Observation and Annihilation: The Discovery of the Antiproton

This paper is the first investigation of the events associated with the discovery of the antiproton. The 1955 observation of the antiproton by Owen Chamberlain, Emilio Segrè, Clyde Wiegand, and Thomas Ypsilantis was “no surprise,” in Chamberlain’s words, and might therefore be understood as a classic example of an experimental proof of an existing theory—except there was no complete theory—at best it was a further validation of Dirac’s 1930 prediction of antiparticles. Instead, I argue, it became a contest between the serendipitous observations of cosmic-ray events and the deliberate observation possible with the new accelerator-based experiments. I show that the discovery was an extended event and was seen by the physicists involved as emerging from a combination of supporting experiments—the counter-based detection of antiprotons was accepted as proof of discovery only with the supporting images of antiproton annihilations.