  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 $\mu_\nu<4.5\times10^{-12}\,\mu_{\rm B}$ (Bohr
magneton $\mu_{\rm B}=e/2m_e$) 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.
