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    Antihydrogen is the antimatter counterpart of hydrogen.Whereas the common hydrogen atom is composed of an electron and proton, the antihydrogen atom is made up of a positron and antiproton.Its (proposed) chemical symbol is H, that is, H with an overbar (pronounced aitch-bar; SAMPA representation: ba:r').
    According to the CPT theorem (CPT stands for charge conjugation, parity reversal,and time/motion reversal) of particle physics, antihydrogen atoms should have manyof the same characteristics as hydrogen atoms, i.e. they should have the same mass, magnetic moment, and transition frequencies between its atomic quantum states(this means that if we were to excite the antihydrogen atom by shooting a laser ormicrowave beam onto it for example, it should glow with the exact same color asthat of hydrogen). Antihydrogen atoms should be attracted to other matter or antimattergravitationally with a force of the same magnitude as ordinary hydrogen atoms would experience.
    When antihydrogen atoms come into contact with ordinary matter, they quickly annihilateand produce energy in the form of gamma rays and high-energy particles calledpions. These pions in turn quickly decay into other particles called muons,neutrinos, positrons, and electrons, and these particlesrapidly dissipate. If antihydrogen atoms were to be suspended in a vacuum,however, they should survive indefinitely.
    Antihydrogen does not occur naturally, and therefore must be manufactured by bringingtogether the necessary building blocks. In 1995 at the CERN laboratory in Geneva, Switzerland, it was first produced by shooting antiprotons, which were produced in a particle accelerator, at xenon clusters. When an antiproton gets close to a xenon nucleus, an electron-positron-pair can be produced, and with some probability the positron will be captured by the antiproton to form antihydrogen. The probability for producing antihydrogen from one antiproton was only about 10-19, so this method is not well suited for the production of substanstial amounts of antihydrogen.
    In recent experiments carried out by the ATRAP and ATHENA collaborations at CERN, positrons from a sodium radioactive source and antiprotons were brought together in a magnetic Penning trap, where synthesis tookplace at a typical rate of 100 antihydrogen atoms per second. Antihydrogen wasfirst produced by these two collaborations in 2002, and by 2004 perhaps a hundredthousand antihydrogen atoms were produced in this way.
    The antihydrogen atoms synthesized so far have a very high temperature (perhaps afew thousand kelvins), and so they hit the walls of the experimentalapparatus and annihilate. The next goal is to produce antihydrogenhaving such low temperature (perhaps a fraction of a kelvin) thatthey can be captured in a magnetic trap. The antihydrogen atoms canthen be interrogated by laser beams, so that their atomic transitionfrequencies can be precisely measured. If any difference between hydrogenand antihydrogen were observed, however small, it would indicate thatmatter and antimatter do not behave in exactly the same way. This may helpexplain why the observable Universe appears to be made entirely of matterand not antimatter.
    Antimatter atoms such as antideuterium(D), antitritium (T), and antihelium (He) are much more difficult to produce than antihydrogen. Among these, only antideuterium nuclei have been produced so far, and these have such very high velocities that synthesis of antideuterium atoms may still be many decades ahead.