According to my notes "any pair of similar quarks must be in identical spin states". What is the reason for this?
Very unclear. I do not expect the quark spin states in your left leg to be anyhow related to my grandmother's right arm quark spin states. So, which pairs are you talking about ?According to my notes "any pair of similar quarks must be in identical spin states". What is the reason for this?
Quarks are fermions. Two identical quarks are undistinguishable inside a hadron. From the spin-statistics theorem, the "wavefunction" of a hadron in terms of quark and gluon degrees of freedom (assuming we can construct such a thing, although it should exist obviously in principle) must be anti-symmetric under the exchange of two quarks. It is one thing that is postulated, but for which people believe there should be a rigorous demonstration, that all hadrons occurring in Nature as free states must be color singlets. The color part of the wavefunction is therefore antisymmetric. So the rest of the wavefunction, in the space of flavor times spin times position for instance, should be symmetric. Note that the ground state, with the space part being obviously symmetric, has spin times flavor symmetric as well. So for two identical flavor, you get the symmetric spin state you were asking about.