Positron

An anti-electron had been theoretically predicted by Dirac but none had yet been observed. Anderson studied cosmic rays using a cloud chamber but devised a lead partition to slow these particles down. Some of these slowed particles behaved like electrons but deflected in the opposite direction, indicating a positive charge.
 Carl David Anderson United States 1932 Positron

An anti-electron had been theoretically predicted by Dirac but none had yet been observed. Anderson studied cosmic rays using a cloud chamber but devised a lead partition to slow these particles down. Some of these slowed particles behaved like electrons but deflected in the opposite direction, indicating a positive charge.