Neutrino and Axion Bounds from the Globular Cluster M5 (NGC 5904)

Abstract

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)<4.3×10(-13) on the axion-electron coupling and μ(ν)<4.5×10(-12)μ(B) (Bohr magneton μ(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.

The brightness of the red giant branch tip

“…We predict M I = −4.14 ± 0.12 or −4.07 ± 0.12 for the RGB-tip depending on whether the Worthey & Lee (2011) or MARCS BCs are used, well within the 1σ observational band M I = −4.17 ± 0.13 determined by Viaux et al (2013b) for M5 that is dominated by a 0.11 uncertainty in the distance modulus. This would basically remove any need for additional cooling, and therefore put very low limits on a possible magnetic dipole moment of neutrinos or the mass of axions, as investigated by Viaux et al (2013b) and Viaux et al (2013a).…”

“…Therefore, the dependence of the prediction on these choices are crucial as well. Raffelt & Weiss (1992) and Viaux et al (2013a) have studied the impact of theoretical uncertainties in stellar models have on photometric predictions, but only for one specific choice of stellar mass and composition in each work. We will review this in Sect.…”

3.55 keV hint for decaying axionlike particle dark matter

“…Interestingly, the emission of ALPs with a decay constant at the upper end of this range from Red Giant (RG) stars, could be responsible for the small amount of nonstandard stellar cooling that seems to be present in the RG population of the globular cluster M5 [14], if we use 2.14) and compare to the upper limit of g aee ¼ 4.3 × 10 −13 . However, the extra cooling observed in white dwarfs [15][16][17] …”

No axions from the Sun

“…A second weak point of the Fraser et al interpretation is the specific choice of the axion-electron coupling constant gae ≃ 2.2 · 10 −12 , which exceeds by about a factor of 5 the most recent upper bound gae < 4.3 · 10 −13 (Viaux et al 2013). A similar problem -even if less significant -concerns the axion-photon coupling constant gaγ ≃ 10 −10 , since the most recent upper bound is gaγ < 0.6 · 10 −10 (Ayala et al 2014).…”