Chronology of Milestone Events in Particle Physics - CHAMBERLAIN 1955
Chronology of Milestone Events in Particle Physics

  Nobel prize to O. Chamberlain and E. Segrè awarded in 1959 "for their discovery of the antiproton''  


Chamberlain, O.; Segrè, E.; Wiegand, C.; Ypsilantis, T.;
Observation of Antiprotons
Phys. Rev. 100 (1955) 947;

Reprinted in
(translation into Russian) Usp. Fiz. Nauk 58 (1956) 685.
R. N. Cahn and G. Goldhaber, The Experimental Foundations of Particle Physics, Cambridge Univ. Press (1991) 92.
The Physical Review - the First Hundred Years, AIP Press (1995) 847.

One of the striking features of Dirac's theory of the electron was the appearance of solutions to his equations which required the existence of an antiparticle, later identified as the positron.
The extension of the Dirac theory to the proton requires the existence of an antiproton, a particle which bears to the proton the same relationship as the positron to the electron. However, until experimental proof of the existence of the antiproton was obtained, it might be questioned whether a proton is a Dirac particle in the same sense as is the electron. For instance, the anomalous magnetic moment of the proton indicates that the simple Dirac equation does not give a complete description of the proton.
The experimental demonstration of the existence of antiprotons was thus one of the objects considered in the planning of the Bevatron. The minimum laboratory kinetic energy for the formation of an antiproton in a nucleon-nucleon collision is 5.6 BeV. If the target nucleon is in a nucleus and has some momentum, the threshold is lowered. Assuming a Fermi energy of 25 MeV, one may calculate that the threshold for formation of a proton-antiproton pair is approximately 4.3 BeV. Another, two-step process that has been considered by Feldman has an even lower threshold.
There have been several experimental events recorded in cosmic-ray investigations which might be due to antiprotons, although no sure conclusion can be drawn from them at present.
With this background of information we have performed an experiment directed to the production and detection of the antiproton. It is based upon the determination of the mass of negative particles originating at the Bevatron target. This determination depends on the simultaneous measurement of their momentum and velocity. Since the antiprotons must be selected from a heavy background of pions it has been necessary to measure the velocity by more than one method. To date, sixty antiprotons have been detected. (Extracted from the introductory part of the paper.).

Accelerator LBL Detectors CNTR, OSPK

Related references
See also
G. Feldman, Phys. Rev. 95 (1954) 1967;
J. Marshall, Ann. Rev. Nucl. Sci. 4 (1954) 141;
Analyse data from
E. Amaldi et al., Nuovo Cim. 1 (1955) 492;
E. Hayward, Phys. Rev. 72 (1947) 937;
H. S. Bridge, H. Courant, H. C. de Staebler, and B. Rossi, Phys. Rev. 95 (1954) 1101;

  p Cu X 4.2,5.1,6.2 GeV (Elab) angp
  p Cu X 4.2,5.1,6.2 GeV (Elab) angp

Particles studied
  ex, mass

Record comments
Experimental evidence for the antiproton.
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