A positron is the antiparticle of the electron. It has exactly the same rest mass as an electron and the same magnitude of spin, but it carries positive electric charge rather than the electron's negative electric charge. The positron is represented by the symbol conventionally written as e plus. It is a fundamental particle and is described by the electron quantum field in relativistic quantum field theory as the electron's antiparticle excitation.
Like the electron, it is a lepton and has spin one half, making it a fermion. Unlike an electron, which has negative electric charge, a positron has positive electric charge. Its existence was predicted theoretically by relativistic quantum mechanics and confirmed experimentally in 1932 by Carl Anderson through observations of cosmic ray tracks in a cloud chamber subjected to a magnetic field.
The theoretical prediction of the positron arose from Paul Dirac's relativistic equation for the electron. The Dirac equation incorporated special relativity and quantum mechanics while correctly accounting for electron spin and magnetic properties. Its mathematical solutions included states that could be interpreted as corresponding to antiparticles. The positron provided an experimentally observable example of this predicted structure and became one of the earliest major confirmations of antimatter.
Its discovery demonstrated that relativistic quantum theory was capable of predicting entirely new particles rather than merely describing known ones. Modern quantum field theory gives a more complete interpretation in which the electron and positron are particle and antiparticle excitations associated with the same underlying electron field.
A positron has positive elementary electric charge, whereas the electron has negative elementary electric charge. Their rest masses are equal to extremely high experimental precision, and their intrinsic spin magnitudes are the same. In a magnetic field, their trajectories curve in opposite directions for comparable momentum because the Lorentz force depends on electric charge. This property was crucial in early cloud chamber identification.
A cloud chamber contains supersaturated vapour in which charged particle tracks can become visible through ionisation and condensation. By analysing the direction and curvature of a track in a known magnetic field, researchers can infer the particle's charge sign and momentum. Anderson's observation of a track with the curvature expected for a positively charged particle but with a mass consistent with an electron was key evidence for the positron.
Positrons are commonly produced through certain radioactive beta plus decays. In beta plus decay, a proton within a nucleus transforms into a neutron while emitting a positron and an electron neutrino, subject to the relevant conservation laws. The emitted positron leaves the nucleus with kinetic energy and subsequently interacts with its environment. Positrons can also be produced in high energy particle collisions when sufficient energy is available.
Pair production is another important mechanism in which a photon or other energetic interaction can produce an electron and positron together under conditions that satisfy energy and momentum conservation. The production mechanism determines the energy distribution and environment of the resulting positron, but the particle's intrinsic mass, charge, spin, and other fundamental properties remain the same.
When a positron encounters an electron, the two can annihilate. At low relative kinetic energy, electron positron annihilation commonly produces two gamma ray photons, with the exact final state determined by conservation laws and the initial quantum state. The photons carry away the energy associated with the particles' rest masses and any remaining kinetic energy while conserving momentum and angular momentum. The positron does not simply disappear without consequence.
Its quantum numbers combine with those of the electron and the final electromagnetic radiation carries the permitted energy and other conserved quantities. Positronium, a temporary bound state of an electron and positron, can also form before annihilation. Different positronium spin configurations have different decay pathways and lifetimes.
Positrons have an important medical application in positron emission tomography. A positron emitting radionuclide is incorporated into a biologically relevant tracer and introduced into the body. The radionuclide undergoes beta plus decay, producing a positron. The positron travels a short distance through tissue, loses kinetic energy through interactions, and eventually encounters an electron. Their annihilation produces photons that travel approximately in opposite directions in the centre of mass frame.
PET scanners detect coincident gamma ray photons and use the measured lines of response to reconstruct the spatial distribution of the tracer. This allows clinicians and researchers to investigate physiological processes such as metabolism, blood flow, receptor distribution, and other biological functions. The imaging method relies directly on the fundamental properties of positron production and annihilation.
Positrons are also important in fundamental physics. Comparing electrons and positrons provides tests of charge conjugation, charge parity, CPT symmetry, magnetic moments, and other predictions of quantum field theory. High precision measurements can search for deviations from the Standard Model or constrain possible new physics. Antihydrogen experiments combine positrons with antiprotons to form neutral atoms consisting of antimatter counterparts to ordinary hydrogen. Researchers can compare antihydrogen spectroscopy and other properties with hydrogen to test fundamental symmetries. Because positrons are charged, they can be confined using electromagnetic fields under appropriate conditions, although storing them requires extremely good vacuum and specialised trapping techniques. These experiments demonstrate that antimatter is not merely a theoretical concept but an experimentally manipulable form of quantum matter.
A positron should not be confused with a proton simply because both have positive electric charge. A proton is a composite hadron containing quarks and gluons and is roughly two thousand times more massive than an electron or positron. A positron is an elementary lepton and has no known internal substructure. Nor is a positron a positive version of an electron created by changing the electron's charge while leaving all other physical properties unrelated.
In quantum field theory, particle and antiparticle relationships involve charge conjugation and the structure of the underlying fields. The positron has the same rest mass and spin as the electron but opposite additive quantum numbers such as electric charge and lepton number. Its discovery, production, annihilation, and applications make it one of the clearest examples of how relativistic quantum theory predicts and explains antimatter.
