New bounds on light scalars from the tip of the red giant branch
Abstract
Light, weakly coupled particles can be copiously produced in the hot and dense interior of stars, and the resulting energy loss can leave observable imprints on entire stellar populations. For instance, additional energy loss from red giant cores delays helium ignition and brightens the tip of the red giant branch (TRGB). We revisit the TRGB bounds on light scalars coupled to electrons, of which the Higgs portal is the prototypical example. In the degenerate cores of red giants, scalar emission is dominated by resonant conversion of longitudinal plasmons, which we compute using finite-temperature field theory. Rather than rescaling neutrino emission rates at fixed benchmark core conditions, we implement this emissivity in the MESA stellar evolution code and self-consistently simulate the evolution of red giants, including the backreaction of the scalar energy loss on the stellar structure. Comparing our predictions with the observed bolometric TRGB magnitudes of 27 Milky Way globular clusters, we exclude scalar-electron couplings αϕee = ge2/4π> 1.6 × 10−34 at 95% confidence for sub-keV masses, improving on previous bounds by nearly two orders of magnitude. In the context of the Higgs portal, this corresponds to an exclusion on values of the mixing angle sinθ> 2.2× 10−11, providing the tightest bound for scalar masses in the ∼ 1 eV − 10 keV range.
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BibTeX
@misc{debru2026bounds,
title = {{New bounds on light scalars from the tip of the red giant branch}},
author = {Natnael Debru and Audrey Fung and Saniya Heeba and Hugo Schérer and Katelin Schutz and Aaron C. Vincent},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.22638},
eprint = {2609.22638},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.22638},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - New bounds on light scalars from the tip of the red giant branch
AU - Natnael Debru
AU - Audrey Fung
AU - Saniya Heeba
AU - Hugo Schérer
AU - Katelin Schutz
AU - Aaron C. Vincent
PY - 2026
DA - 2026/09/18
PB - arXiv
DO - 10.48550/arxiv.2609.22638
UR - https://arxiv.org/abs/2609.22638
AB - Light, weakly coupled particles can be copiously produced in the hot and dense interior of stars, and the resulting energy loss can leave observable imprints on entire stellar populations. For instance, additional energy loss from red giant cores delays helium ignition and brightens the tip of the red giant branch (TRGB). We revisit the TRGB bounds on light scalars coupled to electrons, of which the Higgs portal is the prototypical example. In the degenerate cores of red giants, scalar emission is dominated by resonant conversion of longitudinal plasmons, which we compute using finite-temperature field theory. Rather than rescaling neutrino emission rates at fixed benchmark core conditions, we implement this emissivity in the MESA stellar evolution code and self-consistently simulate the evolution of red giants, including the backreaction of the scalar energy loss on the stellar structure. Comparing our predictions with the observed bolometric TRGB magnitudes of 27 Milky Way globular clusters, we exclude scalar-electron couplings α_ϕee = g_e^2/4π> 1.6 × 10^−34 at 95% confidence for sub-keV masses, improving on previous bounds by nearly two orders of magnitude. In the context of the Higgs portal, this corresponds to an exclusion on values of the mixing angle sinθ> 2.2× 10^−11, providing the tightest bound for scalar masses in the ∼ 1 eV − 10 keV range.
ER -