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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. Antihydrogen began to be produced artificially in accelerator experiments in 1995, but the atoms produced had such "hot" velocities as to collide with matter and annihilate before they could be examined in detail. In November 2010, for the first time, cold antihydrogen was produced and magnetically confined for about a sixth of a second by the Antihydrogen Laser Physics Apparatus (ALPHA) team at CERN.[1]

The standard symbol for antihydrogen is H. When spoken, it is usually pronounced "H-bar".



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 their atomic quantum states. For example, excited antihydrogen atoms are 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 that ordinary hydrogen atoms experience. This would not be true if antimatter had negative gravitational mass, which is considered highly unlikely, though not yet empirically disproven (see Gravitational interaction of antimatter).

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