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For every particle (matter) there is a corresponding antiparticle (antimatter). Antiparticles are the same as the corresponding matter particle in every respect except for their opposite charges. A proton is electrically positive: an anti-proton is electrically negative. They both have the exact same mass, so they are affected in the same way by gravity.

When a particle and its antiparticle meet, they annihilate into pure energy. This energy may then give rise to neutral force-carrier particles, such as photons, Z bosons, or gluons.

Some Terminology:

The symbol for an antimatter particle is a bar over the corresponding matter particle symbol. For example, a proton () has an antiparticle denoted by , pronounced p-bar. The antiparticle of a proton is called an antiproton. An electron's (e-) antiparticle is a positron (e+).

Here's an interesting question about antimatter that puzzles physicists:

If matter and antimatter are exactly equal but opposite, then why is there so much more matter in the universe than antimatter? Why did matter "win" instead of antimatter?

Antimatter and Experiments

Particle physicists use colliding beams of and or e- and e+. They then study the numerous particles that result from the force carrier particle's (a boson) decay.

An example of such an annihilation is the 
electron/positron annihilation that 

Look at visual proof of antimatter's existence 
(an antiproton/proton annihilation) in an old 
bubble chamber photograph (55 kB).

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