Antihydrogen

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 /ˌeɪtʃ ˈbɑr/ aitch-bar).

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Antihydrogen characteristics

According to the CPT theorem of particle physics, antihydrogen atoms should have many of the characteristics regular hydrogen atoms have, i.e. they should have the same mass, magnetic moment, and transition frequencies (see Atomic spectroscopy) between its atomic quantum states. Excited antihydrogen atoms are for example expected to glow with the same color as that of regular hydrogen. Antihydrogen atoms should be attracted to other matter or antimatter gravitationally with a force of the same magnitude as ordinary hydrogen atoms would experience. This would not be true if antimatter has negative gravitational mass, which is considered highly unlikely, though not yet empirically disproven.

When antihydrogen atoms come into contact with ordinary matter, they quickly annihilate and produce energy in the form of gamma rays and high-energy particles called pions. These pions in turn quickly decay into other particles called muons, neutrinos, positrons, and electrons, and these particles rapidly dissipate. If antihydrogen atoms were to be suspended in a perfect vacuum, however, they should survive indefinitely.

Production

In 1995, the CERN laboratory in Geneva first produced antihydrogen 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 substantial 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 took place at a typical rate of 100 antihydrogen atoms per second. Antihydrogen was first produced by these two collaborations in 2002, and by 2004 perhaps a hundred thousand antihydrogen atoms were produced in this way.

The antihydrogen atoms synthesized so far have a very high temperature (a few thousand kelvins); they will hit the walls of the experimental apparatus as a consequence and annihilate. A potential solution to this problem would be to produce antihydrogen atoms at such a low temperature (perhaps a fraction of a kelvin) that they can be captured in a magnetic trap.

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.

Natural occurrence

Today, no conclusive spectral signature for the presence of antihydrogen could be reported, since measuring the spectrum of antihydrogen, especially the 1S-2S interval, is exactly the goal of these CERN collaborations.

See also

• Gravitational interaction of antimatter