Particle-antiparticle creation

The reaction of Eq. (3.6.15) is also possible in the reverse direction, even if relatively infrequent this is particle-antiparticle pair creation. This possibility is what underlies the idea of vacuum polarization and small effects, like the Lamb shift in atomic spectra. Positrons are not that rare Many radioactive nuclei decay by positron emission—for instance, sodium-22 ... 

Giving a rigorous account of relativistic effects is now an important goal in theoretical and experimental studies because of recent progress made in experimental techniques and because of the accuracy currently achievable in measurements, e.g. in atomic and molecular spectroscopy, or in view of newly available laser techniques. Present accessible energies in heavy-ion accelerators allow a new generation of experiments with ultrarelativistic ions, which, for example, enable us to probe the structure of the vacuum via the electromagnetic particle-antiparticle pair creation. 

In this way all contributions to and 7 resulting from the virtual creation of particle-antiparticle pairs in the Furry picture defined by the KS potential (63) are suppressed. 

Interaction with the vacuum (Fig. 3.5a). In contemporary physics theory, the perfect vacuum does not just represent nothing. The electric field of the vacuum itself fluctuates about zero and these instantaneous fluctuations influence the motion of any chaiged particle. When a strong electric field operates in a vacuum, the latter undeigoes a polarization (called vacuum polarizfition), which means a spontaneous creation of matter, and more specifically, of particle-antiparticle pairs. 

What about the creation of other (than e-p) particle-antiparticle pairs from the vacuum the larger the rest mass is, the more difficult it is to squeeze out the corresponding particle-antiparticle pair. And yet we have some tiny effect (see non-QED entry) corresponding to the creation of such pairs as muon-antimuon (jx), pion-antipion (tt), etc. This means that the helium atom is composed of the nucleus and the two electrons only, when we look at it within a certain approximation. To tell the truth, the atom contains also photons, electrons, positrons, muons, pions, and whatever you wish, but with a smaller and smaller probability of appearance. All that has only a minor effect of the order of something like the seventh significant figure (both for the ionization potential and for the polarizability). 

Zeeman effect (p. 132) vacuum polarization (p. 133) particle-antiparticle creation (p. 134) virtual photons (p. 134)... 

Figure 2.2 A gas of electrons and positrons in equilibrium with radiation at very high temperatures. At temperatures over 10 K, particle-antiparticle pair creation and annihilation begins to occur and the total number of particles is no longer a constant. At these temperatures, electrons, positrons and photons are in the state called thermal radiation. The energy density of thermal radiation depends only on the temperature...

When we consider interconversion of particles and radiation, as in the case of particle-antiparticle pair creation and annihilation, the chemical potential of thermal photons becomes more significant (Fig. 11.4). Consider thermal photons in equilibrium with electron-positron pairs ... 

Figure 11.4 Creation of particle-antiparticle pairs by thermal photons...

A gamma-ray line at 0.511 MeV results from the mutual annihilation of an electron and a positron, a particle-antiparticle pair. A number of radioactive decay chains (see Table I) result in the emission of a positron as a decay product, which will annihilate upon first encounter with an electron. Also of astrophysical importance is the production of electrons and positrons via the photon-photon pair-creation process. Such pair plasmas are found in the vicinity of compact objects, such as neutron stars and black holes, that are associated with heated accretion disks and relativistic flows and jets, within which particle acceleration is known to occur. Thus, relatively narrow lines of 0.511-MeV annihilation radiation are expected to arise in the interstellar medium through the decay of dispersed, nucleosynthetic radionuclides, while broadened, Doppler-shifted, and possibly time-variable lines may occur in the high-energy and dense environments associated with compact objects. 

In formulating the second-quantized description of a system of noninteracting fermions, we shall, therefore, have to introduce distinct creation and annihilation operators for particle and antiparticle. Furthermore, since all the fermions that have been discovered thus far obey the Pauli Exclusion principle we shall have to make sure that the formalism describes a many particle system in terms of properly antisymmetrized amplitudes so that the particles obey Fermi-Dirac statistics. For definiteness, we shall in the present section consider only the negaton-positon system, and call the negaton the particle and the positon the antiparticle. 

One could argue, then, that a small excess of particles over antiparticles was produced during the creation of the universe. Suppose, for example, that there were a million antiprotons and a million and one protons created in a section of the universe. Then a million of each particle would have been annihilated as they came into contact with each other, leaving an excess of a single proton. Over time, a small irregularity of this kind might explain the dominance of matter over antimatter in the modern universe. 

The well-established solution to this problem is the reinterpretation of the negative energy states as unoccupied antiparticle states with positive energy —e. The destruction of a particle with < —mc via must then be understood as the creation of an antiparticle and vice versa, which is reflected by a redefinition of the negative energy destruction and creation operators,... 

It is the purpose of this report to review the rich experimental measurements and theoretical understanding of how the change in sign of the charge of a projectile affects reaction cross sections in atomic collisions. We are thus engaged in comparing particle and antiparticle impact, when, in the optimum situation, all other factors such as velocity and mass are held the same. We shall not be concerned with elementary particle effects of antiparticles such as annihilation and creation. 

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