Tom’s Infinite Science Archive: Antimatter
Matter composed of elementary particles that are, in a special sense, mirror images of the particles that make up ordinary matter as it is known on earth. Antiparticles have the same mass as their corresponding particles but have opposite electric charges or other properties related to electromagnetism. For example, the antimatter electron, or positron, is positively charged but is identical in all other respects to the electron. The antimatter equivalent of the chargeless neutron, on the other hand, differs in having a magnetic moment of opposite sign (magnetic moment is another electromagnetic property). In all of the other parameters involved in the dynamical properties of elementary particles, such as mass, spin, and partial decay, antiparticles are identical with their corresponding particles.
The existence of antiparticles was first proposed by the British physicist Paul Adrien Maurice Dirac, arising from his attempt to apply the techniques of relativistic mechanics to quantum theory. In 1928 he developed the concept of a positively charged electron but its actual existence was established experimentally in 1932. The existence of other antiparticles was presumed but not confirmed until 1955, when antiprotons and antineutrons were observed in particle accelerators. Since then, the full range of antiparticles has been observed or indicated. Antimatter atoms were created for the first time in September 1995 at the European Organization for Nuclear Research (formerly known by the acronym CERN). Positrons were combined with antimatter protons to produce antimatter hydrogen atoms. These atoms of antimatter exist only for forty-billionths of a second, but physicists hope future experiments will determine what differences there are between normal hydrogen and its antimatter counterpart.
A profound problem for particle physics and for cosmology in general is the apparent scarcity of antiparticles in the universe. Their nonexistence, except momentarily, on earth is understandable, because when a particle and its antiparticle meet, they annihilate with a great release of pure energy. This energy may then give rise to neutral force-carrier particles, such as photons, Z bosons, or gluons.
Distant galaxies could possibly be made of antimatter, but no direct method of confirmation exists. Most of what is known about the far universe arrives in the form of photons, which are identical with their antiparticles and thus reveal little about the nature of their sources. The prevailing opinion, however, is that the universe consists overwhelmingly of "ordinary" matter, and explanations for this have been proposed by recent cosmological theory.
The symbol for an antimatter particle is a bar over the corresponding matter particle symbol. For example, a proton (
) has an antiparticle denoted by
, pronounced p-bar. The antiparticle of a proton is called an antiproton. An electron's (e- antiparticle is a positron (e+).
Experiments with Antimatter
Particle physicists use colliding beams of and or e- and e+. They then study the numerous particles that result from the force carrier particle's (a boson) decay.
Electron/Positron Annihilation
When an electron and positron (antielectron)
collide at high energy they can annihilate to produce
D+ and D- mesons (charm particles)
Frame 1: The electron and positron zoom towards their certain doom.
Frame 2: They collide and annihilate, releasing tremendous amounts of energy.
Frame 3: The electron and positron have annihilated into a virtual photon, or a virtual Z particle, both of which are virtual force carrier particles.
Frame 4: A charm quark and a charm antiquark emerge from the virtual force carrier particle.
Frame 5: They begin moving apart, stretching the color force field
Frame 6: The quarks move apart, further spreading their force field.
Frame 7: The energy in the force field increase with the separation between the quarks. When there is sufficient energy in the force field, the energy is converted into a quark and an anti-quark (remember 
).
Frames 8-10: The quarks separate into distinct, color-neutral particles: the D+ (a charm and anti-down quark) and D- (an anti-charm and down quark) mesons.
This animation may be deceefing!
Intermediate stages of this process occur in less than almost a billionth of a billionth of a billionth of a second, and are not observable.
To understand why specific quarks need to bind together, it is necessary to understand Quark Confinement
Top Quark Production
Proton + Antiproton --> Top + Antitop
A quark (from within a proton) and an antiquark (from an antiproton) colliding at high energy can annihilate to produce a top quark and a top antiquark, which then decay into other particles.
Frame 1: A quark from a proton and an anti-quark from an anti-proton rush to meet each other.
Frame 2: These quarks collide and annihilate....
Frame 3: ...into virtual gluons.
Frame 4: A top and antitop quark emerge from the gluon cloud.
Frame 5: These quarks begin moving apart, stretching the color force field (gluon field) between them.
Frame 6: Before the top quark and antiquark have moved very far, they decay into a bottom and antibottom quark (respectively) with the emission of W force carrier particles.
Frame 7: The new bottom quark and antibottom quark rebound away from the emitted W force carrier particles.
Frame 8: An electron and neutrino emerge from the virtual W- boson, and an up quark and down antiquark emerge from the virtual W+ boson.
Frame 9: The bottom quark and bottom antiquark,
electron, neutrino, up quark, and down antiquark all move away from one another.
Each such decay yields a set of color neutral particles made from the quarks that materialize from the energy in the color force field. Many different outcomes are possible.
Charm / Anticharm Annihilation
The charm quark and the charm antiquark in a 
(eta-c) meson annihilate into a pion (
) and two kaons.
Frame 1: A charm and anticharm quark head towards each other....
Frame 2: ...annihilate...
Frame 3: ...into virtual gluons.
Frame 4: A strange quark and an antistrange quark emerge from the gluon cloud.
Frame 5: As the quarks move apart, a color force field develops between them.
Frame 6: The energy in the field increases as the quarks further separate, until there is sufficient energy in the force field to be converted into an up quark and and an up antiquark.
Frame 7: The up and antistrange quarks begin to separate.
Frame 8: There is sufficient energy in the color field between the up quark and the strange antiquark to produce another quark-antiquark pair (down and antidown).
Frame 9: The anti-up and strange quark segregate into a color neutral meson (a kaon).
Frame 10: All the quarks have formed color neutral pairs. The up - antidown particle is a pion, and the down - antistrange particle is another kaon.
Bubble Chamber Photograph
This is an actual bubble chamber photograph of an antiproton
(entering from the bottom of the picture) colliding with a proton (at rest) and annihilating. Eight pions were produced in this annihilation. One decayed into a 
+ and a 
. The positive and negative pions curve different ways in the magnetic field.
The bubble chamber is a more advanced detector than the cloud chamber, but much less powerful than modern detectors.
Definitions & Descriptions:
Virtual Particles
In many decays and annihilations, a particle decays into a very high-energy force-carrier particle, which almost immediately decays into low-energy particle. These high-energy, short-lived particles are virtual particles.
The conservation of energy seems to be violated by the apparent existence of these very energetic particles for a very short time. However, the Heisenberg Uncertainty Principle, if the time of a process is exceedingly short, then the uncertainty in energy can be very large. Thus, due to the Heisenberg Uncertainty principle, these high-energy force-carrier particles may exist if they are short lived. In a sense, they escape reality's notice.
The bottom line is that energy is conserved. The energy of the initial decaying particle and the final decay products is equal. The virtual particles exist for such a short time that they can never be observed.
In summery the virtual particles include the W and Z particles, graviton, and vertual photon, were origanaly postulated to account for some of the observed atomic phenomena and make them comply with quantum theory.
Recent experiments have demonstrated the existence of X and Z particles.
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