An antiparticle is the counterpart of an ordinary matter particle, sharing many properties but with specific quantum characteristics reversed. Every known particle in the Standard Model has a corresponding antiparticle, such as the positron for the electron. An antiparticle carries the same mass and lifetime as its normal twin but is its perfect opposite. The existence of antiparticles confirms a deep symmetry within the laws of physics, even though they are exceedingly rare in the observable universe.
Defining Characteristics
The defining feature of an antiparticle is the reversal of its intrinsic additive quantum numbers compared to its particle counterpart. While an antiparticle possesses the exact same mass and spin as its partner, all “charges” are of the opposite sign. For example, the electron has a negative electric charge, while the positron has an identical mass but a positive electric charge.
This reversal extends to other properties that govern particle interactions, such as baryon and lepton numbers. A proton has a baryon number of \(+1\), while an antiproton carries a baryon number of \(-1\). Similarly, leptons have a lepton number of \(+1\), while their antiparticles have a lepton number of \(-1\). The systematic reversal of these internal quantum numbers fundamentally distinguishes matter from antimatter.
The Annihilation Reaction
The annihilation reaction occurs the moment a particle and its antiparticle meet. In this process, the two particles vanish entirely, and their combined mass is converted completely into pure energy. This transformation is a direct demonstration of Albert Einstein’s mass-energy equivalence equation, \(E=mc^2\).
The energy released is in the form of high-energy photons, gamma rays. When an electron and a positron annihilate at rest, their combined rest mass energy is released as two gamma rays, each carrying 511 kilo-electron volts (keV) of energy. These two photons are always emitted in exactly opposite directions to conserve energy and momentum.
How Antiparticles Are Created and Observed
Antiparticles are created when energy is converted into mass, a process known as pair production. This occurs when a high-energy photon, a gamma ray, passes close to an atomic nucleus and transforms into a particle-antiparticle pair. The photon must possess energy at least equal to the combined rest mass energy of the pair, which is \(1.022 \text{ MeV}\) for an electron-positron pair. The nearby nucleus ensures that both momentum and energy are conserved during the transformation.
Antiparticles are naturally produced in minute quantities by high-energy phenomena like cosmic rays striking Earth’s atmosphere. They are also emitted through the radioactive decay of certain unstable isotopes, such as the beta-plus decay of Potassium-40. In a laboratory setting, scientists use particle accelerators to smash particles together, generating antiparticles from the collision energy.
The creation and annihilation cycle is utilized in the medical imaging technique called Positron Emission Tomography (PET). A patient is injected with a radioactive tracer that emits positrons, which travel a short distance before encountering an electron in the body. The resulting annihilation produces the two characteristic 511 keV gamma rays, which are detected by the PET scanner. Mapping the source of these gamma rays allows physicians to observe metabolic activity inside the body.
The Cosmic Riddle of Antimatter
One of the greatest unresolved questions in physics is why the universe today is composed almost entirely of matter, with very little naturally occurring antimatter. Cosmological models suggest that the Big Bang should have created nearly equal amounts of matter and antimatter. If this symmetry had been perfect, the two would have completely annihilated each other, leaving behind a universe filled only with radiation.
This discrepancy is known as the Baryon Asymmetry Problem. To explain the survival of matter, physicists hypothesize that a slight imbalance developed in the early universe, favoring matter over antimatter by about one part per billion. Theoretical work by physicist Andrei Sakharov established conditions necessary for this asymmetry, including a difference in how matter and antimatter interact.
This difference is called Charge-Parity (CP) violation, a subtle process where certain physical laws do not treat a particle and its antiparticle identically. While CP violation has been observed in certain particle decays, the effect is not strong enough within the Standard Model to account for the matter dominance seen today. The search for additional sources of CP violation remains a primary goal for particle physicists, as it may hold the key to understanding the formation of the cosmos.