2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/186075We derive general constraints on the relic abundances of a long-lived particle which mainly decays into a neutrino (and something else) at cosmological time scales. Such an exotic particle may show up in various particle-physics models based on physics beyond the standard model. The constraints are obtained from big-bang nucleosynthesis, cosmic microwave background and diffuse neutrino and photon fluxes, depending on the lifetime and the electromagnetic and hadronic branching ratios.33 pages, 23 figuresHigh Energy Physics - PhenomenologyAstrophysicstext