Stars as Laboratories for Fundamental Physics - MPP Theory Group

Chapter 12
Radiative Particle Decays
from Distant Sources
If neutrinos, axions, or other low-mass particles had radiative decay
channels, the decay photons would appear as x- or γ-ray fluxes from
stellar sources where these particles can be produced by nuclear or
plasma processes. This chapter is devoted to limits on such decays,
including decays into charged leptons, that are based on observational
limits on photon or positron fluxes from stellar sources, notably the Sun
and supernova 1987A. For comparison, laboratory and cosmological
limits are also reviewed.
12.1 Preliminaries
This book is largely about the properties of electrically neutral particles
whose electromagnetic interactions are correspondingly weak. However,
because they can virtually dissociate into charged states, they will still
interact with photons through higher-order amplitudes. The focus of
the present chapter is the possibility of radiative decays of the form
ν → ν ′ γ (neutrinos) or a → γγ (axions), but also ν → ν ′ e + e − and
ν → ν ′ e + e − γ. Cowsik (1977) was the first to recognize that the huge
path lengths available in the astrophysical environment allow one to
obtain much more restrictive limits on such decays than from laboratory
experiments. For example, the absence of single-photon counts in
a detector near a fission reactor indicates a bound 69 τ γ /m νe > 22 s/eV
69 In this chapter τ γ will always denote the partial neutrino decay time into radiation
while τ tot is the total decay time if hypothetical invisible channels are included.
Then τγ
−1 = B γ τtot −1 with the branching ratio B γ .
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