ABSTRACT
Protons, neutrons, and electrons are examples of ordinary matter. In 1932, less than a year after the discovery of the neutron, American physicist Carl Anderson discovered the positron, a form of antimatter. Anderson was studying the eects of cosmic rays, which are high-energy particles that reach Earth from sources mostly outside our solar system. Anderson noticed that when cosmic rays interact with matter, they sometimes produce particles with the same mass as an electron but opposite charge (i.e., a charge +e, compared with the electron’s −e). Anderson originally referred to the new particle as a “positive electron,” but today we call it a positron.
