Isochrone fitting of Galactic globular clusters - IX. Tip of the red-giant branch for 27 clusters and constraints on new particle physics
Abstract
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, together with cluster-specific MESA calculations. Our primary likelihood uses 24 clusters after excluding three hot-flasher systems and explicitly accounts for stellar variability in identifying the brightest RGB stars. 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. The observed TRGB is fainter than the standard-cooling prediction by 0.078 (+0.036,-0.037) mag; to avoid an artificially strong bound from this downward fluctuation, we use the power-constrained profile likelihood method and obtain g_{ae}<6.4e-14 (95% CL). Sensitivity estimates for the neutrino magnetic dipole moment and for millicharged particles are also presented.
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BibTeX
@misc{gontcharov2026isochrone,
title = {{Isochrone fitting of Galactic globular clusters - IX. Tip of the red-giant branch for 27 clusters and constraints on new particle physics}},
author = {G. A. Gontcharov and A. M. Kudashov and O. S. Ryutina and S. V. Troitsky},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2608.04801},
eprint = {2608.04801},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2608.04801},
}
RIS
TY - UNPB
TI - Isochrone fitting of Galactic globular clusters - IX. Tip of the red-giant branch for 27 clusters and constraints on new particle physics
AU - G. A. Gontcharov
AU - A. M. Kudashov
AU - O. S. Ryutina
AU - S. V. Troitsky
PY - 2026
DA - 2026/10/22
PB - arXiv
DO - 10.48550/arxiv.2608.04801
UR - https://arxiv.org/abs/2608.04801
AB - 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, together with cluster-specific MESA calculations. Our primary likelihood uses 24 clusters after excluding three hot-flasher systems and explicitly accounts for stellar variability in identifying the brightest RGB stars. 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. The observed TRGB is fainter than the standard-cooling prediction by 0.078 (+0.036,-0.037) mag; to avoid an artificially strong bound from this downward fluctuation, we use the power-constrained profile likelihood method and obtain g_{ae}<6.4e-14 (95% CL). Sensitivity estimates for the neutrino magnetic dipole moment and for millicharged particles are also presented.
ER -