A pseudoscalar or scalar particle φ that couples to two photons but not to leptons, quarks and nucleons would have effects in most of the experiments searching for axions, since these are based on the aγγ coupling. We examine the laboratory, astrophysical and cosmological constraints on φ and study whether it may constitute a substantial part of the dark matter. We also generalize the φ interactions to possess SU(2) × U(1) gauge invariance, and analyze the phenomenological implications.

A light pseudoscalar coupled to two photons would be copiously emitted by the core of a supernova. Part of this flux would be converted to γ−rays by the galactic magnetic field. Measurements on the SN1987A γ−ray flux by the Gamma-Ray Spectrometer on the Solar Maximum Mission satellite already imply a bound on the coupling g < 3×10 −12 GeV −1 . The improved generation of satellite-borne detectors, like EGRET or the project GLAST, could be able to detect a pseudoscalar-tophoton signal from a nearby supernova, for allowed values of g.

It has been speculated that the recently detected ultra-high energy cosmic rays may originate from the decays of relic particles with mass of order $10^{12}$ GeV clustered in the halo of our Galaxy. This hypothesis can be tested through forthcoming measurements of the spectra of both high energy cosmic nucleons and neutrinos, which are determined in this model by the physics of QCD fragmentation, with no astrophysical uncertainties. We evolve fragmentation spectra measured at LEP energies up to the scale of the decaying particle mass by numerical solution of the DGLAP equations. This enables incorporation of the effects of supersymmetry on the development of the cascade and we also allow for decays into many-particle states. The calculated spectral shape agrees well with present cosmic ray data beyond the Greisen-Zatsepin-Kuzmin energy.Comment: 30 pages (revtex), 12 figures (eps); Small revisions; Accepted for publicatio