New physics and the black hole mass gap

In the Standard Model, the so-called pair instability from creation of electron-positron pairs reduces the mass of low-metallicity population-III stars as they collapse, unless they are heavy enough to make the process inefficient, leading to an expected mass gap in the black hole spectrum. In this work, the authors argue that future observations of the black hole population will allow one to test this prediction, and use it to put constraints on new particles, such as axions, that would cause an additional instability.