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Comment on Neutrino Radiative Decay Limits from the Infrared Background

Recent observations of TeV gamma rays from distant sources provide new limits on the cosmic infrared background and thus on radiative decays of background neutrinos. I translate these limits into bounds on transition dipole moments and compare them with limits from the energy-loss of globular-cluster stars. The latter are more restrictive for neutrino masses below 1-2 eV.

Recent observations of TeV γ rays from the active galaxies Markarian 421 and 501 have provided new limits on the cosmic background of infrared photons which are an efficient opacity source due to the pair process γ TeV γ infrared → e + e − . In a recent Letter New Limits to the Infrared Background: Bounds on Radiative Neutrino Decay and on Contributions of Very Massive Objects to the Dark Matter Problem [1] these limits were interpreted, inter alia, as limits on the radiative decay of cosmic background neutrinos with sub-eV masses. However, it was overlooked that in this mass range much more restrictive limits on neutrino radiative decay channels already exist, preventing neutrino decays from contributing significantly to the infrared background.
Neutrino dipole and transition moments are also constrained to avoid excessive stellar energy-losses by the plasmon decay process γ → νν, an idea going back to Ref. [3], with more recent refinements in Refs. [4][5][6]. For neutrino masses below a few keV the limit is µ eff < ∼ 3 × 10 −12 µ B , shown as a hatched bar across Fig. 1. (For Dirac neutrinos the limit would be slightly more restrictive because the number of final states in γ → νν doubles.) The m 3 ν phase-space factor in Eq. (1) is so punishing that the direct radiative decay limits quickly become irrelevant for m ν < ∼ 2 eV. The recent indications for neutrino masses from the solar and atmospheric neutrino anomalies, the LSND experiment, and the cosmological hypothesis of hot plus cold dark matter suggest that neutrino masses are indeed below 1-2 eV.

FIG. 1. Exclusion range for neutrino transition moments.
The light-shaded region was previously excluded from the contribution of radiatively decaying neutrinos to the cosmic photon background radiations [2], the dark-shaded region is the new exclusion range from the infrared background, case 10 TeV [1]. Values above the hatched bar are excluded from the plasmon decay process in globular-cluster stars [4][5][6].
Therefore, unless something is unexpectedly wrong with the globular-cluster limit, radiative neutrino decays cannot produce infrared background photons at the level of current detection limits. Interpreting measurements of photon backgrounds as constraints on radiative neutrino decays is still important, of course, as it is an independent method with other experimental and theoretical uncertainties than the stellar-evolution arguments.
I thank Dr. S. Biller (for the authors of Ref. [1]) for a collegial correspondence where they express agreement with my analysis. Partial support by the Deutsche Forschungsgemeinschaft under grant No. SFB 375 is acknowledged.