By Sarah R.

Over time, science has gone to far stretches to answer the complex questions of what we are made of. Most people know that we are made of atoms, which are composed of electrons, protons, and neutrons. But what may people don't know is the particles that make up protons and neutrons, particles that are 1000 times smaller than the nucleus itself. These tiny particles are called quarks. Quarks and Leptons are the fundamental particles that make up all atomic matter in our universe. 

History of Quarks

In the 1960's, physicists theorized that there was a possibility that protons and neutrons were made out of smaller particles. In 1964, physicists Murray Gell-Mann and George Zweig came up with the Quark Model. At the time, the "particle zoo" was thought to be made up of leptons and what are called hadrons. Gell-Mann and Zweig went against this, saying Hadrons were not elementary particles, but were actually made up of different ratios of antiquarks and quarks. They postulated three different types, or flavors,  of quarks, known as Up, Down, and Strange. Many physicists had conflicted feelings about Quarks, many feeling they were just theoretical and just a replacement until we could understand the concept better. Soon after the quark model was released, two more physicists, Sheldon Lee Glashow and James Bjorken, agreed with the quark model and came up with a fourth flavor, called Strange. In 1968, experiments at the Stanford Linear Accelerator Center proved that Protons and Hadrons were indeed made up of smaller particles and not elementary. By 1973, all six quark flavors had been proposed. In 1974, the quark model was finally recognized for it's correctness. It wasn't until 1995 that the sixth quark, the top quark, was observed.

Different Quarks, Antiquarks, and Combinations

We currently have six different flavors of quarks, seperated into three generations. The first generation contains the Up and Down quarks. These quarks have the lowest mass of the other quarks, making them stable and the most common in the universe. In contrast, the other quarks are more massive, so they decay quickly. These four other quarks are in the second and third generation. The second generation quarks are known as Charm and Strange, and the third generation quarks are known as Top and Bottom. Because of their properties that make them rapidly decay, these four quarks can only be made in high energy collisions, which are what particle accelerators are sometimes used for. The Up, Charm, and Top quarks all have a charge of +2/3, while the Down, Strange, and Bottom quarks all have a charge of -1/3 (Shown by chart).

http://universe-review.ca/F15-particle.htm

Next on the list are antiquarks. With every particle, there is an opposite, or antiparticle. So there are anti-bottom particles, anti-top, anti-up, anti-strange, etc. Each quark has it's relative antiquark. The relative antiquark has the exact same mass as the quark is is relative to, but it's charge is the exact opposite. So an Anticharm particle has a charge of -2/3 because a Charm particle has a charge of +2/3.

Many different types of combinations can be made with different quarks and antiquarks. Each type of combination has a name. Hadrons are made up of 2 or more quarks that are held together by a strong nuclear force including gluons (the color force). Mesons are a combination of a quark and an antiquark. A baryon is a combination of three quarks. With each different way of putting quarks and antiquarks together, you can come up with a amazing amount of combinations.

http://www.phys.uu.nl/~thooft/quarks.gif

What Keeps Quarks Together?

Before understanding what keeps quarks together, it helps to know about the different force carriers and the forces they carry. There are four primary forces; Gravitational force, Weak Nuclear force, Electromagnetic force, and Strong Nuclear force. For each force, there is a particle that carries it. The theorized (and not yet observed) Graviton carries the Gravitational force, Bosons carry the weak nuclear force, photons carry the electromagnetic force, and gluons carry the strong nuclear force. 

Quarks have a property called color charge. This actually has nothing to do with visible color at all. Instead, it's a way of describing the force holding quarks together. There are three colors, red, blue, and green, as well as the relative anticolors, such as antired, antiblue,and antigreen. The color charges cause bindings and repulsion between quarks, known as strong interaction. This area of physics is known as quantum chromodynamics. A quark with a color charge can be bound to it's relative anticolor, or three quarks with different color charges each can be held together. In baryon, there is always a red, green, and blue charged quark. The idea is that the combination of colors will cancel each other out, making it so the object as a whole has no color charge. So how do quarks obtain their color charge? The reason quarks react this way is because of gluons, the particles that carrie the strong nuclear force (as mentioned before). Gluons are always being exchanged between quarks. When a gluon is exchanged, a color change happens to the emitting and receiving quarks. So even though quark colors are constantly changing, the balance of attraction stays the same. 

On that same note, the force that gluons carry actually tend to be weak when gluons are close together. Quarks act almost as if they were freely moving, called asymptotic freedom. If a quark is knocked by an accelerated particle and is moving away from it's companions, though, the force of the gluons increase. This is due to the fact that gluons can create more gluons while being exchanged in between quarks. So when the quark is knocked away, the gluons utilize the energy from the moving quark and produce more gluons, creating a stronger force that prevents the quark from moving outside of the proton. 

Tying it Up

So, why would we want to know about quarks? Quarks give us a better understanding of how things work and how things are put together. If we know about elementary particles, we can extend our knowledge by combining them in accelerators and laboratories. We may even learn more about mysteries of science, such as dark matter or dark energy. From quarks, we can also understand different force carries and particle interactions. They may be small, but quarks give us a huge amount of information and almost infinite possibilities. 

References

http://school.eb.com/all/eb/article-9062172

http://en.wikipedia.org/wiki/Quark