Missing in axion: Where are XENON1T’s big black holes?

Abstract We pioneer the black hole mass gap as a powerful new tool for constraining new particles. A new particle that couples to the Standard Model—such as an axion—acts as an additional source of loss in the cores of population-III stars, suppressing mass lost due to winds and quenching the pair-instability. This results in heavier astrophysical black holes. As an example, using stellar simulations we show that the solar axion explanation of the recent XENON1T excess implies astrophysical black holes of ∼ 56 M ⊙ , squarely within the black hole mass gap predicted by the Standard Model.