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7

When it comes to fundamental charges, the (left-handed) up-type quarks actually have either the same values of the charge as the down-type quarks, or exactly the opposite ones. It just happens that the electric charge isn't a fundamental charge in this sense. Let me be more specific. All the quarks carry a color – red, green, or blue – the charge of the ...

0

If you are looking for symmetry, I think one should point out that there IS a particle with a -2e/3 charge and a particle with a +e/3 charge. They are the up antiquark and down antiquark respectively. Now, following that, you would very reasonably ask the question why we observe more up quarks than up antiquarks, and other follow-up questions like ...

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The names up and down don't refer to electric charge $Q$ but are rather references to isospin charge $I_3$.

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A neutron consists of three quarks $u d d$(up down down quarks). The up quark(u) carries charge $2e/3$ and the down quark(d) carries a charge $-e/3$. Thus $2e/3-e/3-e/3=0$

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The up quark has a charge of $+2/3$, the down has a charge of $-1/3$. If you have a bound state of charged particles, the total charge is just the charge of the elementary constituents. The neutron consists of one up quark and two down quarks, so the total charge $Q$ is: $$Q = 2/3 + 2 \times (-1/3) = 0$$

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Because $2/3-1/3-1/3=0$.

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Re your question 1, have a look at my answer to Conversion of mass and energy. You might also want to look at What keeps mass from turning into energy?, which is related. These give a basic desciption of how the energy of motion can turn into particles of matter like top quarks. Re questions 2 and 3: the discovery of the top quark was made at the Tevatron ...

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