1) Cosmic ray component 2) Nucleon 3) Particle discovered in 1955 4) The antiparticle of a proton 
The antiproton was experimentally confirmed in 1955 by University of California, Berkeley physicists Emilio Segrè and Owen Chamberlain, for which they were awarded the 1959 Nobel Prize in Physics. An antiproton consists of two up antiquarks and one down antiquark ({SubatomicParticle|link=yes|Up antiquark}{SubatomicParticle|link=yes|Up antiquark}{...
subatomic particle of the same mass as a proton but having a negative electric charge and oppositely directed magnetic moment. It is the proton`s ... [8 related articles]
The anti-matter partner of the proton.
[n] - an unstable negatively charged proton
noun an unstable negatively charged proton; the antiparticle of a proton
an elementary particle having negative charge equal in magnitude to that of the electron and having the same mass and spin as a proton; the antiparticle of the proton.