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. 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 eh/(2Mpc), where Mp is the nucleon mass.] The neutron's magnetic moment, which would be zero if the neutron were an ordinary Dirac particle, was measured by L. Alvarez and F. Bloch in 1940 to have a value of −1.91 nuclear magnetons. The second reason was based on a cosmological argument. Where were the antigalaxies one expected if the Universe had baryon–antibaryon symmetry?

One of the motivations for the choice of the energy for the Bevatron was the hope that the antiproton could be found. The momentum chosen, 6.5 GeV/c, was above threshold for antiproton production on free protons, p + p → p + p + p +, to occur.