positron and antiproton

If antimatter is the mirror inventory of matter, the positron and the antiproton are its first two confirmed entries. The positron is the antiparticle of the electron: same tiny mass, but positive charge instead of negative. The antiproton is the antiparticle of the proton: same mass as a proton, but negative charge instead of positive, and built from antiquarks instead of quarks. Discovering each was a milestone that turned antimatter from an equation into something you could detect, count, and store.

The positron came first. In 1932 Carl Anderson, photographing cosmic rays in a cloud chamber inside a magnetic field, saw a track that curved like an electron's but in the opposite direction — the signature of a particle with the electron's mass and the opposite charge. The antiproton was much harder: a proton is about two thousand times heavier than an electron, so making its antiparticle takes far more energy. In 1955 Emilio Segre and Owen Chamberlain built a dedicated accelerator at Berkeley, the Bevatron, smashed protons hard enough to create antiprotons in the debris, and sifted them out by their mass and negative charge. Together the two discoveries showed that both the lightest charged matter particle and a heavy composite one have real antimatter counterparts.

These two particles are the workhorses of antimatter physics. Positrons are used every day in hospitals: in a PET scan (positron emission tomography), a tracer emits positrons that annihilate with electrons in your tissue and produce detectable photons. Antiprotons are captured and cooled at CERN's Antiproton Decelerator and combined with positrons to build antihydrogen atoms. A frequent confusion is to call the positron simply a 'positive electron' as if it were ordinary matter with a flipped charge — it is genuinely antimatter, and it annihilates when it meets an ordinary electron.