New bounds on light millicharged particles from the tip of the red-giant branch

TL;DR

This paper refines stellar bounds on light millicharged particles by self-consistently modeling their emission and impact on stellar evolution, leading to improved constraints based on observations of globular clusters.

Contribution

It introduces a comprehensive modeling approach for MCP emission and stellar evolution, providing more accurate bounds on MCP properties from red-giant branch observations.

Findings

MCP emission causes significant brightening of the red-giant tip.

New bounds exclude MCPs with masses below 100 keV.

Enhanced modeling improves the robustness of stellar constraints.

Abstract

Stellar energy loss is a sensitive probe of light, weakly coupled dark sectors, including ones containing millicharged particles (MCPs). The emission of MCPs can affect stellar evolution, and therefore can alter the observed properties of stellar populations. In this work, we improve upon the accuracy of existing stellar limits on MCPs by self-consistently modelling (1) the MCP emission rate, accounting for all relevant in-medium effects and production channels, and (2) the evolution of stellar interiors (including backreactions from MCP emission) using the MESA stellar evolution code. We find MCP emission leads to significant brightening of the tip of the red-giant branch. Based on photometric observations of 15 globular clusters whose bolometric magnitudes are inferred using parallaxes from Gaia astrometry, we obtain robust bounds on the existence of MCPs with masses below 100 keV.