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Quarks are fundamental particles that make up composite particles like protons, neutrons and other hadrons by way of exchanging force mediating particles known as gluons. This takes place through what is known as the strong interaction. There are six types (or flavors) of quarks: up, down, charm, strange, top, and bottom.
Quarks are the most active among the fundamental particles, participating in strong, weak, electromagnetic and gravitational interactions through practically all mediating particles, gluons, photons, W and Z bosons, and gravitons respectively.
Although quarks have not been found to exist in isolation, their existence is proven through experimental data that are consistent with mathematical predictions. When we say a particle exists in isolation it means that it can be found on its own. For example, we can play around with free electrons in cathode ray tubes even if they are known to be parts of atoms.
In the case of quarks, they are only known to exist within hadrons. Quark masses are so light. To give you an idea how light they are, consider the mass of a proton at 0.938 GeV/c2. The combined masses of one up quark and one down quark, which make up the proton, sum up to only 0.02 GeV/c2. These are just mere estimates of course, determined through, again, mathematical predictions and experimental data.
Now, the only way to reconcile the difference between the two masses mentioned above is by assuming that the energies that hold them together are very large. Hence, the term strong interaction. That should be a very strong interaction indeed.
Quarks also have charges and are expressed as fractions of e, the elementary charge. This elementary charge is simply the magnitude of the charge carried by the proton or electron, which you may be familiar with from high school physics. Just to refresh your memory, this magnitude is around 1.602 x 10-19 Coulombs.
Thus, typical quark charges are like (+2/3)e for up, top, and charm quarks, and -(1/3)e for down, strange, and bottom quarks.
Among the physicists who have contributed to the collection of information that we know regarding quarks are: Murray Gell-Mann, George Zweig, Yuval Ne’eman, Sheldon Lee Glashow, James Bjorken, Makato Kobayashi, Toshihide Maskawa, and of course, Richard Feynman.
Experiments that led to the accumulation of information regarding quarks were performed in prominent high energy laboratories like the Stanford Linear Accelerator (SLAC) and Fermilab, as well as less familiar ones (to the general public, that is) like Brookhaven National Laboratory.
You can read more about quarks here in Universe Today. Want to know how Fermilab scientists discovered a rare single top quark? You may also join us as we ponder on the question: Could quark stars explain magnetars’ strong magnetic field?
There’s more about it at the SLAC website and Physics World. Here are a couple of sources there:
Here are two episodes at Astronomy Cast that you might want to check out as well: