Annu. Rev. Astron. Astrophys. 1991. 29: 59-88 Copyright © 1991 by Annual Reviews. All rights reserved

6.1 The Radiative Decay of Massive Neutrinos and Other Particles

Stecker (1980) and Kimble, Bowyer, and Jakobsen (1981) used the intensity of the far ultraviolet background to derive constraints on the radiative lifetime of massive neutrinos, assuming these particles form a uniform cosmic sea of material. Although these (and later) authors framed their discussion in terms of massive neutrinos, their arguments are valid for any particles conforming to the underlying assumptions. These authors assumed that radiative decay dominates the decay routes available (this is the favored route in many of the relevant particle physics models), and the secondary particle is very light compared to the parent (as would be expected for neutrinos given the limited number of neutrino families). Neutrinos in the mass range 10-100 ev/c² were considered, which covered the range of mass suggested by the available experimental data. Stecker interpreted a possible step in the spectrum of the far ultraviolet background as the product of neutrino decay and derived a lifetime for this decay. Given subsequent data, this result is surely incorrect. Kimble et al. examined the range of lifetimes for which the far ultraviolet data were relevant; they included the effects of attenuation of the decay radiation through a clumpy intergalactic medium. These authors were able to obtain upper limits to the radiative decay of the neutrino for lifetimes between 10¹³ and 10²² sec. Martin, Hurwitz, and Bowyer (1990) searched for an edge, or step, in their lowest intensity, high Galactic latitude spectrum obtained with the Berkeley UVX experiment and were able to improve upon the results of Kimble et al. by a factor of about three.

Shipman and Cowsik (1981) pointed out that observations of assemblages of matter (such as clusters of galaxies) would provide more stringent limits (though for a narrower range of potential masses) with the assumption that these local potential wells can capture the particles. Shipman and Cowsik (1981), and Holberg and Barber (1985) used Voyager far ultraviolet data from the Coma cluster to set limits of ~ 10²⁴ to 10²⁶ s on the radiative lifetimes of neutrinos with masses from ~ 5 to 15 ev/c². Martin, Hurwitz, and Bowyer (1990) considered effects of massive neutrinos trapped in, or forming the core of, gravitational potential wells associated with large-scale structure in the universe. In this case, large-scale fluctuations in the spatial distribution of the particles, comparable to those observed in the Harvard-Smithsonian Center for Astrophysics redshift survey (de Lapparent, Geller, and Huchra 1986), would produce neutrino radiative decay emission smeared by the velocity dispersions present in the most concentrated regions. This structure will have a characteristic scale of 2500 km s^-1 and will produce continuum fluctuations in ~ 15 to 30 Å bins. In a search of the Berkeley UVX experimental data, Martin et al. found no evidence for these fluctuations. The results of these investigations are shown in Figure 8.

Figure 8. Lower limits to the radiative lifetime of neutrinos as derived from studies of the far ultraviolet background. Different lines reflect different types of data and/or different effects based on different underlying assumptions. KBJ is from Kimble et al. (1981), HB is from Holberg & Barber (1985), MHB is from Martin et al. (1990), and SC is from Shipman and Cowsik (1981). The results of Shipman & Cowsik include various other data in addition to far ultraviolet observations.