upsilon meson
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upsilon meson
n.
An electrically neutral meson consisting of a bottom quark and a bottom antiquark and having a mass 18,513 times that of the electron and a mean lifetime of about 1.2 × 10-20 second.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.