https://doi.org/10.1351/goldbook.E01975
Elementary particle not affected by the strong force having a spin quantum number 12, a negative
elementary charge
and a rest mass of 0.000 548 579 903(13) u.
Source:
Green Book, 2nd ed., p. 93 [Terms] [Book]
See also:
PAC, 1982, 54, 1533. (Glossary of terms used in nuclear analytical chemistry (Provisional)) on page 1540 [Terms] [Paper]