Abstract
No direct experimental evidence is known for the existence of free quarks. Searches1–4 in cosmic rays for particles of charge e/3 and 2e/3 have set an upper limit to their cosmic ray flux of 
This limiting flux bombarding the Earth for the Earth's life-time of 5 × 109 yr, stopping in a depth of 300 kg/cm2 of the Earth's surface, provides 
in this layer.
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LIBBY, L., THOMAS, F. New Method to Limit Abundance of Cosmic Ray Quarks. Nature 219, 711–712 (1968). https://doi.org/10.1038/219711a0
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DOI: https://doi.org/10.1038/219711a0
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