The main constituents of atoms are protons, neutrons, and electrons. After the revision of Dalton’s atomic theory, these particles were considered to be elementary particles, the basic constituents of matter. Later, Yukawa recognized that the nucleons interact with each other through the meson field, and a new elementary particle, the meson, had to be postulated. Moreover, several different kinds of mesons with different rest masses and charges have been discovered. In addition, new elementary particles have been observed in different nuclear and cosmic processes. Today, more than 300 ele­mentary particles are known (this fact raises the ironic question: How can something be called elementary if there are hundreds of them?).

The elementary particles can be classified on the basis of rest mass: light and heavy elementary particles are called leptons and hadrons, respectively. Hadrons can be divided into two groups: mesons (with medium rest mass) and baryons (with large rest mass). They are characterized similarly to the nuclei; as listed at
the end of Section 2.3, they have rest mass, electric charge, spin, parity, statistics, magnetic moment, and electric quadruple moment. In addition, an important prop­erty of the elementary particle is the mean lifetime.

The heavy particles, hadrons, consist of more fundamental particles, which are called “quarks.” Particles are referred to as fundamental if they exhibit no inner structure. Quarks can be experimentally demonstrated, for example, by irradiating protons with 50 GeV electrons. The magnetic momentum of neutrons implies the presence of charged particles inside the neutron as well.

The physics of the elementary particles postulate six types and three families of quarks (up—down, charm—strange, top—bottom). Within the atomic nucleus, the up and down quarks are the most important. The rest mass of up and down quarks is about 1/3 a. m.u. and their charge is +2/3 and —1/3, respectively. The proton consists of two up quarks and 1 down quark; the neutron contains one up quark and two down quarks. The sum of the rest masses of the three quarks gives 1 a. m.u. for both proton and neutron. In addition, the net charge of the nucleons (+1 for protons and 0 for neu­trons) is the summation of the individual charges of the quarks. Table 2.3 illustrates the important properties of some elementary particles. The particles with half-integral spin (fermions) are the fundamental constituents of matter; the particles with integral spin (bosons) are the exchange particles between quarks, which are similar to the exchange of photons in the electromagnetic force between two charged particles.

Interactions in the last column of Table 2.3 can be ordered on the basis of their relative strength as follows:

Interaction Relative Strength

Strong 1

Electromagnetic 10—2 Weak 10—14

Gravitation 10—39

The range of the interactions is inversely proportional to their relative strength. In nuclear processes, strong interactions are dominant. The range of strong interac­tions is about 10—15 m.

The antiparticles of all the particles listed in Table 2.3 could and should exist. The electric charge of these antiparticles is the opposite of their corresponding par­ticles. When the particle—antiparticle pairs interact with each other, they form other particles with lower or zero rest masses. As an example, the annihilation of positron and electron could be mentioned, which have a great practical importance (as discussed in Section 5.3.3).