## Elementary Particles and the World of Planck Scale

### Elementary Particles

Although the table for elementary particles in Figure 15-02 is somewhat reminiscent to the periodic table discovered way back in
1869, it represents the cumulative efforts of theoretical and experimental research in physics over the last fifty years. The theoretical frame work is more sophisticated and the experimental tools are highly complex and expensive. Figure 15-04a is a pictorial view. Figure 15-03 lists the fundamental interactions, which take place between the various particles. To understand nature at its fundamental level we have to examine these two tables in detail.

#### Figure 15-03 Fundamental Interactions [view large image]

• The mass of the particles are listed on the lower right-hand corner. These data are taken from the 2004 Edition of Review of Particle Physics.
• The lifetime of the particle is indicated on the left. Stable means infinite lifetime. Most of the quarks are labelled as variable. Their lifetimes depend on the nature of the individual meson or baryon in which the quarks reside. The gauge bosons with mass are unstable. In fact, elementary particles having mass over the range of 5 Mev tend to be unstable.
• Each member in the fermion generations has its own anti-particle with opposite electric charge and/or colour charge, e.g., the anti-particle of electron is the positron with one unit of positive charge and identical mass.
• The electric charge for quarks appears to be either -1/3 or 2/3. However, when three of them combine to form a baryon (fermion affected by strong interaction); it is always in such a way that the total charge is +2, +1, 0, or -1. When a quark and anti-quark combine to form a meson (boson affected by strong interaction); it is always in such a way that the total charge is +1, 0, or -1.
• Each quark can carry either one of the three varieties of colour charge -- red (+1/2), green (-1/2), and blue (0). This is just a label (a name) and has nothing to do with optics. In order to form a baryon the three quarks must each carry a different colour to add up to white (0). While in a meson the colour would be balanced by the anti-colour (such as red and anti-red).
• The force carriers are gauge bosons, which mediate the various kinds of interaction between the fermions. While photon does not carry electric charge, the gluons and the W bosons do carry colour charge, and electric charge respectively. The range and relative strength of these force carriers are indicated at the right in Figure 15-02.
• #### Figure 15-04a Particle Chart [view large image]

• For quarks, the anti-particles are typically represented by a bar placed over the letter that symbolizes them.

• Collections of quarks and antiquarks form composite particles known as hadrons, some of them are shown in Figure 15-04b. Hadrons are divided into mesons and baryons: mesons comprise a quark and an anti-quark; baryons (including the proton and the neutron) are three-quark states.

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