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

Particle-physics constraints from the globular cluster M5: Neutrino Dipole Moments