Isochrone fitting of Galactic globular clusters - IX. Tip of the red-giant branch for 27 clusters and constraints on new particle physics
Red-giant evolution is sensitive to non-standard channels of energy loss in stellar cores and therefore provides a probe of new physics. Such losses increase the luminosity of the tip of the red-giant branch (TRGB), the brightest point reached by a red giant in the color-magnitude diagram. Previous analyses of TRGB luminosities in globular clusters placed stringent bounds on the axion-electron coupling g_ae, but were limited by observational and theoretical uncertainties. To obtain a homogeneous and systematically controlled constraint, we use new parameters and TRGB estimates for 27 globular clusters, derived from Stetson, Hubble Space Telescope, and Gaia photometry and cluster-specific MESA calculations. Our primary likelihood uses 24 clusters after excluding three hot-flasher systems and adopts the brightest non-variable or well-sampled variable red giants. Finite sampling of the upper RGB is described by the full conditional distribution, and shared observational and stellar-physics uncertainties are retained as correlated nuisance parameters. A 95% power-constrained profile-likelihood limit is g_ae<6.4e-14. The median TRGB bolometric absolute magnitude is -3.54+-0.05 mag. We briefly discuss the implications for the neutrino magnetic dipole moment, millicharged particles, and other new physics.