider

Abstract

We study early and late time signatures of both QCD axion strings and hyperlight axion strings (axiverse strings). We focus on charge deposition onto axion strings from electromagnetic fields and subsequent novel neutralizing mechanisms due to bound state formation. While early universe signatures appear unlikely, there are a plethora of late time signatures. Axion strings passing through galaxies obtain a huge charge density, which is neutralized by a dense plasma of bound state Standard Model particles forming a one dimensional “atom”. The charged wave packets on the string, as well as the dense plasma outside, travel at nearly the speed of light along the string. These packets of high energy plasma collide with a center of mass energy of up to 109 GeV. These collisions can have luminosities up to seven orders of magnitude larger than the solar luminosity, and last for thousands of years, making them visible at radio telescopes even when they occur cosmologically far away. The new observables are complementary to the CMB observables for hyperlight axion strings that have been recently proposed, and are sensitive to a similar motivated parameter range.

Observing Invisible Axions with Gravitational Waves

“…The black points correspond to the simulations used in this work, while for comparison we also show data from [17,21] starting from different initial conditions and reaching larger log. 60 As discussed in [17,21], the data clearly rules out any The initial conditions that we use for the GW analysis, starting close to the attractor, are plotted in black. The evolution of the system starting from other initial conditions (studied in [17,21]) is shown for comparison.…”

“…Moreover, friction on the string due to interactions with the thermal bath, which is relevant at high temperatures if the axion couples to standard model particles (but we do not include in our analysis of the scaling regime), will be negligible by this point for axion decay constants that lead to observable signals. In particular, the friction is expected to be irrelevant for temperatures 10 11 GeV(f a /10 14 GeV) 2 [56], which is safely far above those corresponding to observable frequencies (further analysis would be required to determine the temperature at which friction becomes irrelevant precisely, see also [57][58][59][60]).…”

“…See[147] for an analysis demonstrating that the lattice version of the projector does not introduce systematic errors 60. As mentioned, in our present simulations, evolving the fields u ij adds to the computational cost, so to gain sufficient statistics we use slightly smaller grids.…”

Measures of non-Gaussianity in axion-string-induced CMB birefringence

“…The important aspect of birefringence from axion strings [6] is that the typical axion field excursion is large ∆a ≈ 2πf a , thereby evading a suppression factor that appears for other models, such as axion dark matter. Several recent studies [19][20][21][22][23][24][25][26] have explored the signatures of axion-string-(and domain wall)-induced birefringence, calculated the angular power spectrum, and assessed compatibility with the various measurements of CMB birefringence (including a claimed detection of isotropic birefringence [27][28][29][30][31][32]). To summarize, these studies conclude that the current generation of CMB telescopes (Planck, SPTpol, ACTpol,…”

Chiral fermions on lattice axion strings

“…These systems are of interest to both high energy and condensed matter physics. For instance, cosmic axion strings are considered to be important for astrophysical signature of new physics at high energy [2][3][4][5][6]. They are also of relevance to the physics of early universe.…”