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- n. Used attributively to describe the state of a positronium exotic atom in which the positron and electron have parallel spins
Sorry, no etymologies found.
The existence of the mirror world can contribute to invisible decay of orthopositronium via orthopositronium-mirror orthopositronium oscillations, see here, but Randall-Sundrum type extra dimensions can also lead to the same effect.
It was realized some time ago by Glashow  that the orthopositronium system provides one sensitive way to search for the mirror universe.
The idea is that small kinetic mixing of the ordinary and mirror photons may exist which would mix ordinary and mirror ortho positronium, leading to maximal ortho positronium - mirror orthopositronium oscillations.