ALP Dark Matter Mini-Clusters from Kinetic Fragmentation

Abstract

We show that very compact axion mini-clusters can form in models where axion-like-particle (ALP) dark matter is produced via the kinetic misalignment mechanism, which is well-motivated in pre-inflationary U (1) symmetry breaking scenarios. This is due to ALP fragmentation. We predict denser halos than what has been obtained so far in the literature from standard misalignment in post-inflationary U (1) breaking scenarios or from large misalignment. The main reason is that adiabatic fluctuations are significant at early times, therefore, even if amplification from parametric resonance effects is moderate, the final size of ALP fluctuations is larger in kinetic misalignment. We compare halo mass functions and halo spectra obtained in kinetic misalignment, large misalignment and standard misalignment respectively. Our analysis does not depend on the specific model realization of the kinetic misalignment mechanism. We present our results generally as a function of the ALP mass and the ALP decay constant only. We show that a sizable region of this ALP parameter space can be tested by future experiments that probe small-scale structures.

ALP dark matter from kinetic fragmentation: opening up the parameter window

“…One main model-independent consequence is a distinct prediction for dense compact miniclusters that is presented in detail in a companion paper [26]. There is also a stochastic gravitational wave background generated by axion fragmentation as we discuss in our section 6.…”

“…The distinctive feature in this scenario is the momentum distribution of the axion particles. Ultimately, our analysis sets the basis to derive the phenomenological implications of this modified power spectrum, which will be presented in [26]. We will provide predictions in the explicit UV completions in another companion publication [27], deriving the precise model parameter space regions for ALP dark matter through kinetic fragmentation.…”

“…If the only thing we want to know is whether at a given point on the ALP parameter space the relic density is dominated by fragmented axions or by the homogeneous mode, then the back-reaction effects might not be important. However, ultimately we are interested in observational consequences of fragmentation such as miniclusters [26]. For this JCAP10(2022)053 we need to calculate the density power spectrum of the ALP field after fragmentation, and in order to get accurate results we need to take the back-reaction into the account.…”

QCD Axion Kinetic Misalignment without Prejudice

“…It has been shown that fragmentation might play an important role in the dynamics, as it introduces an additional scale which could alter the description above [125]. For a model of the axion-like particle, fragmentation occurs in a sizable part of the parameter space, leading to heavier miniclusters than in the standard scenario [126]. Fragmentation is expected to occur also for the QCD axion [125], although a study of this is not yet available.…”