[ ŭp′sə-lŏn′, yōōp′- ]
An electrically neutral meson having a mass 18,513 times that of the electron and a mean lifetime of approximately 8.0 X 10-20 seconds. See Table at subatomic particle.
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The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.