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. Antihydrogen has been produced artificially in particle accelerators, albeit at energies too high for detailed study. CERN experiments created low energy antimatter and trapped atoms for precision studies. Scientists hope studying antihydrogen may shed light on the baryon asymmetry problem or why there is more matter than antimatter in the universe.

The standard symbol for antihydrogen is H.

Experimental history

Accelerators detected hot antihydrogen in the 1990s. ATHENA studied cold H in 2002. It was first trapped by the Antihydrogen Laser Physics Apparatus (ALPHA) team at CERN in 2010, who then measured the structure and other important properties. ALPHA, AEGIS, and GBAR plan to further cool and study H atoms.

Characteristics

The CPT theorem of particle physics predicts antihydrogen atoms have many of the characteristics regular hydrogen has; i.e. the same mass, magnetic moment, and atomic state transition frequencies (see atomic spectroscopy). For example, excited antihydrogen atoms are expected to glow the same color as 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 has negative gravitational mass, which is considered highly unlikely, though not yet empirically disproven (see gravitational interaction of antimatter).