Search for Grand-Unified-Theory Magnetic Monopoles at a Flux Level below the Parker Limit

We report the results of the first direct search for grand-unified-theory magnetic monopoles with adequate sensitivity to detect a flux as small as the Parker flux limit. If stable monopole-nucleus bound states exist then the observed absence of monopole tracks in our 4.6×${10}^8$-yr-old mica detector places an upper limit of ${10}^{-17\mathrm{}}$ to ${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^{-2\mathrm{}}$ ${\mathrm{sr}}^{-1\mathrm{}}$ ${\mathrm{s}}^{-1\mathrm{}}$ on the flux of grand-unified-theory monopoles having velocity $3\times {10}^{-4\mathrm{}}c$ to $1.5\times {10}^{-3\mathrm{}}c$.