  Stellar evolution is modified if energy is lost in a "dark channel" similar
to neutrino emission. Comparing modified stellar evolution sequences with
observations provides some of the most restrictive limits on axions and other
hypothetical low-mass particles and on non-standard neutrino properties. In
particular, a putative neutrino magnetic dipole moment mu_nu enhances the
plasmon decay process, postpones helium ignition in low-mass stars, and
therefore extends the red-giant branch (RGB) in globular clusters (GCs). The
brightness of the tip of the RGB (TRGB) remains the most sensitive probe for
mu_nu and we revisit this argument from a modern perspective. Based on a large
set of archival observations, we provide high-precision photometry for the
Galactic GC M5 (NGC5904) and carefully determine its TRGB position. On the
theoretical side, we add the extra plasmon decay rate brought about by mu_nu to
the Princeton-Goddard-PUC stellar evolution code. Different sources of
uncertainty are critically examined. The main source of systematic uncertainty
is the bolometric correction and the main statistical uncertainty derives from
the distance modulus based on main-sequence fitting. (Other measures of
distance, e.g., the brightness of RR Lyrae stars, are influenced by the energy
loss that we wish to constrain.) The statistical uncertainty of the TRGB
position relative to the brightest RGB star is less important because the RGB
is well populated. We infer an absolute I-band brightness of M_I=-4.17+/-0.13
mag for the TRGB compared with the theoretical prediction of -3.99+/-0.07 mag,
in reasonable agreement with each other. A significant brightness increase
caused by neutrino dipole moments is constrained such that
mu_nu<2.6x10^-12mu_B(68% CL), where mu_B is the Bohr magneton, and
mu_nu<4.5x10^-12 mu_B(95% CL). In these results, statistical and systematic
errors have been combined in quadrature.
