Antiproton, Discovery of

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The notion of the existence of antimatter in general, and of antiprotons in particular, can be traced at least as far back as the 1930s. The first unambiguous identification of the antiproton, however, did not occur until September 1955 at the University of California, Berkeley's Radiation Laboratory (later renamed the Lawrence Berkeley National Laboratory). In an article published in the journal Physical Review, Owen Chamberlain, Emilio Segrè, Clyde Wiegand, and Thomas Ypsilantis described how they employed Berkeley's new proton accelerator, the Bevatron, to record the presence of these negatively charged pieces of antimatter, whose mass is identical to that of the positive proton. In 1959 the Royal Swedish Academy of Sciences awarded Chamberlain and Segrè the Nobel Prize in Physics for their efforts.

Theoretical Context

As early as 1928, British theoretical physicist Paul Dirac (1902–1984) realized that solutions to his equations—which described behaviors of negatively charged electrons quite successfully—contained a puzzling feature. The equations allowed particles of negative energy to exist in addition to their positive energy counterparts. According to the equations, such particles would have a positive charge. Few knew what to make of this strange property. During the next four years, many, including Dirac, speculated that the well-known and positively charged proton might somehow account for these odd solutions. However, the significantly greater mass of the proton (two thousand times that of the electron), among other things, cast doubt on such assumptions. In 1931, Dirac proposed "a new kind of particle, unknown to experimental physics, having the same mass and opposite charge to an electron," the "anti-electron" (Dirac, p. 61). Such a particle would be rare in nature since it would tend to recombine into a state of pure energy with any of the many electrons present, but it should otherwise be stable. Observed experimentally in 1932, the anti-electron soon became known as the positron; its presence suggested that other fundamental particles, like the proton, might also have antimatter counterparts. Some theoreticians proceeded to find a place for further antimatter in their equations, and some experimentalists proceeded to seek further antimatter in their observations.

Early Observations

The tendency of antiparticles to recombine with their particle counterparts presented a challenge to any experimental physicist wishing to record the presence of these elusive tidbits. The relatively high energy necessary to produce a proton-antiproton pair meant that physicists in the 1930s and 1940s could only expect to find an antiproton during cosmic ray observations. These measurements utilized the high-energy particles bombarding the Earth daily from extraterrestrial sources. In performing the experiments, scientists used cloud chambers and nuclear emulsions to visually record the paths of the incoming particles and their collisions. By analyzing pictures of particle movements in a magnetic field, observers made claims about the masses, energies, and charges of particles that had passed through these devices. From 1946 through 1955, cosmic ray experimenters suggested that one or another event might be traced to an antiproton, but they never completely convinced themselves or their colleagues. Copious production of antiprotons would have to wait for an artificial source, namely, a particle accelerator. Before 1954, however, no accelerator in the world was able to achieve more than half of the minimum energy predicted to be necessary.

The Accelerator and the Experiment

In 1946 and 1947 physicists at both the Radiation Laboratory at the University of California, Berkeley and the Brookhaven National Laboratory on Long Island, New York, lobbied for larger and more expensive accelerators to probe the finer structure of matter and energy. Given political and funding constraints, the laboratories had to moderate their requests. Brookhaven agreed to the quicker construction of a machine optimized for lower-energy particles, the Cosmotron. Berkeley embraced the longer-term task of building a machine whose particle energies could be expected to produce enough antiprotons for reliable measurements.

Berkeley's Bevatron beam, however, would also produce much larger amounts of another negatively charged particle, the negative pion. Therefore, the challenge of detecting antiprotons from an accelerator focused on convincingly identifying the presence of antiprotons from among the formidable noise of the pion background. The physicists needed to invent a scheme whereby the lighter pions would be excluded from their detectors and only negatively charged particles of a mass equal to the proton would be selected. To accomplish this, they set up a series of magnetic fields and detectors which indicated only particles whose charge, momentum, and velocity all matched the readings expected for antiprotons.

The selection was performed as follows (see Figure 1): the Bevatron's proton beam was steered into a copper target. The negatively charged particles from the ensuing collision passed through a magnet, M1, which was designed to bend the path of the particles with the appropriate momentum through focusing magnets, Q1, and through shielding into the first of three scintillation counters, S1. The particles then continued along through a second set of focusing magnets, Q2, and a second bending magnet, M2, until they reached the second scintillation counter, S2. If the particles made it through both M1 and M2, then they possessed the desired momentum. However, the heavier antiprotons would travel more slowly between S1 and S2 (51 billionths of a second compared to the pions' 40 billionths), so only particles with this longer time-of-flight would be recorded. The Cerenkov counter, C2, double-checked the velocity by registering only when triggered by the slower particles. (Slower, in this case, meant about 75 percent rather than 99 percent of the speed of light.) The remaining counters ensured that particles made it all the way through the apparatus and that two pions traveling some distance apart were not mistaken for a single, slower antiproton. The apparatus could also be adjusted to select for other particles as a test of its effectiveness. When tuned for antiprotons, it registered negatively charged particles with a mass identical to that of the proton. Thus, the first convincing


evidence for the antiproton was recorded electronically rather than visually and originated from an artificial source. Later, some of the earlier cosmic ray and nuclear emulsion observations were confirmed, and nuclear emulsion images were created in conjunction with the electronic apparatus.

Disputed Credit

In 1972 Italian émigré Oreste Piccioni brought a well-publicized lawsuit against Chamberlain and Segrè, claiming that he had originated the idea for the apparatus and had communicated it to them at a meeting in Berkeley in December 1954. The suit was dismissed on technicalities in 1973.

See also:Antimatter; Conservation Laws; Dirac, Paul


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Elizabeth Paris