Grand Unification Signal from Ultra High Energy Cosmic Rays?
  The spectrum of ultrahigh energy (above \approx 10^{9} GeV) cosmic rays is consistent with the decay of GUT scale particles. The predicted mass is m_X=10^b GeV, where b=14.6_{-1.7}^{+1.6}. 
Z. Fodor and S.D. Katz