Neutrino and axion bounds from the globular cluster M5 (NGC 5904)

The red-giant branch (RGB) in globular clusters is extended to larger brightness if the degenerate helium core loses too much energy in "dark channels." Based on a large set of archival observations, we provide high-precision photometry for the Galactic globular cluster M5 (NGC 5904), allowing for a detailed comparison between the observed tip of the RGB with predictions based on contemporary stellar evolution theory. In particular, we derive 95% confidence limits of $g_{ae}<4.3\times10^{-13}$ on the axion-electron coupling and $μ_ν<4.5\times10^{-12}\,μ_{\rm B}$ (Bohr magneton $μ_{\rm B}=e/2m_e$) on a neutrino dipole moment, based on a detailed analysis of statistical and systematic uncertainties. The cluster distance is the single largest source of uncertainty and can be improved in the future.

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