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Fundamental particles and forces

A major goal of physics over the centuries has been the search for the most fundamental building blocks of matter. We have progressed from earth, air, fire and water, through the atoms of Democritus, to Mendeleev's realization that the elements showed a regularity that was eventually explained by saying that the elements are not fundamental, but are made up of even smaller structures - nuclei and electrons. We have seen that the nuclei are made up of protons and neutrons.

21.3.1 Fundamental particles

Since the 1950s, physicists studying elementary particles have been able to use accelerators, like that shown in Fig. 21.18, to bring particles together at high energy. (Before the advent of accelerators, naturally produced cosmic rays were used.) When the energy of the collisions is just right, particles can be created out of the excess energy. As accelerators with more energy became available, increasing numbers of fundamental particles were found. Physicists began to suspect that these particles were no more fundamental than the 92 elements are.

To follow these results, we divide the various particles into groups, according to their ability to interact via the strong nuclear force. This is shown in Table 21.1. Particles that do not interact by the strong force, but still obey the Pauli exclusion principle, are called leptons. The most familiar lepton is the electron. The strongly interacting particles are called hadrons. We further divide the hadrons into the more massive baryons, and the less massive mesons. The most familiar baryons

Fig 21.18.

Aerial view of Fermilab, the large particle accelerator outside Chicago.The ring has a diameter of over 6 km. [Fermilab photo]

Fig 21.18.

Aerial view of Fermilab, the large particle accelerator outside Chicago.The ring has a diameter of over 6 km. [Fermilab photo]

Table 21.1.

Classification of elementary

particles.

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