Axion strings are superconducting

Abstract

We explore the cosmological consequences of the superconductivity of QCD axion strings. Axion strings can support a sizeable chiral electric current and charge density, which alters their early universe dynamics. We examine the possibility that shrinking axion string loops can become effectively stable remnants called vortons, supported by the repulsive electromagnetic force of the string current. We find that vortons in our scenario are generically unstable, and so do not pose a cosmological difficulty. Furthermore, if a primordial magnetic field (PMF) exists in the early universe, a large current is induced on axion strings, creating a significant drag force from interactions with the surrounding plasma. As a result, the strings are slowed down, which leads to an orders of magnitude enhancement in the number of strings per Hubble volume. Finally, we study potential implications for the QCD axion relic abundance. The QCD axion window is shifted by orders of magnitude in some parts of our parameter space.

Electroweak axion string and superconductivity

“…Our present argument that the string in the DFSZ model can carry a large amount of the supercurrent using the bosonic zero mode allows the studies in Ref. [82] to be applied to the DFSZ model as well. Thus our present work is complementary to theirs.…”

“…After we completed the present work, an analysis on cosmological consequences of the superconducting axion string appeared in Ref. [82], in which the authors discussed the KSVZ model as a benchmark model. As we stated in introduction, the DFSZ model has no heavy fermion, and thus the supercurrent carried by fermionic zero modes is not significant compared to the string tension.…”

Axion string signatures: a cosmological plasma collider

“…Collisions between these bound state particle and the zero mode fermions can scatter the zero mode fermions, and therefore charge overdensities, efficiently into a bulk fermions, discharging the string. Thus, the axion string core never obtains a charge density larger than m P Q , which is required to stabilize axion string loops, aka vortons [29][30][31]. Therefore, macroscopic axionic vortons which couple to gauge fields with light charged matter cannot be stabilized by charge or current.…”

Axiverse Strings

“…In particular, as some of the axiverse ALP masses can be as small as 10 −20 eV or even H 0 ∼ 10 −33 eV (giving a favoured implementation of quintessence) [19], the network can survive to the post-recombination epoch, or even the present. If the axion couples to electromagnetism via aF F then strings can give rise to quantised polarisation rotation of CMBR photons, as well as zero modes mandated by index theorems and associated forms of superconductivity [31][32][33][34][54][55][56][57][58].…”

“…Cosmologically, however, the inter-string separation provides an IR cutoff, and a network of such strings and domain walls is important for both relic axion DM production [22][23][24][25][26] and the generation of stochastic gravitational wave backgrounds [21,[27][28][29][30]. They can also lead to a variety of other striking, potentially observable, phenomena [31][32][33][34].…”