Stable superstring relics and ultrahigh energy cosmic rays

One of the most intriguing experimental results of recent years is the obse rvation of Ultrahigh Energy Cosmic Rays (UHECRs) above the GZK cutoff. Plau sible candidates for the UHECR primaries are the decay products of a meta-s table matter state with mass of order O(10(12-15) GeV), which simultaneousl y is a good cold dark matter candidate. We study possible meta-stable matte r states that arise from Wilson line breaking of GUT symmetries in semi-rea listic heterotic string models. In the models that we study the exotic matt er states can be classified according to patterns of SO(10) symmetry breaki ng. We show that cryptons, which are states that carry fractional electric charge +/-1/2, and are confined by a hidden gauge group cannot produce viab le dark matter. This is due to the fact that, in addition to the lightest n eutral bound state, cryptons give rise to meta-stable charged bound states. However, these states may still account for the UHECR events. We argue tha t the uniton, which is an exotic Standard Model quark but carries "fraction al" U(1)(Z') charge, as well as the singleton, which is a Standard Model si nglet with "fractional" U(1)(Z') charge, do provide viable dark matter cand idates and can at the same time explain the observed UHECR events. (C) 2001 Elsevier Science B.V. All rights reserved.