Antimatter : History

The history of antimatter begins in 1928 with a young physicist named Paul Dirac and a strange mathematical equation...
The equation, in some way, predicted the existence of an antiworld identical to ours but made out of antimatter. Was this possible? if so, where and how could we search for antimatter?
From 1930, the search for the possible constituents of antimatter, antiparticles, began, and it has been the main influence behind a major scientific and technical evolution over the last 70 years.


From 1928 to 1995
1928: the beginning

The history of antimatter begins with a young physicist named Paul Dirac and the strange implications of a mathematical equation...
It was the beginning of the 20th century, an exciting time when the very foundations of physics were shaken by the appearance of two important new theories: relativity and quantum mechanics.
In 1905 Albert Einstein unveiled his theory of Special Relativity, explaining the relationship between space and time, and between energy and mass in his famous equation E=mc². Meanwhile experiments had revealed that light sometimes behaved as a wave, but other times behaved as if it were a stream of tiny particles. Max Planck proposed that each light wave must come in a little packet, which he called a "quantum": this way light was not just a wave or just a particle, but a bit of both.
By the 1920s, physicists were trying to apply the same concept to the atom and its constituents, and by the end of the decade Erwin Schrodinger and Werner Heisenberg had invented the new quantum theory of physics. The only problem now was that quantum theory was not relativistic - meaning the quantum description worked only for particles moving slowly, and not for those at high (or "relativistic") velocity, close to the speed of light.
In 1928, Paul Dirac solved the problem: he wrote down an equation, which combined quantum theory and special relativity, to describe the behaviour of the electron. Dirac's equation won him a Nobel Prize in 1933, but also posed another problem: just as the equation x²=4 can have two possible solutions (x=2 OR x=-2), so Dirac's equation could have two solutions, one for an electron with positive energy, and one for an electron with negative energy. But in classical physics (and common sense!), the energy of a particle must always be a positive number!
Dirac interpreted this to mean that for every particle that exists there is a corresponding antiparticle, exactly matching the particle but with opposite charge. For the electron, for instance, there should be an "antielectron" identical in every way but with a positive electric charge. In his Nobel Lecture, Dirac speculated on the existence of a completely new Universe made out of antimatter!

1930: nature's helping hand
From 1930, the hunt for the mysterious antiparticles began...
Earlier in the century, Victor Hess (Nobel Prizewinner in 1936) had discovered a natural source of high energy particles: cosmic rays. Cosmic rays are very high energy particles that come from outer space and as they hit the Earth's atmosphere they produce huge showers of lower energy particles that have proved very useful to physicists.

In 1932 Carl Anderson, a young professor at the California Institute of Technology, 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 one 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. Confirmed soon after by Occhialini and Blackett, the discovery gave Anderson the Nobel Prize in 1936 and proved the existence of antiparticles as predicted by Dirac.

For many years to come, cosmic rays remained the only source of high energy particles. A steady stream of discoveries was made but for the next sought-after antiparticle, the antiproton (antipartner of the proton and much heavier than the positron), physicists had to wait another 22 years...

1954: power tools
The search for antiprotons heated up in the 1940s and 1950s, as laboratory experiments reached ever higher energies...

In 1930, Ernest Lawrence (Nobel Prizewinner in 1939) had invented the cyclotron, a machine that eventually could accelerate a particle like a proton up to an energy of a few tens of MeV. Initially driven by the effort to discover the antiproton, the accelerator era had begun, and with it the new science of "High Energy Physics" was born.
It was Lawrence that, in 1954, built the Bevatron at Berkeley, California (BeV, at the time, was what we now call GeV). The Bevatron could collide two protons together at an energy of 6.2 GeV, expected to be the optimum for producing antiprotons. Meanwhile a team of physicists, headed by Emilio Segre', designed and built a special detector to see the antiprotons.

In October 1955 the big news hit the front page of the New York Times: "New Atom Particle Found; Termed a Negative Proton". With the discovery of the antiproton, Segre' and his group of collaborators (O. Chamberlain, C. Wiegand and T. Ypsilantis) had succeeded in a further proof of the essential symmetry of nature, between matter and antimatter.
Segre' and Chamberlain were awarded the Nobel Prize in 1959. Only a year later, a second team working at the Bevatron (B. Cork, O. Piccione, W. Wenzel and G. Lambertson) announced the discovery of the antineutron. 

1965:antinuclei

By now, all three particles that make up atoms (electrons, protons and neutrons) were know 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 do 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 an atom's nucleus?
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. 

1995: from antiparticles to antimatter
After making antinuclei, naturally the next question was: can antielectrons stick to antinuclei to make antiatoms?

In fact the answer was only revealed quite recently, thanks to a very special machine, unique to CERN, the Low Energy Antiproton Ring (LEAR). Contrary to an accelerator, LEAR actually "slowed down" antiprotons. Physicists could then try to force a positron (or antielectron) to stick to an antiproton, making an antihydrogen atom, a real antimatter atom.
Towards the end of 1995, the first such antiatoms were produced at CERN by a team of German and Italian physicists. Although only 9 antiatoms were made, the news was so thrilling that it made the front page of many of the world's newspapers.
The achievement suggested that the antihydrogen atom could play a role in the study of the antiworld similar to that played by the hydrogen atom in over more than a century of scientific history. Hydrogen makes up three quarters of our universe, and much of what we know about the cosmos has been discovered by studying ordinary hydrogen.

But does antihydrogen behave exactly like ordinary hydrogen ? To answer this question CERN decided to build a new experimental facility: the Antiproton Decelerator (AD). 

Source : Cern