QCD Axion Dark Matter in String Theory: Haloscopes and Helioscopes as Probes of the Landscape

TL;DR

This paper explores how laboratory searches for the QCD axion can constrain string theory models by analyzing a large set of Calabi-Yau compactifications and correlating axion properties with topological features.

Contribution

It provides the first comprehensive scan of axion masses across extensive string theory compactifications and links experimental detection prospects to the string landscape.

Findings

Axion mass correlates with the topological Hodge number $h^{1,1}$.

Detection at any mass range disfavors 80% of models with $h^{1,1}=491$.

Solar axion measurements are key probes for models with high $h^{1,1}$.

Abstract

Laboratory experiments have the capacity to detect the QCD axion in the next decade, and precisely measure its mass, if it composes the majority of the dark matter. In type IIB string theory on Calabi-Yau threefolds in the geometric regime, the QCD axion mass, $m_a$, is strongly correlated with the topological Hodge number $h^{1,1}$. We compute $m_a$ in a scan of $185{,}965$ compactifications of type IIB string theory on toric hypersurface Calabi-Yau threefolds. We compute the range of $h^{1,1}$ probed by different experiments under the condition that the QCD axion can provide the observed dark matter density with minimal fine-tuning. Taking the experiments DMRadio, ADMX, MADMAX, and BREAD as indicative on different mass ranges, the $h^{1,1}$ distributions peak near $h^{1,1}=24.9, \ 65.4, \ 196.8,$ and $310.9,$ respectively. We furthermore conclude that, without severe fine tuning, detection of the QCD axion as dark matter \emph{at any mass} disfavours 80% of models with $h^{1,1} =491$, which is thought to have the most known Calabi-Yau threefolds. Measurement of the solar axion mass with IAXO is the dominant probe of all models with $h^{1,1}\gtrsim 250$. This study demonstrates the immense importance of axion detection in experimentally constraining the string landscape.