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The Black Hole Mass Gap as a New Probe of Millicharged Particles

Damiano F. G. Fiorillo, Giuseppe Lucente, Jeremy Sakstein, Edoardo Vitagliano, Matteo Cantiello

Abstract

We investigate the impact of millicharged particles (MCPs) on massive stars undergoing pulsational pair-instability supernovae and on the location of the lower edge of the black hole mass gap. We find that energy losses due to MCP emission weaken the pulsations, allowing the star to retain more mass and thereby shifting the lower edge of the mass gap to higher black hole masses. The mass gap is sensitive to a region of MCP parameter space with masses $35\,{\rm keV}\lesssim m_χ\lesssim 200\,{\rm keV}$ and charges $10^{-10}\lesssim q \lesssim 10^{-9}$, which remains unconstrained by existing astrophysical probes. If confirmed, recent gravitational wave observations placing the lower edge of the mass gap near $45\,{\rm M}_\odot$ would translate directly into bounds on this parameter space.

The Black Hole Mass Gap as a New Probe of Millicharged Particles

Abstract

We investigate the impact of millicharged particles (MCPs) on massive stars undergoing pulsational pair-instability supernovae and on the location of the lower edge of the black hole mass gap. We find that energy losses due to MCP emission weaken the pulsations, allowing the star to retain more mass and thereby shifting the lower edge of the mass gap to higher black hole masses. The mass gap is sensitive to a region of MCP parameter space with masses and charges , which remains unconstrained by existing astrophysical probes. If confirmed, recent gravitational wave observations placing the lower edge of the mass gap near would translate directly into bounds on this parameter space.

Paper Structure

This paper contains 1 section, 2 equations, 3 figures.

Table of Contents

  1. Acknowledgments

Figures (3)

  • Figure 1: Sensitivity of the Black Hole Mass Gap (BHMG) to the fractional charge $q$ as a function of the MCP mass $m_\chi$. The hatched region will be excluded if the location of the lower edge of the BHMG is confirmed to be $45^{+5}_{-4}\,{\rm M}_\odot$ ($90\%$ credibility), as inferred from a recent analysis of the LVK GWTC-4 catalog Tong:2025wpz. Previous constraints from the cooling of SN 1987A Fiorillo:2024upk and the tip of the red-giant branch (TRGB) Fung:2023euv are shown in gray.
  • Figure 2: Black hole mass as a function of initial helium core mass for fixed $q=10^{-9}$ with varying MCP mass $m_\chi$ (left), and for fixed $m_\chi=75$ keV with varying $q$ (right). Both panels were obtained assuming median $^{12}{\rm C}(\alpha,\gamma)^{16}{\rm O}$ rates.
  • Figure 3: Lower edge of the black hole mass gap as a function of the MCP mass $m_\chi$ ($x$-axis) and $q$ assuming the $+3\sigma$$^{12}{\rm C}(\alpha,\gamma)^{16}{\rm O}$ rate. Each colored point corresponds to a different value of $q$ indicated in the figure. The black dashed line is the Standard Model prediction, and the gray region indicates the 95% C.L. reported by Tong:2025wpz. The hatched red region in Fig. \ref{['fig:bounds']} was found by interpolating each colored curve and finding the value of $q$ for which it intersects the gray region.