New upper bound on the flux of cosmic magnetic monopoles

Abstract

Grand unification theories (GUTs) suggest that massive magnetic monopoles (mass ∼10¹⁶ GeV) were created in the very early stages of the formation of the Universe. The report, 3 years ago¹, of a candidate cosmic magnetic monopole therefore stimulated intense experimental and theoretical activity^2,3. These monopoles should have the magnetic charge h/e predicted by Dirac⁴, or perhaps small integer multiples of h/e, but zero electric charge. Surviving magnetic monopoles are thought to be moving so slowly, v/c = 10⁻³ or less, that they interact exceedingly weakly with matter. For this reason, conventional particle detectors (whose mechanism is electronic ionization or excitation) are expected to be rather insensitive⁵, so that inductive detectors are particularly attractive. We describe here the results of 6 months observation with a large inductive detector. Some putative events have been seen, but none of them seem to have been induced by a monopole. Our total exposure (detector area–time product) is now about 230 times that of Cabrera's first experiment, so that it seems increasingly likely that his original candidate event was spurious.