An Introduction to Quarks
Quarks. You may have heard of them in high school or college, little particles that make protons and neutrons. Quarks are subatomic particles making up matter at the smallest level. As of right now, nothing has been found to suggest that quarks are made up of even more minute particles, suggesting that they are the fundamental building blocks to the universe.
History
Quarks were discovered by Dr. Murray Gell-Man and Dr. Georgie Zweig in 1964 through independent experiments of the two. Dr. Murray Gell-Man had come up with a classification system for the zoo of particles at the time. It was based on SU(3) mathematical symmetry and called the eightfold way. His model helped to explain protons’ and neutrons’ magnetic properties, but it needed new particles to complete it. These are the quarks. The name quarks came from a book that Dr. Gell-Man came across: Finnegan’s Wake by James Joyce. The line in question was “Three quarks for Muster Mark”. The name has stuck ever since.
Dr. George Zweig made his contribution to the discovery of quarks after visiting the CERN. He suggested that baryons are composed of a triplet of quarks, which helped to explain their properties.
In 1968, deep inelastic scattering experiments were done at the Stanford Linear Acceleration Center, which proved the existence of quarks. Later experiments at the CERN showed that quarks had charges of ⅔ and ⅓.
Flavor & Color
Quarks have two classifications: flavor and color. These classifications help denote certain properties of the quarks and help to explain their behavior as well. Flavor comes from the quark’s mass and charge as well as some more arcane quantum numbers. For instance, an up quark contains a charge of +⅔, while a down quark contains a charge of -⅓. Since quarks all have either ⅔ or ⅓ charges, mass helps to further differentiate them. An example of flavors of quarks affecting the particles they make up can be seen in neutrons and protons. A neutron is made of 2 down quarks and 1 up quark. The resulting charge is 0, explaining the neutron’s neutrality. A proton is made of 2 up quarks and 1 down quark, explaining the proton’s charge of +1.
However, the model still had problems; quarks seemed to violate the Pauli Exclusion Principle. The Pauli Exclusion Principle states that two fermions or particles with a half-integer spin cannot exist in the same quantum state in the same system. Quarks have half-integer spins, and when making up particles like neutrons and protons, there seemed to be at least two identical quarks. This conundrum was solved with the proposal of colors. Colors are a property of quarks identifying how they stay together through the strong force. These colors are red, blue, and green or antired, antiblue, and antigreen for the antiquarks. A baryon, which is a particle composed of three quarks, would have quarks with the colors red, green, and blue to cancel each other out to create white. A meson, which is a particle composed of two quarks, would have the color red and antired, which would cancel each other out.
Strong Force
Color is the property of quarks to remain bound together. This represents gluons being transferred around the quarks. Gluons are particles that transmit the strong force among quarks. The mechanism with which gluons hold the quarks together is a very interesting one. When quarks are close together such as in a neutron, they act as though they are free particles. When a quark is knocked away, gluons use the energy from the quark’s movement to create more gluons. This pulls the quark back into the fold, restricting its movement.