Millicharged Particle Constraints from Asymptotic Giant Branch Stars

We investigate the effect of millicharged particles (MCPs) with electric charge $qe\ll e$ and mass $m_{\chi }$ on the late-stage evolution phases of low-mass stars in globular clusters. We predict the $R_2$ parameter – the ratio of the number of stars in the asymptotic giant branch (AGB) phase to the number of stars in the horizontal branch (HB) phase – and compare it against globular cluster data. While the production of MCPs shortens both the HB and AGB lifetimes, a larger reduction in the AGB phase arises from the higher temperatures in the helium-burning shell. We find the strongest bounds in the range $10\mathrm{k}\mathrm{e}\mathrm{V}\lesssim m_{\chi }\lesssim 100\mathrm{k}\mathrm{e}\mathrm{V}$, reaching charges as small as $q\simeq 5\times {10}^{-13}$ and surpassing existing constraints by up to two orders of magnitude.