Quark I INTRODUCTION Quark, smallest known building block of matter.

Quark I INTRODUCTION Quark, smallest known building block of matter. Quarks never occur alone; they always are found in combination with other quarks in larger particles of matter. By studying these larger particles, scientists have determined the properties of quarks. Protons and neutrons, the particles that make up the nuclei of atoms, consist of quarks. Without quarks there would be no atoms, and without atoms, matter would not exist as we know it. Six types of quarks exist. They are designated up, down, charm, strange, top, and bottom. All quarks have a certain mass and electric charge. Ordinary matter--that is, matter made up of atoms--contains only the two lightest quarks, up and down. The next lightest quarks--charm and strange--are found in particles called cosmic rays, which originate in space. Scientists have produced top and bottom, the heaviest quarks, in the laboratory, but they have not found these quarks in nature. Many physicists and astronomers believe that right after the big bang, the explosion that originated the universe, all six types of quark existed. The heavier quarks, however, immediately decayed into the lighter types. Quarks have antiparticle counterparts called antiquarks, which combine to compose antimatter. Antimatter does not exist in nature on Earth, and most scientists believe that it is fairly rare in the universe, but physicists have produced it in the laboratory. Antiquarks have many of the same properties as their corresponding quarks, but some of their properties are opposite to that of their counterparts. II CHARACTERISTICS AND BEHAVIOR 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. Strong Force and the Creation of a Particle The strong force holds together particles called quarks inside protons. When a fast-moving particle collides with a proton, the strong force can convert the energy of the collision into matter, resulting in the creation of a new particle. © Microsoft Corporation. All Rights Reserved. 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. Gluons and Color Charge Gluons are particles of energy that carry the strong nuclear force. They hold together particles called quarks and antiquarks, which combine to form hadrons. Examples of hadrons include protons and neutral kaons. As gluons bind together quarks, or quarks and antiquarks, they affect a property of quarks and antiquarks called color charge. The relationship between color charge and the strong force is similar to that between electric charge and the electromagnetic force. © Microsoft Corporation. All Rights Reserved. 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. Constituents of Matter Matter is composed of tiny particles called quarks. Quarks come in six varieties: up (u), down (d), charm (c), strange (s), top (t), and bottom (b). Quarks also have antimatter counterparts called antiquarks (designated by a line over the letter symbol). Quarks combine to form larger particles called baryons, and quarks and antiquarks combine to form mesons. Protons and neutrons, particles that form the nuclei of atoms, are examples of baryons. Positive and negative kaons are examples of mesons. © Microsoft Corporation. All Rights Reserved. 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. Family of Major Elementary Particles Elementary particles are thought to be the smallest units of matter. They are classified by mass, spin, and electric charge. © Microsoft Corporation. All Rights Reserved. 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. III HISTORY Scientists of the early 20th century believed that all matter was composed of three types of particles--protons, neutrons, and electrons--and that these particles could not be split into anything smaller. By the late 1950s, however, this simple model could no longer explain all the particles that physicists had discovered. Scientists needed a new model to make sense of their findings. In 1964 American physicists Murray Gell-Mann and George Zweig independently developed a theory of particle physics that proposed quarks as the building blocks of protons and neutrons. Gell-Mann borrowed the word quark from James Joyce's novel Finnegans Wake (1939), which contains the phrase "three quarks for Muster Mark." The two physicists needed only two types of quarks to describe the proton and neutron accurately: the up quark and the down quark. At about the same time, physicists also had discovered new elementary particles, including kaons, that they called strange. The explanation of these particles required a third type of quark, so physicists named it the strange quark. Physicists continued to use the three-quark model through the 1960s. Experiments conducted at the Stanford Linear Accelerator in Stanford, California, during the early 1970s supported the model. In these experiments, high-energy electrons collided with a target of protons. These experiments were analogous to experiments performed by British physicist Ernest Rutherford during the early 1900s. Rutherford had determined the inner structure of atoms by observing how particles scattered off atoms. In the 1970s scientists looked at how electrons scattered off protons to learn about the internal structure of the proton. These experiments conclusively supported the existence of quarks and gluons inside protons. Even though physicists didn't need more than three quarks to describe existing particles, American physicist Sheldon Glashow, Greek physicist John Iliopoulis, and Italian physicist Luciano Maiani developed a theory in 1970 that predicted the existence of a fourth quark, called the charm quark. A particle containing the charm quark, the second-generation partner of the strange quark, was discovered at the Stanford Linear Accelerator and at the Brookhaven National Laboratory in Brookhaven, New York, in 1974. The discovery in 1975 of a third-generation lepton, another building block of matter, led scientists to predict the existence of a third generation of quarks. The bottom quark was discovered in 1977 and, after a long search by physicists from all over the world, the top quark was found in 1995. The long-awaited discovery of the top quark filled a hole in the standard model, a theory that physicists had developed to explain particles and their interactions. The standard model predicted that three generations of quarks should exist, each one containing two different quarks. Current theory and data suggest that the six quarks scientists have discovered are all that exist and that quarks are fundamental--that is, they cannot be split into anything smaller. Physicists continue to conduct experiments, however, to discover whether new data will dispute their findings and to learn more about the properties of quarks. Physicists do not know how long single quarks exist (since they are always found in other particles), or whether quarks could combine in any other ways. Scientists suspect that particles containing one lepton and one quark, called leptoquarks, may exist, but they have yet to find any. Nor have experiments yet indicated how quarks acquire their masses. The standard model predicts the existence of a particle, called the Higgs boson, that would give quarks their mass, but scientists have yet to detect a Higgs boson in any experiment. Answers to these questions will have to wait for future experiments and theories that go beyond the principles of the standard model. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.