The BASE collaboration, publishes today in Nature a new measurement of the magnetic moment of the antiproton, with a precision exceeding that of the proton. Thanks to a new method involving simultaneous measurements made on two separately-trapped antiprotons in two Penning traps, BASE succeeded in breaking its own record presented last January. 

The magnetic moment of the antiproton is found to be 2.792 847 344 1(42), to be compared to the figure of 2.792 847 350(9) that the same collaboration of researchers found for the proton in 2014, at the BASE companion experiment at Mainz, in Germany.

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CERN issues a press release announcing the first storage of antiprotons. It reads: 

Antimatter, in the form of antiprotons, has been stored for the first time in history. 

This scientific first occurred at CERN, the European Organisation for Nuclear Research, at the end of July during tests conducted in view of using the SPS European accelerator as a colliding device between protons and antiprotons. 

Several hundred antiprotons of 2.1 GeV/c were produced by protons from the PS accelerator and were kept circulating in a machine called ICE (Intial Cooling Experiment) for a period of 85 hours i.e. about 300, 000 seconds (3 × 10⁵). The previous best experimental measurement of antiproton lifetime, acquired during bubble chamber experiments, was about 10^-4 second, i.e. a ten-thousandth of a second. 

In 1964, James Cronin and Val Fitch at Brookhaven National Laboratory in the US did an experiment with particles called neutral K-mesons, or "kaons". The types of kaons they chose to study can be regarded to consist of one half ordinary matter and the other half antimatter. They started with two types of kaon that had seemingly identical masses but different lifetimes. Kaons of the long-lived type exist for 5.2 × 10^-8 seconds before each decays into 3 pions. Kaons of the short-lived type exist for only 0.89 × 10^-10 seconds before each decays into 2 pions.

Cronin and Fitch shot the two types of kaon down a 17-metre beamline and detected the resulting pion-decays at the other end.

Given the different lifetimes of the kaon types and the length of the beamline, you would expect only to see decays from the long-lived kaon type at the detector. Cronin and Fitch expected the short-lived kaon type to decay long before it reached the end of the beamline, and so its decay products would not be detected. In other words you would expect to detect only 3-pion decays and no 2-pion decays at all.

But in their experiment, Cronin and Fitch did detect 2-pion decays: 45 of them, out of a total of 22,700 decay events – a ratio of about 1 in 500. The result violated a fundamental principle of physics – the symmetry between matter and antimatter.

The pair announced their result in the paper "Evidence for the 2-pion Decay of the K Meson", published in the journal Physical Review Letters on 27 July 1964. They shared the 1980 Nobel prize in physics "for the discovery of violations of fundamental symmetry principles in the decay of neutral K-mesons."

By 1965, all three particles that make up atoms (electrons, protons and neutrons) were known to each have an antiparticle. So if particles, bound together in atoms, are the basic units of matter, it is natural to think that antiparticles, bound together in antiatoms, are the basic units of antimatter.

But are matter and antimatter exactly equal and opposite, or symmetric, as Dirac had implied? The next important step was to test this symmetry. Physicists wanted to know how subatomic antiparticles behave when they come together. Would an antiproton and an antineutron stick together to form an antinucleus, just as protons and neutrons stick together to form the nucleus of an atom?

The answer to the antinuclei question was found in 1965 with the observation of the antideuteron, a nucleus of antimatter made out of an antiproton plus an antineutron (while a deuteron – the nucleus of the deuterium atom – is made of a proton plus a neutron). The goal was simultaneously achieved by two teams of physicists, one led by Antonino Zichichi using the Proton Synchrotron at CERN, and the other led by Leon Lederman, using the Alternating Gradient Synchrotron (AGS) accelerator at the Brookhaven National Laboratory, New York.

The CERN paper, Experimental Observation of Antideuteron Production was published in the Italian particle-physics journal Il nuovo cimento on 1 September 1965 (the journal ended when it was merged into the European Physical Journal in 1999.

The journal Physical Review receives the paper Antineutrons Produced from Antiprotons in Charge-Exchange Collisions by a second team working at the Bevatron – Bruce Cork, Glen Lambertson, Oreste Piccioni and William Wenzel. The paper – which announces the discovery of the antineutron – is published in the issue dated November 1956.

