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Question:
If protons and neutrons are made up of quarks, what are electrons made? Why cannot you isolate a quark? What is the mass of a quark?



Replies:
As far as is known (this being Arthur Smith again...) electrons and quarks (as well as neutrinos, photons, the W and Z bosons and gluons, along with other particles probably still undetected) are "elementary" - they are not composed of any smaller objects. The evidence for this is that they do not show any "structure". The neutron and proton, when bombarded with electrons at high energy, show evidence of having an internal structure (the quarks) but when you collide electrons with other electrons or positrons, there is no such internal structure to be seen. But that may be just because we cannot experimentally reach high enough energies - it still could be that there is something else in there (according to the "string" theories everything is really made up of strings which can only be seen at billions of times higher energy than we have been able to achieve so far).

The accepted reason why you cannot isolate a quark is related to the reason why it is hard to build up a lot of charge on an object. Trying to separate positive and negative charges requires a lot of energy, and even in high-capacity capacitors only a very small fraction of the electrons have moved from one capacitor plate to the other. Inside the nucleus, the quarks carry a color charge which all cancels out. If you try to separate them out, this net "color" charge creates an energy that increases many times faster than electrical charge energy (remember it is called the "strong" force). So the only things that can be seen a large distance apart are things that are "color" neutral - these are the nucleons (protons and neutrons) and the mesons (pions etc.) and similar beasts.

The masses of the small quarks are not known very well, as I recall, although I think they are around the 100 MeV level (the electron is 500 or so times lighter). That is for the up and down quarks in the nucleus. The heavy quarks (strange, charm, bottom, and the recently discovered top) get increasingly more massive, with the top quark more than 1000 times heavier (around 180 GeV).

Arthur Smith



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