New Limit on Magnetic Monopole Flux

Abstract

The underground Soudan-1 nucleon decay experiment can measure the velocities of slowly moving particles if they can be detected by ionization at a level 0.5 times that of a relativistic muon. Magnetic monopoles predicted by grand unified theories should satisfy this condition if their velocities are greater than ${10}^{-3\mathrm{}}c$. During a live time of 0.41 yr, no candidate has been observed, implying a 90%-confidence-level limit on the flux of such objects of <4.1×${10}^{-13\mathrm{}}$ ${\mathrm{cm}}^{-2\mathrm{}}$ ${\mathrm{sr}}^{-1\mathrm{}}$ ${\mathrm{s}}^{-1\mathrm{}}$ for velocities between ${10}^{-3\mathrm{}}c$ and ${10}^{-2\mathrm{}}c$.