Journal ArticleWe report on a search for deeply penetrating particles in the > 10^17 eV cosmic-ray flux using the University of Utah Fly's Eye detector. No such events have been found in 6 x 106 sec of running time. We consequently set limits on the following: quark matter in the primary cosmic-ray flux, high-energy long-lived weakly interacting particles produced in proton-air interactions, such as τ's; astrophysical neutrino flux; and other hypothetical high-energy weakly interacting components of the cosmic-ray flux such as photinos
