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toponium

toponium Particle Physics.
  (tɒˈpəʊnɪəm)
  [f. top n.1 + -onium, after positronium.]
  A bound state of a top quark and a top anti-quark.

1978 Nature 28 Sept. 268/2 This gives the prospect of detailed investigation of quark dynamics by studying transitions among these levels of ‘toponium’ analogous to transitions among levels of positronium and hydrogen in atomic physics. 1984 New Scientist 17 May 15/1 Theory suggests that if there is an electrically neutral Higgs boson whose mass is less than that of toponium, then we should be able to observe toponium decaying into a photon and a Higgs.

Oxford English Dictionary

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