Possible Implications of QCD Axion Dark Matter Constraints from Helioscopes and Haloscopes for the String Theory...

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 185965 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 at any mass disfavors 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}≳250. This Letter demonstrates the immense importance of axion detection in experimentally constraining the string landscape.