Carl Anderson’s 1932 cloud-chamber experiment discovered the positron in cosmic rays.
Anderson photographed tracks made by charged particles in a cloud chamber placed in a magnetic field. One track curved in the direction expected for a positively charged particle, but its curvature and energy loss showed that it had the mass of an electron rather than the mass of a proton.
This particle was the positron, the electron’s antimatter counterpart. It has the same mass as an electron and an equal positive charge. Its observation provided the first direct evidence for antimatter predicted by Paul Dirac’s relativistic quantum theory.
The positron is often confused with the proton because both are positively charged. They are fundamentally different: a proton is a composite baryon, while a positron is an elementary antilepton. Anderson received the 1936 Nobel Prize in Physics for the discovery.