Abstract
We generalize gravitational particle production in a radiation-dominated $CPT$-symmetric universe to non-standard, but also $CPT$-symmetric early universe cosmologies. We calculate the mass of a right-handed "sterile" neutrino needed for it to be the cosmological dark matter. Since generically sterile neutrinos mix with the Standard Model active neutrinos, we use state-of-the-art tools to compute the expected spectrum of gamma rays and high-energy active neutrinos from ultra-heavy sterile neutrino dark matter decay. We demonstrate that the sterile neutrinos are never in thermal equilibrium in the early universe. We show that very high-energy Cherenkov telescopes might detect a signal for sterile neutrino lifetimes up to around 10$^{27}$ s, while a signal in high-energy neutrino telescopes such as IceCube could be detectable for lifetimes up to 10$^{30}$ s, offering a better chance of detection across a vast landscape of possible masses.
Highlights
Dark matter (DM) continues to stand as one of the greatest mysteries at the interface of particle physics and cosmology [1]
Since no particle in the standard model (SM) can account for the bulk of the cosmological dark matter, one must extend the particle content of the theory to include at least an additional particle species, or a bound state thereof, that can account for a matter species making up around 5 times the baryonic matter
Adding righthanded (RH) neutrino states is a well-motivated and oftfollowed route that can at once address the origin of active neutrino masses, explain the matter-antimatter asymmetry, and provide a viable, arguably minimal, DM candidate [2]
Summary
Dark matter (DM) continues to stand as one of the greatest mysteries at the interface of particle physics and cosmology [1]. Besides generalizing the results of [3,4] to any CPT-symmetric early-Universe cosmology, we relax a key assumption in the dark matter sector: that the righthanded neutrino dark matter be absolutely stable because of a discrete symmetry Relaxing such an assumption produces the interesting prospect of testing experimentally this production mechanism by searching for the decay products of massive right-handed neutrinos, with a lifetime related to the mixing angle between sterile and active neutrinos. The remainder of this work is structured as follows: in Sec. II we describe in detail the calculation of the dark matter number density generated in arbitrary CPTsymmetric cosmologies in the early Universe; Sec. III discusses the right-handed neutrino sector and the decay modes thereof; Sec. IV explores the issue of thermalization of right-handed neutrinos; Sec. V addresses constraints and prospects for the detection of decaying massive righthanded neutrinos as predicted in the present scenario; Sec. VI concludes
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