A B-factory is an electron-positron collider tuned to produce pairs of B-mesons, i.e. mesons carrying a b- or bottom-quark. Unlike a conventional collider, where the counter-rotating beams have identical energies, here the electron and positron beams have different energies: the particles produced in these "lop-sided" electron-positron collisions emerge at relativistic speeds (close to the speed of light).
Quarks are fundamental particles that carry fractional electric charge and are strongly interacting. They also have a quantum number called "colour". The three quark colours are red, yellow and blue, and all known particles are "colourless", which means that each of the three colours is present in equal proportions.
Strong Nuclear Interactions
The Strong Nuclear interaction is an interaction between quarks, mediated by particles called gluons, that serve as carriers of "colour". It is a short range force operating at the femtometer range. Leptons do not experience the strong interaction, but gluons interact among themselves. As Yukawa explained, Strong interactions are responsible for holding the nucleus together through a reaction that manifests itself as an exchange of mesons between neutrons and protons.
Weak Nuclear Interactions
Weak interactions are experienced by both quarks and leptons. These are short range forces mediated by the so-called intermediate bosons, W± and Z0. The vector bosons interact among themselves. The weak interaction is currently believed to be a manifestation of the electroweak interaction. Weak interactions are responsible for beta decay, and it is only by weak interactions that neutrinos can be located.
Electromagnetic interactions are interactions between particles mediated by photons. Photons do not interact among themselves as they are chargeless. Among other things, this interaction is responsible for various resonance decays. The electromagnetic interaction is a manifestation of the electroweak interaction.
The Gravitational interaction is a long-range force. Gravity is not an important effect at accelerator energies. It is described in terms of the Newtonian constant, GN. At very high mass scales of around the Planck Mass (~1.22 * 1019GeV), they are conjectured to become important. Supergravity theories attempt to combine the gravitational interaction with the strong and the electroweak.
Unified Field Theories
It was Albert Einstein that first suggested that all fundamental forces can be described by a single field theory, after he developed the general theory of relativity to describe gravity. He attempted to construct one, but he failed. At subatomic distances, interactions are described by quantum field theories, but no one has succeeded so far in unifying all the fields convincingly.
In the late 1960s, Weinberg, Salam and Glashow, building on other developments. described how it would be possible to treat electromagnetic and weak interactions as different aspects of a single interaction. They predicted that this symmetry between electromagnetic and weak interactions would be manifest at very large momentum transfers. At low energies, this symmetry would be broken. Of the four mediators, one (the photon) would be massless, while the others (the vector bosons) would be massive. Because of this, weak interactions would be short-range and apparently feeble as compared to Electromagnetic interactions. Like Maxwell's electromagnetic theory before it, this theory contains one arbitrary parameter, the weak mixing angle given by sin2ΘW, which has to be determined by experiment. This model has been verified experimentally over the last three decades with increasing accuracy.
The Higgs Mechanism
To explain the fact that leptons (which are believed to be fundamental, point particles) have different masses, as well as to solve the problem of the divergence of the electroweak model at very high energies, the Scottish Physicist, Peter Higgs postulated a mechanism - the Higgs Mechanism - which explained mass as the result of an interaction between a particle and the Higgs Field. This required the existence of the Higgs boson. Physicists have been searching for the Higgs boson for a while, but all the various experiments at LEP (Large Electron Positron collider) have found possible indications of the existence of such a particle at around 115 GeV/c2. It is hoped that this particle will be well within reach LEP's successor, the LHC (Large Hadron Collider).