Detecting the $\pi$-axiverse through parametric resonance

Abstract

Axions are a leading dark matter candidate. In this work, we study the detectability of a multi-axion-like model, dubbed the $\pi$-axiverse, that is distinguishable from the string axiverse. The dark matter candidates are $N^2-1$ pseudo-Nambu-Goto modes (pion- and kaon-like states) stemming from spontaneous breaking of a global $SU(N)$ flavor symmetry. The low energy theory includes $N-1$ axionic couplings with additional couplings to the Standard Model photon kinetic energy, reminiscent of the string theory dilaton-photon coupling. We explore the parametric resonance of photons interacting with such a dark sector. Axions are well known to form macroscopic solitonic-like objects (axion stars), which experience instabilities due to overdensities stemming from mergers or accretion processes. The instabilities produce high-intensity bursts of radiation via parametric resonance that may be detected at observatories such as MeerKAT, the Square Kilometre Array (SKA), and the next generation Very Large Array (ngVLA). Using numerical methods, we systematically explore the multi-dimensional parameter space of the $\pi$-axiverse to search for regions where such signals are detectable, which generically differ from single axion models. We identify regions of the parameter space where MeerKAT, SKA, and ngVLA can resolve such signals, assessing the potential of transient searches to constrain the model. Our results provide a significant step forward in understanding the phenomenology and indirect detection of multi-axion-dilaton dark matter.