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?
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).
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Update: June 2012