A paper titled "Observation of antiprotons," by Owen Chamberlain, Emilio Segrè, Clyde Wiegand, and Thomas Ypsilantis, members of what was then the Radiation Laboratory of the University of California at Berkeley in the US, appeared in the 1 November 1955 issue of Physical Review Letters. It announced the discovery of a new subatomic particle, identical in every way to the proton – except its electrical charge was negative instead of positive.

A month before the paper appeared, The New York Times had put the news on the front page:

New Atom Particle Found; Termed a Negative Proton

With the discovery of the antiproton, Segrè and colleagues had further proof of the essential symmetry of nature, between matter and antimatter. Segrè and Chamberlain were awarded the Nobel prize in physics in 1959 "for their discovery of the antiproton".

The Bevatron in 1958 (Image: Lawrence Berkeley National Laboratory)

In 1954, Ernest Lawrence oversaw the building of a proton accelerator called the Bevatron at the radiation laboratory in Berkeley, California. The machine's name comes from BeV, the symbol used at the time for "billion electronvolt", or 10⁹ electronvolts. We now call this unit the gigaelectronvolt, symbol GeV – BeV is no longer used. The Bevatron was designed to collide protons at 6.2 GeV, the expected optimum energy for creating antiprotons.

The following is from Experiences with the Bevatron by then Berkeley physicist Edward Lofgren, who was present for the start-up of the machine:

Finally, on April 1, 1954, a feeble pulse was obtained at a magnetic field corresponding to 6 BeV. The intensity was measured by counting the tracks in nuclear emulsion that had been inserted into the beam. The intensity was in the range of 10⁴ to 10⁶ protons per pulse.

A team of physicists headed by Italian-American physicist Emilio Segrè designed and built a detector specialized to look for antiprotons. The Bevatron was up and running.

In 1929 Ernest Lawrence – then associate professor of physics at the University of California, Berkeley, in the US – invented the cyclotron, a device for accelerating nuclear particles to high velocities without the use of high voltages. Lawrence was granted US patent 1948384 for the cyclotron on 20 February 1934. The machine was used in the following years to bombard atoms of various elements with swiftly moving particles. Such high-energy particles could disintegrate atoms, in some cases forming completely new elements. Hundreds of artificial radioactive elements were formed in this manner.

Eventually, the cyclotron was able to accelerate particles such as protons to the energy of a few tens of megaelectronvolts (symbol: MeV. One MeV equals one million electronvolts). Initially driven by the effort to discover the antiproton, the accelerator era had begun, and with it the science of high-energy physics was born.

In 1939 Lawrence won the Nobel prize in physics, "for the invention and development of the cyclotron and for results obtained with it, especially with regard to artificial radioactive elements".

In the 1920s, physicists were trying to apply Planck's concept of energy quanta to the atom and its constituents. By the end of the decade Erwin Schrödinger and Werner Heisenberg had invented the new quantum theory of physics. The Physical Institute of the University of Zürich published Schrödinger's lectures on Wave Mechanics (the first from 27 January 1926) and in 1930 Heisenberg's book The physical principles of the quantum theory appeared.

The problem now was that quantum theory was not relativistic; the quantum description worked for particles moving slowly, but not for those at high or "relativistic" velocities, close to the speed of light.

In 1932 Carl Anderson, a young professor at the California Institute of Technology in the US, was studying showers of cosmic particles in a cloud chamber and saw a track left by "something positively charged, and with the same mass as an electron". After nearly a year of effort and observation, he decided the tracks were actually antielectrons, each produced alongside an electron from the impact of cosmic rays in the cloud chamber. He called the antielectron a "positron", for its positive charge and published his results in the journal Science, in a paper entitled The apparent existence of easily deflectable positives (1932).

The discovery was confirmed soon after by Occhialini and Blacket, who in 1934 published Some photographs of the tracks of penetrating radiation in the journal Proceedings of the Royal Society A. Anderson's observations proved the existence of the antiparticles predicted by Dirac. For discovering the positron, Anderson shared the 1936 Nobel prize in physics with Victor Hess.

For years to come, cosmic rays remained the only source of high-energy particles. The next antiparticle physicists were looking for was the antiproton. Much heavier than the positron, the antiproton is the antipartner of the proton. It would not be confirmed experimentally for another 22 years.