Subatomic Particles

New Discoveries of Subatomic Particles Taken from “Conceptual Physics” text by Paul Hewitt The discovery that all atoms were made of just three types of particles showed a great simplicity in the structure of matter. Unfortunately. this simplicity was soon proven false. In the decades following the discovery of the neutron, scientists discovered more subatomic particles. These particles were not stable; they did not exist in ordinary matter. Instead they turned up when atomic nuclei were struck by high-energy particles or radiation. A target nucleus would shatter, giving rise to one or more new particles. Such collisions happened very infrequently in nature. In order to have more collisions to study, scientists built cyclotrons, synchrotrons, and linear accelerators. These machines differ in the manner by which the particle “bullets” are accelerated, As a result of these new tools, however, hundreds of new subatomic particles have been discovered. This huge family of particles can now be explained in terms of more truly elementary particles. Physicists now refer to two families of particles. The first, the leptons, includes the electron, the mu-meson (or muon), the tau-meson, and three types of neutrino. Each neutrino is matched to each of the three particles mentioned. Neutrinos have no charge and no detectable mass. These six particles are grouped as a family because they are affected only by the electroweak force. This is a force that affects charged particles and is also involved in certain kinds of radioactive decay. Leptons seem to be truly elementary. No evidence yet exists that leptons are made of smaller particles. The second family of particles is called the hadrons. These particles are affected by the strong force that holds the nucleus of an atom together. Hundreds of hadrons are known, including the familiar proton and neutron. These particles are not elementary. Physicists have discovered that hadrons consist of smaller particles called quarks. The six types, or, “flavors,” of quarks are called up and down, top and bottom (or truth and beauty), and strange and charm. Each type of quark can have one of three “colors.” These are not colors in the normal sense but they are more like a kind of charge. Up, top, and charm quarks have a charge of +2/3. The remaining three quarks have a charge of -1/3. The proton is made of two up quarks and one down quark; thus its charge is +1. The neutron is made of two down quarks and one up; its charge is 0. Each lepton and quark also has a corresponding “antiparticle.” For example, the anti-electron is called the positron. It has the same mass as the electron but a charge of +1. The anti-down quark has the same mass as a down quark but the opposite charge and color. When a particle collides with its antiparticle, both are annihilated. A burst of energy is released, and new particles are formed. The study of the forces within atoms is equally important and fascinating. Physicists have identified the particles that carry the atomic forces. The photon, which has no charge and no mass, carries the electromagnetic force. Other particles, called bosons, carry the strong and weak forces mentioned earlier. The mass less particles that carry the strong force, holding quarks together, are called gluons. Those that carry the weak force are known as the W+, W-, and Z particles. Unlike photons and gluons, these particles are very heavy. Their mass is as much as 100 times the mass of the proton. Again the number of “elementary particles” has grown. They now include 6 types of quarks, 6 leptons, 24 antiparticles, 8 gluons, the W and Z bosons, and the photon. The physicists’ hope for simplicity is not satisfied by these numerous subatomic particles. Hence, the search for truly fundamental particles continues.