Standard Model?
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| 1b.
What
is the
Standard Model?
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In the last half century, many particles and features
of their interactions have been discovered. Out of the confusing
and terrifying proliferation of hundreds of particles, a coherent picture
gradually emerged. This picture, now known as the Standard Model,
has been stable for two decades and explains everything (so far) in terms
of a dozen elementary particles and three of the forces, Strong,
Electromagnetic,
and Weak (gravitation hasn't been included effectively in the theory
yet). There are several excellent descriptions of the Standard Model,
to which you can link from our links page, so here is just a brief outline,
with the emphasis being on weak interactions and CP.
According to the Standard Model, the true elementary particles - the building blocks of matter as we know it - are quarks and leptons. They interact through the Strong, Electromagnetic, and Weak forces. The most common example of a lepton is the electron. Leptons are not susceptible to the strong force (they have a strong charge of zero). They include electrons, muons, tau's, which all have the same negative electric charge, commonly denoted -e, and an associated neutrino for each, a total of six particles, summarized in the table. All of the neutrinos have no electric charge. So they have zero strong and electric charges, but not zero weak charge. Quarks have strong charge and are thought to be the constituents of all the observed strongly interacting particles, known collectively as hadrons (neutrons, protons, mesons, hyperons). The Strong force holds the quarks together so strongly inside of hadrons that they are unable to exist as free particles. We now know that there are six types of quarks, each given a name, or flavor. The six flavors of quarks, have the names up (u), down (d), strange (s), charm (c), bottom (b), and top (t), in order of increasing mass. All quarks have fractional electric charge, +2/3e for u, c, and t, and -1/3e for d, s, and b, where e is the magnitude of the electron's charge. All quarks also carry weak charge. Each of the forces has associated with it one or more carrier particles. The carrier of the electromagnetic for, for example, is called the photon, and is familiar in one of its forms as visible light. The carrier of the strong interaction is called the gluon - there are 8 of them - and the carriers of the weak interaction are called Z0, W+ and W-. |