The discovery of antimatter resulted from the strange implication of a mathematical equation which was discovered by a young physicist named Paul Dirac in 1928. In the beginning of the 20th century, two important new theories, relativity and quantum mechanics, made a revolution in the physics world. However, these two theories had not been treated together until Dirac solved the problem. He combined quantum theory and special relativity to describe the behavior of the electron. Despite his triumph, he posed another problem that he got extra solution with negative energy except normal positive energy state. Dirac interpreted the result that for every particle there exists a corresponding antiparticle.
After Dirac’s prediction, the hunt for the mysterious antiparticle began. 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 one year of research, he identified that those tracks were actually antielectrons and he called the antielectron “positron” for its positive charge.
From many years to come, physicists hoped to find the antiproton. However, physicists had to wait another 22 years until power tools were invented. In 1930, Ernest Lawrence 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. The accelerator era had begun, and the new science of "High Energy Physics" was born. In 1954, at Berkeley, Lawrence built the Bevatron which could collide two protons together at an energy of 6.2 GeV. In October of 1955, Emilio Segre and his group had succeeded in discovering a proof of the antiproton. The essential symmetry between matter and antimatter was identified for the first time.
Since antiparticle was discovered, it is natural for physicists to think of antiatoms which are made up by positrons, antiprotons and antineutrons. Physicists wanted to know whether those matter and antimatter are symmetric as Dirac had implied, as well as the behavior when they come together. The observation of the antideuteron was made in 1965-- antinucleus made out of an antiproton plus an antineutron.
After making antinuclei, physicists naturally wanted to know whether antielectrons stick to antinuclei to make antiatoms. The answer was revealed quite recently. Toward the end of 1995, the first such antiatoms were produced at Conseil Européen pour la Recherche Nucléaire (CERN) by a team of German and Italian physicists. Although only nine antiatoms were made, this was another triumph for physicists. The next question is: does an antiatom behave exactly like an ordinary atom? To answer this question, CERN decided to build a new experimental facility, the Antiproton Decelerator.[1,2]
Besides an accelerator, physicists tried to study antimatter from another perspective. Antimatter could exist somewhere in outer space. Dirac was the first person to consider the existence of antimatter on an astronomical scale after the confirmation of his theory. However, the antimatter would annihilate normal matter and become energy so that the antimatter is discovered in our galaxy. In the late 1950s, physicists calculated that the amount of antimatter in our galaxy would be less then one part in a hundred million (0.00000001). Therefore, if there exists an isolated system consisting of antimatter, it must have no interaction with our galaxy and our observation could not disturb this system. Thus, even if we could not observe this kind of antimatter system, it is still possible that an extra-galaxy of antimatter exists in our universe. In the ensuing years, it was believed that our universe must consist of both matter and antimatter in equal amounts because of the physical symmetry of matter and antimatter.
How to detect this antimatter galaxy? If there exists natural antimatter, like antinuclei, from an antimatter galaxy, while they fly through earth atmosphere they would be annihilated with nuclei and emit photons. During the past twenty years, in order to overcome this annihilation problem, physicists have tried to take their instruments as high as possible in the atmosphere. Originally physicists used balloons and later used satellites; however, such efforts were costly and difficult. Presently, physicists plan to implement experiment on satellites. In 1998, for instance, a high-energy particle detector, the Alpha Magnetic Spectrometer (AMS), was flown on the Space Shuttle Discovery for a ten-day mission. Now AMS is being redesigned and upgraded, and physicists will install it on the International Space Station in a few years. Orbiting around the Earth above the atmosphere, one of its goals is to measure the fluxes of charged antiparticles and antinuclei to search for any form of cosmic antimatter.
However, we have to think of another possibility: that there is no antimatter galaxy. The numbers of matter and antimatter were not equal at the birth of the universe. There is a strange interaction named CP Violation. (C is charge and P is parity.) It is not easy to explain here. Briefly, the symmetry of matter and antimatter is violated. The characters of both of them are not same so that the decays of matter and antimatter are not equal. James Cronin and Val Fitch first detected CP-violation in the decay of particles, called neutral kaons, at the U.S. Brookhaven laboratory in the summer of 1964. CP-violation is necessary for a matter-antimatter imbalance to arise. However, the magnitude of symmetric violation of the neutral Kaon system is only 0.2%. This small number is not enough to explain the different amount of matter and antimatter in our universe. Physicists tried to study CP-violation in B-meson system. It is expected that CP-violation will be easily visible in the B-system. The B-meson has been studied in two types of experiments in various accelerator centers all over the world. One is with electron-positron colliders; as soon as the energy of the center of mass exceeds the rest mass of a B-meson pair, such a pair can be created in electron-positron annihilation. The decay products of B-mesons are subsequently observed in particle detectors. The two main experiment groups are at Stand Linear Accelerator Center (SLAC) Barbar (Stanford, USA), and at National Laboratory for High Energy Physics (KEK) Belle (Tsukuba, Japan). The other one is with proton-antiproton colliders; here the quarks inside the proton collide with the antiquarks of the antiproton and produce the necessary b-quark pairs. Until recently this research was done at the Super Proton Synchrotron (SPS) accelerator at CERN and presently it is at the Fermilab Tevatron near Chicago (the t-quark was discovered in the same experiment).
It can be seen there are two kinds of experiments trying to approach antimatter, and their basic ideas contradict. It is normal for physicists since physicists do not understand antimatter so much, even though they have already studied antimatter in the past one hundred years. There is no important application for antimatter in the real world; however, studying antimatter can help us to realize the birth of universe and its destiny.
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