Missing in Action: New Physics and the Black Hole Mass Gap

TL;DR

This paper explores how new light particles could alter stellar evolution and black hole formation, potentially expanding the black hole mass gap and impacting gravitational wave observations.

Contribution

It introduces the idea that new light particles can suppress pair instability, allowing formation of heavier black holes within the mass gap, a novel probe of fundamental physics.

Findings

Heavy black holes (>72 solar masses) can form inside the mass gap due to new particles.

The upper edge of the black hole mass gap can exceed 130 solar masses.

New instabilities may occur in stars with heavy particles, affecting black hole formation.

Abstract

We demonstrate the power of the black hole mass gap as a novel probe of fundamental physics. New light particles that couple to the Standard Model can act as an additional source of energy loss in the cores of population-III stars, dramatically altering their evolution. We investigate the effects of two paradigmatic weakly coupled, low-mass particles, axions and hidden photons, and find that the pulsational pair instability, which causes a substantial amount of mass loss, is suppressed. As a result, it is possible to form black holes of $72\msun$ or heavier, deep inside the black hole mass gap predicted by the Standard Model. The upper edge of the mass gap is raised to $>130{\rm M}_\odot$, implying that heavier black holes, anticipated to be observed after LIGO's sensitivity is upgraded, would also be impacted. In contrast, thermally produced heavy particles would remain in the core, leading to the tantalizing possibility that they drive a new instability akin to the electron-positron pair instability. We investigate this effect analytically and find that stars that avoid the electron-positron pair instability could experience this new instability. We discuss our results in light of current and upcoming gravitational wave interferometer detections of binary black hole mergers.