Discovery and Creation of Antiparticles
– Antiparticles were predicted by Paul Dirac in 1928.
– Carl D. Anderson discovered positrons in 1932.
– Emilio Segrè and Owen Chamberlain found antiprotons and antineutrons in 1955.
– Antiparticles of many other subatomic particles have been created in particle accelerator experiments.
– Complete atoms of antimatter have been assembled using antiprotons and positrons.
Experimental Observations and Measurements
– Positrons were first observed in a cloud chamber by Carl D. Anderson.
– The charge-to-mass ratio of particles can be measured by observing their paths in a magnetic field.
– Antiprotons and antineutrons were discovered at the University of California, Berkeley.
– Antiparticles have been created in particle accelerator experiments.
– Antiprotons and positrons have been collected in electromagnetic traps to form complete atoms of antimatter.
Dirac’s Theory and the Existence of Antiparticles
– The Dirac equation predicted negative energy quantum states and the possibility of infinite energy radiation.
– Dirac proposed a sea of negative-energy electrons filling the universe to prevent unphysical situations.
– Negative-energy particles lifted from the Dirac sea were interpreted as positrons.
– Dirac’s theory initially implied an infinite negative charge for the universe.
– Dirac later modified his theory and postulated the existence of the positron.
Particle-Antiparticle Annihilation
– Particle-antiparticle annihilation can occur if they are in the appropriate quantum states.
– Annihilation can produce other particles, such as two photons in the case of electron-positron annihilation.
– Single-photon annihilation of an electron-positron pair is impossible in free space due to energy and momentum conservation.
– In the Coulomb field of a nucleus, single-photon annihilation may occur.
– Annihilation processes are allowed in quantum field theory as intermediate quantum states.
Symmetry, Conservation, and Quantum Field Theory
– Particle-antiparticle pairs have opposite charges, conserving total charge.
– Laws of nature are nearly symmetrical with respect to particles and antiparticles.
– Antiparticles can be used to form antimatter atoms with similar properties to their matter counterparts.
– The predominance of matter over antimatter in the universe is still a question in cosmology.
– Charge conservation prevents the creation of an antiparticle without the simultaneous creation or destruction of a particle.
– The quantization of a quantum field theory involves the use of annihilation and creation operators.
– Fermionic fields, such as the electron field, require canonical anti-commutation relations for their operators.
– The introduction of charge conjugate antiparticle fields allows for the proper quantization of fermionic fields.
– Scalar fields can describe neutral bosons (real scalar fields) or charged bosons (complex scalar fields).
– The energy of the vacuum state is defined as the reference point for measuring all other energies in the system. Source: https://en.wikipedia.org/wiki/Antiparticle
In particle physics, every type of particle is associated with an antiparticle with the same mass but with opposite physical charges (such as electric charge). For example, the antiparticle of the electron is the positron (also known as an antielectron). While the electron has a negative electric charge, the positron has a positive electric charge, and is produced naturally in certain types of radioactive decay. The opposite is also true: the antiparticle of the positron is the electron.
Some particles, such as the photon, are their own antiparticle. Otherwise, for each pair of antiparticle partners, one is designated as the normal particle (the one that occurs in matter usually interacted with in daily life). The other (usually given the prefix "anti-") is designated the antiparticle.
Particle–antiparticle pairs can annihilate each other, producing photons; since the charges of the particle and antiparticle are opposite, total charge is conserved. For example, the positrons produced in natural radioactive decay quickly annihilate themselves with electrons, producing pairs of gamma rays, a process exploited in positron emission tomography.
The laws of nature are very nearly symmetrical with respect to particles and antiparticles. For example, an antiproton and a positron can form an antihydrogen atom, which is believed to have the same properties as a hydrogen atom. This leads to the question of why the formation of matter after the Big Bang resulted in a universe consisting almost entirely of matter, rather than being a half-and-half mixture of matter and antimatter. The discovery of charge parity violation helped to shed light on this problem by showing that this symmetry, originally thought to be perfect, was only approximate. The question about how the formation of matter after the Big Bang resulted in a universe consisting almost entirely of matter remains an unanswered one, and explanations so far are not truly satisfactory, overall.
Because charge is conserved, it is not possible to create an antiparticle without either destroying another particle of the same charge (as is for instance the case when antiparticles are produced naturally via beta decay or the collision of cosmic rays with Earth's atmosphere), or by the simultaneous creation of both a particle and its antiparticle (pair production), which can occur in particle accelerators such as the Large Hadron Collider at CERN.
Particles and their antiparticles have equal and opposite charges, so that an uncharged particle also gives rise to an uncharged antiparticle. In many cases, the antiparticle and the particle coincide: pairs of photons, Z0 bosons,
π0
mesons, and hypothetical gravitons and some hypothetical WIMPs all self-annihilate. However, electrically neutral particles need not be identical to their antiparticles: for example, the neutron and antineutron are distinct.
