The question is in the title: What are quarks made of? Strings? Thanks.
Quarks are made of quarks... They are the fundamental blocks of all the things that exist in this world. They are indivisible because they are dimensionless.
Quarks are made of bunch of tiny strings that exist throughout 11 dimensions. What I heard was that 4th~11th spatial dimension (excluding time in here), they are extremely small, so small that it rarely interacts with the ordinary 3 spatial dimensions hence we don't feel their existence. Since these dimensions are so small, they manage to fit strings in there. I haven't taken a rigorous course on String theory, so don't take what I said to be 100% fact cause that's just what I heard from the talks of other scientists and physics students. I probably be missing some other important information of the theory.
I'd say the most important thing you didn't point out is that string theory is JUST theory with zero experimental results. It MAY turn out to represent the real world, or it may turn out to be a mathematically elegant dead end.
I can remember when Protons, Neutrons and electrons were the fundamental building blocks of the universe. Now its quarks until we find a way to look deeper into the quarks. we may end op with "x", "y" and "z" as the building blocks of quarks and then they will be the "building blocks" of the universe, until we take the next step.
Hi, I'd like to make an addition. Quarks, leptons (e.g. electrons, neutrinos) and the fundamental bosons (e.g. photons, W and Z bosons) are what we call elementary particles. This means they have no known substructure, and they are (today) considered to be point-size (zero size). For the electron there are no signs of any substructure down to about 10-18 m (or less, maybe, I'm not sure about the exact number).
My quick answer to the question "What are quarks made of?" is "nothing we know of" (and the case is the same for the electron). Whether (any of) the elementary particles have substructure or not is an open question. I also second what Phinds said above concerning String Theory.
These threads contain discussions related to sizes and substructure:
Sub Atomic Particle Sizes (Orders of Magnitude related)
What exactly is an electron? (page 2 in particular)
There's also John Baez FAQ about open questions in particle physics.
Well, what is the result of quark--anti-quark collision (the so called "annihilation")?
Photons. The same as for any annihilation.
That isn't really relevant to the question "what are quarks made of?".
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