Search for supermassive relics with a 2000-m2array of plastic track detectors

A direct search has been made for supermassive relics (heavier than about ${10}^{12}$ GeV/${\mathit{c}}^{2}$) with a 2000-${\mathrm{m}}^{2}$ array of CR-39 track-etch detectors deployed underground for 2.1 yr. The nonobservation of penetrating tracks places a new upper limit for the velocity-dependent flux at 3.2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}16}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{sr}}^{\mathrm{\ensuremath{-}}1}$, for magnetic monopoles carrying various magnetic charges, for electrically charged relics, and strange matter. This flux limit corresponds to a limit on the relic abundance of order ${\mathit{n}}_{\mathit{x}}$/${\mathit{n}}_{\ensuremath{\gamma}}$=${10}^{\mathrm{\ensuremath{-}}29}$ relative to the 3-K photons.