eta meson

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An electrically neutral meson having a mass 1,071 times that of the electron and a mean lifetime of about 5 × 10⁻¹⁹ second.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physics Either of two mesons made of a mixture of up, down and strange quarks and their antiquarks