Particle-physics constraints from the globular cluster M5: neutrino dipole moments ^⋆

¹ Pontificia Universidad Católica de Chile, Departamento de Astronomía y Astrofísica, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
e-mail: nviaux@astro.puc.cl
² Pontificia Universidad Católica de Chile, Centro de Astroingeniería, Av. Vicuña Mackena 4860, 782-0436 Macul, Santiago, Chile
³ The Milky Way Millennium Nucleus, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
⁴ National Research Council, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
⁵ Max-Planck-Institut für Physik (Werner-Heisenberg-Institut), Föhringer Ring 6, 80805 München, Germany
⁶ Arnold Sommerfeld Center, Ludwig-Maximilians-University, Theresienstr. 37, 80333 München, Germany
⁷ Universidade Federal do Rio Grande do Norte, Depto. de Física, 59072-970 Natal, RN, Brazil
⁸ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany

Received: 31 May 2013
Accepted: 19 August 2013

Abstract

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 μ_ν 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 μ_ν 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 (NGC 5904) and carefully determine its TRGB position. On the theoretical side, we add the extra plasmon decay rate brought about by μ_ν to the Princeton-Goddard-PUC (PGPUC) 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 μ_ν < 2.6 × 10^-12  μ_B (68% CL), where μ_B ≡ e/2m_e is the Bohr magneton, and μ_ν < 4.5 × 10^-12  μ_B (95% CL). In these results, statistical and systematic errors have been combined in quadrature.