Antimatter
ANTIMATTER
The proton in a hydrogen atom carries one unit of positive charge, the electron one unit of negative charge. Imagine reversing the signs of the charges, producing a negative proton and a positive electron. One would have a sample of antimatter, in this case an atom of antihydrogen.
Concept and Confirmation
The existence of antimatter was predicted by Paul Dirac in 1928. While attempting to make quantum mechanics consistent with special relativity, Dirac encountered negative-energy solutions. In special relativity, when a free particle of mass m moves with momentum p , its energy E is given by E2 = (mc2)2 + (pc )2 where c denotes the speed of light in a vacuum. The square root yields a spectrum of positive and negative energies (Figure 1): E = ±[(mc2)2 + (pc2]1/2
FIGURE 1
With no ground state, the negative energies offered no immediate physical interpretation. But for spin-1/2 particles (fermions) such as the electron, which are constrained by the Pauli exclusion principle, Dirac conceptualized a way to make sense of these states, with observable consequences. Dirac supposed that the vacuum consists of no positive- energy particles and the negative-energy states filled with electrons. The exclusion principle prohibits the negative-energy electrons from dropping further into the bottomless energy well. This vacuum thus carries infinite negative charge and energy, but Dirac postulated that only departures from this situation are observable.
Suppose a negative-energy electron receives energy from an external source, such as an electromagnetic field, sufficient to jump the 2mc2 energy gap. This boosts the electron into a positive-energy state, leaving a hole in a negative-energy state (Figure 2). The absence of a negative-energy electron (the hole) would be observed in the laboratory as the presence of a particle of positive energy and positive charge. In the laboratory, this process is observed as pair production: Electromagnetic Energy ? e- + e+ where e denotes the electron and e the antielectron (the positron). The electron will eventually drop back into the hole in pair annihilation: e- + e+ ? Electromagnetic Energy The electromagnetic energy is carried by gamma-ray photons . The positron was first observed in cloud chamber tracks of cosmic rays, photographed by Carl Anderson in 1932.
Pair production and annihilation respect the conservation of energy, linear and angular momentum, and electric charge. Charge conservation requires particle-antiparticle pairs in production and annihilation, and momentum conservation requires more than one gamma ray.
Because the proton is a fermion, Dirac's argument requires the antiproton to exist. In 1955 Emilio Segrè and others demonstrated its existence with an accelerator at Berkeley. Since the proton is a bound state of constituent fermions called quarks, anti- quarks evidently exist as well. Today antiprotons are
FIGURE 2
produced in the accelerators of Fermilab in Batavia, Illinois, and the European Laboratory for Particle Physics (CERN) in Geneva, Switzerland, in collisions that convert kinetic energy into mass, such as p + p ? p + p + p + p? where p? denotes the antiproton. The antiprotons are stored and then collided head-on with a proton beam. The pair annihilations result in gamma rays and other final states because the proton and antiproton participate in the weak and strong interactions, in addition to the electromagnetic interction.
The exclusion principle does not apply to integer spin particles (bosons) such as photons or pions. In 1948 Richard Feynman created another way to think of the negative energy states that applies to fermions and bosons. In Feynman's version of relativistic quantum electrodynamics, the negative-energy particles move backward in time. This concept is interpreted physically as a positive-energy antiparticle moving forward in time. This interpretation is analogous to the description of electric current. In an electric circuit, positive charges moving from A to B (the so-called conventional current) and the same amount of negative charge moving from B to A produce identical changes in the charge distribution. Likewise, when the relativistic quantum mechanics equations have a negative-energy particle moving backward in time, the phenomena show a positive- energy particle of the opposite charge moving ahead in time—the antiparticle (Figures 3a and 3b).
For every particle species P, there is a corresponding antiparticle P
FIGURE 3(a)
FIGURE 3(b)
have identical mass but opposite charge, opposite magnetic dipole moment, and (if applicable) opposite signs of all other conserved quantum numbers (such as strangeness or baryon number that extend the charge concept to nonelectromagnetic interactions). What about uncharged bosons such as the photon or the Z0? For these species the particle and antiparticle are indistinguishable—they are their own antiparticles.
What Happened to All the Antimatter?
In high-energy collisions, where kinetic energies exceed particle masses, photons and particle- antiparticle pairs readily change into one another. The ultimate high-energy "laboratory" was the early universe immediately following the Big Bang. That ultra-relativistic environment witnessed dynamic equilibrium between the photons and particle- antiparticle pairs (here denoted generically as P? ) through the reaction P + P? ? ? + ?
As the universe expanded and cooled, the production of particles and antiparticles of mass m was quenched when the photon energy dropped below mc2. This occurred for the protons and neutrons (collectively called baryons) about 10-6 s after the Big Bang. However, pair annihilations continued (until the onset of nucleosynthesis, about 3 minutes later) without pair production to replenish the supply of baryon-antibaryon pairs. Since particles and antiparticles appear and annihilate as pairs, why did not all baryons annihilate with antibaryons, leaving the universe a gas of photons? That the universe today consists of matter with little anti- matter suggests a lack of symmetry in their reactions. The decay channels available to antiparticles differ slightly from those for the corresponding particles. For example, suppose that, left to themselves, particle P is stable, but its antiparticle can decay, with a small probability , into something else: ? a + b . The probability of not decaying is 1-? . Every that decays is unavailable to annihilate with a P particle, so when all the antiparticles have disappeared through decay or annihilation, some P particles are left over. Such reactions are indeed observed in the laboratory, as in the decay of the kaons. They demonstrate matter-antimatter asymmetry, a mechanism for producing a universe dominated by matter.
See also:Annihilation and Creation; Antiproton, Discovery of; Big Bang Nucleosynthesis; Positron, Discovery of
Bibliography
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Dwight E. Neuenschwander