Consequences of Multiple Axions in Theories with Dark Yang-Mills Groups

TL;DR

This paper explores the phenomenological implications of multiple axions arising from dark Yang-Mills sectors, analyzing constraints on dark confinement scales, axion interactions, and cosmological bounds in models with one or multiple Standard Model copies.

Contribution

It provides the first detailed analysis of multiple axions in theories with dark Yang-Mills groups, including bounds on confinement scales, axion mixing, and cosmological implications.

Findings

Dark confinement scale must be > 1 eV for dark matter axions.

Upper bound on number of SM copies from misalignment without fine-tuning.

Axion kinetic mixing leads to two distinct axion states with specific properties.

Abstract

General consistency requirements of Quantum Gravity demand the existence of one axion per Yang-Mills group. In this work, we consider theories with dark Yang-Mills sectors and investigate general phenomenological implications of these necessary axions. We carry out computations for two simple models, namely a pure Yang-Mills sector and $N$ exact Standard Model copies. For the former, the misalignment mechanism results in a minimal dark confinement scale $\Lambda_{\rm conf} \gtrsim 1 \, {\rm eV}$ if the dark sector axion is supposed to make up the dark matter. For the latter, the misalignment mechanism without fine-tuning of the initial misalignment angle places an upper bound on $N$ below the species bound. When the PQ symmetries are broken during inflation, the collective isocurvature fluctuations do not necessarily tighten the bound on the inflationary Hubble scale arising from a single axion. We also point out that axion stars collectively made from axions of different dark sectors with a suppressed mass spectrum are not possible. Lastly, for the two models at hand, intersector interaction through axion kinetic mixing leads to the existence of two distinct axion states. For a single dark YM sector, the upper bound $\Lambda_{\rm conf} \lesssim 10^{12} \, {\rm GeV}$ emerges from the stability requirement of the dark sector axion. For $N$ exact SM copies, the mass and photon coupling of the second state is completely determined after a potential measurement of the analogous parameters of the first axion.