The discovery of the antiproton and other antimatter, 1955–1959.
While the existence of antiparticles was established with Anderson’s discovery of the positron in 1932, it was not clear in 1955 whether the pattern of each fermion having an antiparticle, suggested by the Dirac equation, would hold for baryons, the heavy particles p, n, Λ, Σ, and Ξ. There were two arguments raising doubts about such particles. One was that nucleons had an anomalous magnetic moment that differed markedly from the Dirac moment. Measurements by Otto Stern in 1933, later improved by I. I. Rabi, had shown that the proton had a magnetic moment of 2.79 nuclear magnetons. [One nuclear magneton is eħ/(2Mpc), where Mp is the nucleon mass.] The neutron’s magnetic ...