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Quantum Theory

The Quantum Atom

pentaquark, kaons, strange quark, strong nuclear force, hadrons

Scientists divide the six different types, or flavors, of quarks into three categories called generations. The up and down quarks belong to the first generation, the charm and strange belong to the second generation, and the top and bottom belong to the third generation. Unlike other elementary particles, quarks have electric charges that are a fraction of the standard charge—that is, the charge (e) of one proton. Other particles have an integer multiple of this charge.

The top quark is very massive compared to the other quarks—for example, it is over 30,000 times more massive than the up quark. Its great mass made the top quark difficult to produce in the laboratory. To create such heavy elementary particles, physicists use particle accelerators. These machines speed up smaller particles and collide them with one another, changing their energy of motion into matter. The more massive something is, the more energy required to produce it. Scientists usually give the mass of particles as small as quarks in giga-electron-volts (GeV). An up quark has a mass of about 0.005 GeV. By comparison, the mass of the proton is about 1 GeV.

Quarks combine with each other to form a class of particles called hadrons. Only two types of hadrons have been observed in nature: the baryon and the meson. Baryons contain three quarks, while mesons contain a quark and an antiquark. The theory of quantum chromodynamics does not forbid other combinations from occurring, and in 2003 physicists searching for new combinations announced the production of a particle called the pentaquark. As its name suggests, the pentaquark consists of five quarks.

Protons and neutrons are baryons. Protons contain two up quarks and a down quark. The electric charges on these quarks combine to give the proton a charge of 1. Neutrons contain one up quark and two down quarks. The charges on these quarks combine to give the neutron a charge of 0.

Physicists discovered mesons while studying cosmic rays, high-energy particles that originate in space. Mesons that enter Earth’s atmosphere are called pions. Pions may consist of an up quark and a down antiquark (with an electric charge of 1), a down quark and an up antiquark (with a charge of –1), or an up quark and an up antiquark (with a charge of 0). Pions exist for only a brief time, then quickly decay into other particles. Physicists have created another type of meson, called a kaon, in the laboratory. All kaons contain either a strange quark or a strange antiquark, as well as another antiquark or quark. Like pions, they can have an electric charge of 1, -1, or 0. They also exist for only a brief time before decaying into other particles.

A force called the strong nuclear force binds quarks together in hadrons and binds hadrons to one another. The strong force holds protons and neutrons together in the nuclei of atoms. Quarks affect one another through the strong force by exchanging gluons, particles of energy that carry the strong force. Gluons bind quarks together by changing a property of quarks called color charge. Quarks can have a color charge of red, green, or blue, while antiquarks can have a color charge of antired (cyan), antigreen (magenta), or antiblue (yellow). Anticolors are complementary to the colors of quarks. The color charges of the quarks in a baryon are all different and add up to 0, or white, making the baryon “colorless.” The color charges of the quark and antiquark in a meson are complementary and also add up to 0 (white), making the meson colorless. Because they are colorless, the strong force does not directly affect baryons or mesons.

In certain circumstances, quarks cannot interact through the strong force, in which case either the weak force or the electromagnetic force takes over. When the weak force is at work, quarks decay into other quarks. Rules of particle physics govern which quarks can decay into which, but the main rule is that quarks can only decay from a heavier one to a lighter one. For example, the weak force causes the top quark to decay into the bottom quark.

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