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Quantum Chromodynamics

An explanation of the Strong Interaction. Hint: it has nothing to do with color.
by

Alexander Constant

on 23 May 2011

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Transcript of Quantum Chromodynamics

Here is an atom Because the nucleus has only
positive and neutral charges
(protons and neutrons) it has a
net positive charge. The protons in the
nucleus have a
mutual repulsion, and should
be unable to clump together. However, as the nucleus does exist,
there must be a force holding
it together, something much
stronger than the electric force. The theory that describes this force is called Quantum Chromodynamics In the current model of hadronic structure (the structure of neutrons, protons, and various "exotic particles") hadrons are composed of either two or three quarks. Quarks, proposed independently in 1964 by Murray Gell-Mann and George Zweig, are the most fundamental known constituents of matter. Shown below are the six types of quark.
The up and down quarks are the least massive, and most stable of the quarks. The other four types of quarks are only created during high energy interactions and quickly decay into either up or down quarks. Proton Neutron Nucleons Color Charge Quarks take on a color charge, which is somewhat
similar to the electric charge of protons and electrons. Unlike in electric charge, where there exists only one charge (a positive charge and its anti-charge, negative) there are three different "color" charges, red, blue, and green, each with its own anti-charge: anti-red, anti-blue, and anti-green. As shown in the graphic, each stable hadron is composed of one quark of each "color". These three different colors cancel each other out and give the hadron a net color charge of 0. The force between quarks is transmitted by a
"force carrying" particle, much like the photon,
called a gluon. Unlike the electrically neutral photons which mediate the electric force, gluons carry a color charge themselves and therefore participate in the color force. The evidence for "color" lies in the fact that two fermions cannot occupy the same state at the same time, as explained in the Pauli Exclusion Principle. Therefore, the quarks' quantum states must differ - thus, three different "colors". The term "color" is used to describe these quantum states and has nothing to do with actual color. Gluons comprise any combination of a color and a different anti-color (i.e. red and anti-blue), and therefore there are eight types of gluon. Inside the bounds of about . 00000000000001 meters, quarks are free to move about as they please, called asymptotic freedom. This occurs because at short distances, the color force WEAKENS. Therefore, once a quark is knocked out of a hadron, as in particle accelerators, it becomes more energetically favorable to pull an anti-quark from the vacuum than to continue to lengthen the flux tube. The anti-quark then pairs with the separated quark and they form a meson, which then decays into an electron, a neutrino, or a photon. In this way it is impossible to study quarks individually - they only exist for periods longer than a few 100 millionths of a second inside of hadrons. The strong interaction (color force)
is the strongest of the four fundamental forces - it is 100 times stronger than the electric force, 100,000,000,000,000 times as strong as the weak force, and 10,000,000,000,000,
000,000,000,000,
000,000,000,000,000
times as strong as gravitation. Color Confinement The nuclear force, which holds protons and neutrons together in the nucleus, is the residuum of the color force. Because gluons are able to interact with one another, they form "flux tubes" between two quarks at larger distances than this. This confines quarks inside of hadrons. Even when a quark is knockfed out of a hadron, in its moment when it is unpaired, it is observed not as a free quark but as a "jet" of hadrons. This is one of the least understood occurences in particle physics.
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