Axion fragmentation

Abstract

We investigate the production of axion quanta during the early universe evolution of an axion-like field rolling down a wiggly potential. We compute the growth of quantum fluctuations and their back-reaction on the homogeneous zeromode. We evaluate the transfer of kinetic energy from the zero mode to the quantum fluctuations and the conditions to decelerate the axion zero-mode as a function of the Hubble rate, the slope of the potential, the size of the barriers and the initial field velocity. We discuss how these effects impact the relaxion mechanism. arXiv:1911.08472v1 [hep-ph] 19 Nov 2019 1 As this work was being completed, Ref.[15] appeared, which considers the production of axion quantum fluctuations during oscillations about the minimum of the potential, as in [14]. This effect is important only if the initial position of the axion field is tuned very close to the top of the barrier of the axion potential, at the level of ∼ 10 −7 . Such peculiar initial position was recently motivated by some inflation dynamics in [16] and by anthropic arguments in [17].

Relaxion fluctuations (self-stopping relaxion) and overview of relaxion stopping mechanisms

“…This effect is studied in detail in ref. [5] and is relevant in the regime where the relaxion initial velocity is large enough to overpass the potential barriers. We summarise the main results in this section.…”

“…Ref. [5] shows that (in general for any rolling axion-like field) the dynamics of axion fluctuations accompanying the evolution of the axion zero-mode rolling down its potential while passing through a large number of wiggles can be described by the Mathieu equation, -3 -…”

“…The purpose of this work is to work out the implications of relaxion particle production on the relaxion mechanism. Our conclusions are based on the results of a companion paper [5] that is general and applies to any axion-like particle in the early universe subject to rolling along a potential which also features a large number of wiggles, as ubiquitous in string axion monodromy models for instance. If the field fast-rolls and overpasses many barriers, it can produce an exponentially large number of its quanta which backreact on the homogeneous field.…”

“…In section 2, we summarise the main results of [5]. Concretely, the only inputs from [5] which we use in this analysis are the expression for the time scale of efficient relaxion fragmentation ∆t frag , the field excursion during this time ∆φ frag , and the conditions on the Hubble expansion rate and on the potential slope which allow efficient relaxion fragmentation. In section 3, we take the original relaxion proposal where the barriers of the relaxion periodic potential are generated by a new (non-QCD) strongly interacting sector and are Higgs-dependent.…”

Searching for Earth/Solar axion halos

“…It is known that throughout the cosmological history, axions may form gravitationally-bound objects, whose density can be orders of magnitude larger than the local dark matter density. Typical examples include axion miniclusters [23,24] and boson stars [25][26][27] (see also [28][29][30][31] for recent discussions). Being much denser than the average galactic DM density, these small-scale objects could boost the discovery potential of aforementioned axion dark matter experiments, should they exist.…”