n. An electrically neutral meson having a mass 1,071 times that of the electron and a mean lifetime of about 5 × 10-19 second.
eta meson
eta meson
[′ād·ə ′mā‚sän] (particle physics) Neutral pseudoscalar meson having zero isotopic spin and hypercharge, positive charge parity and G parity, and a mass of about 549 MeV; decays via electromagnetic interactions.