What is it that quarks are actually made of? Will they decay into this substance?
As the up and down quarks are the lightest type of quark do they not decay?
I was thinking that if this could happen, there must be lighter quarks?
In response to the comment:
I know that they don't exist on their own but a quark must surely decay at some point into what makes them up?
So I wanted to know: if they decay what do they decay into, and if they do not what are quarks made of?
Quarks as we know them are fundamental particles, which means that they do not have smaller constituents. This however does not imply that they cannot decay. A particle in quantum field theory does not need to have constituents to decay into, it can in principle decay into any particle its corresponding field couples to (interacts with), as long as it obeys certain conservation laws. A top quark for example can decay through the weak interaction into a W-boson and another, lighter quark (bottom, down or strange). Up and down quarks can decay into each other by emission of a W boson (this is the origin of beta decay due to the fact that the W can, depending on its type, decay into electrons, positrons and electron (anti-)neutrinos, ).
The current understanding of quarks is, that they are a fundamental particle. This means for the energy scales currently available in particle accelerators all quarks have behaved like point-like particles. Due to the strange nature of the color-field (the energy stored in it increases with distance instead of decreasing) if you break a proton apart (which is made up of two up-quarks and one down-quarks) there will be jets of particles which the detector actually detects, this doesn't mean though, that the quark is not a fundamental particle in the currently understood sense. Hope this helps!
The $u$ & $d$ quarks decay into $d$ & $u$ quarks and bosons (e.g., W bosons)--this is effectively what happens to the hadrons in weak interactions. This (incomplete) chart shows, for instance, $$ u\to d+W^+\\ d\to u+W^- $$ There isn't anything sub-quark, as far as the standard model goes.
I think that we may add that in the confining phase, the QCD-string descritpion of quarks (say, mesons, which are bound states of quark/anti quarks) is that these particles sit at endpoints of "QCD-strings" (I use "" to distinguish this from the normal superstring which is a well defined object, though it failed for the moment at describing exactly confinement in non-supersymmetric theories). This QCD-string is then a sort of "flux-tube" for the strong interaction, and in this picture, it is a more fundamental object than the quarks which sit at the endpoints, and it rather describes "partons" (i.e. it includes gluons in addition to quarks and mix them up in a string).
Of course this is not a well understood formulation, were it be the case confinement would have been solved.
Very insightfull steps in this direction are obtained for supersymmetric QCD like theories for which you take the number of colors N to infinity and the coupling constant of your theory to zero (the so called 't Hooft limit).
