Possible new hadrons containing massive stable quarks (e.g., color-sextet quarks) surviving as relics of the early stages of the big bang should be present in $Z>1$ nuclei at levels accessible to experiment ({~}10^-10). Grand unified theories that explain the observed baryon asymmetry of the universe should not contain these new stable quarks unless they are prevented from evolving asymmetrically. Otherwise, the new hadrons would be as common as nucleons, which is clearly not the case.
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Dover et al. (1979) studied this question.
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