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The spectrum of ultrahigh energy (above ≈10⁹0ex0exGeV) cosmic rays is consistent with the decay of grand unification scale particles. The predicted mass is mX0ex0ex=0ex0ex10ᵇ0ex0exGeV, where b0ex0ex=0ex0ex14.6_-1.7+1.6.
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Fodor et al. (2001) studied this question.
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