Implications of a solar-system population of massive 4th generation neutrinos for underground searches of monochromatic neutrino-annihilation signals

It has been recently pointed out that any primary galactic population of Weakly Interacting Massive Particles (WIMP) generates, through collisions with solar matter, a secondary population of ``slow'' WIMPs trapped in the inner solar system. We show that taking into account this ``slow'' solar-system population dramatically enhances the possibility to probe the existence of stable massive neutrinos (of a 4th generation) in underground neutrino experiments. Our work suggests that a reanalysis of existing underground neutrino data should be able to bring extremely tight constraints on the possible existence of a stable massive 4th neutrino.