Antihelium

In particle physics, antimatter is a material composed of the antiparticle "partners" to the corresponding particles of ordinary matter. A particle and its antiparticle have the same mass as one another, but opposite electric charge and other quantum numbers. For example, a proton has positive charge while an antiproton has negative charge. A collision between any particle and its antiparticle partner leads to their mutual annihilation, giving rise to various proportions of intense photons (gamma rays), neutrinos, and sometimes less-massive particle–antiparticle pairs. The total consequence of annihilation is a release of energy available for heat or work, proportional to the total matter and antimatter mass, in accord with the mass–energy equivalence equation, E = mc².

Formally, antimatter particles can be defined by their negative baryon number or lepton number, while "normal" (non-antimatter) matter particles have positive baryon or lepton number. These two classes of particles are the antiparticle partners of one another.

Antimatter particles bind with one other to form antimatter, just as ordinary particles bind to form normal matter. For example, a positron (the antiparticle of the electron) and an antiproton (the antiparticle of the proton) can form an antihydrogen atom. Physical principles indicate that complex antimatter atomic nuclei are possible, as well as anti-atoms corresponding to the known chemical elements. Studies of cosmic rays have identified both positrons and antiprotons, presumably produced by collisions between particles of ordinary matter. Satellite-based searches of cosmic rays for antideuteron and antihelium particles have yielded nothing.

There is considerable speculation as to why the observable universe is composed almost entirely of ordinary matter, as opposed to an even mixture of matter and antimatter. This asymmetry of matter and antimatter in the visible universe is one of the great unsolved problems in physics. The process by which this inequality between matter antimatter particles developed is called baryogenesis.

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