At first sight these so-called elementary particles form a very inhomogeneous group. The mass of a particle may be large (as ME-S1321 Mev) or may be very small (as the mass of y = 0). The lifetime may be very short (as the lifetime of 2° = 10-20 sec) or may be very long (as in the case of a proton which has infinite lifetime). The particle may be electrically charged or may be neutral. Except for a very few of them, we do not know the spin and parity of these particles. Again except for those between a very few of these particles, we do not know their interactions. In fact there is still confusion as to the identity of the particles that have already been observed. Some of these particles that look very different may turn out to be the same particles. Some of them that look very similar may indeed be several different particles. In spite of this almost complete lack of understanding, some general patterns and general rules have been found. These properties will be our main topics of discussion.
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