If a baryon wavefunction is $\Psi = \psi_{spatial} \psi_{colour} \psi_{flavour} \psi_{spin}$,
and we consider the ground state (L=0) only.
We know that the whole thing has to be antisymmetric under the interchange of two quarks. We know that colour is antisymmetric (always colourless) and spatial is symmetric. Therefore, the combination of flavour and spin has to be symmetric.
That's fine, I understand that. However, I'm very uncertain about what the 'interchange of two quarks' actually means. Interchange how?
For the 'easy' example of |uuu>. How is the flavour symmetric?
Basically, what does it mean to be symmetric in the quark model?
Best Answer
Halzen, Martin: Quarks and Leptons has a good introduction into that matter in Chapter 2.
$|uuu\rangle$ is actually an abbreviation of the idea that we keep an eye on three quarks in particular order, call them "first", "second" and "third"; and then first quark is in $u$ flavour state, of $SU(2)$ or $SU(3)$ flavour group, that is, $$\left(\begin{array}{c}1\\0\end{array}\right)\qquad\text{or}\qquad\left(\begin{array}{c}1\\0\\0\end{array}\right),$$ and second and third quarks are in the same state (so we have to take a tensor product of three such columns, like a 3D matrix).
To check the symmetry of this state, we exchange the states of first and second quarks. (In terms of the 3D matrix, we transpose it on 1st and 2nd axes, in terms of the tensor product we permutate its indices.) Since we had (only a 2D slice is shown) $$\left(\begin{array}{cc}1&0\\0&0\end{array}\right)\qquad\text{or}\qquad\left(\begin{array}{ccc}1&0&0\\0&0&0\\0&0&0\end{array}\right),$$ our state is immune to such exchange, and we thus call it symmetric. Other possible outcomes would be antisymmetric, or neither of these two symmetries. For example, $|udu\rangle$ is neither symmetric nor antisymmetric, and $\tfrac{1}{\sqrt{2}}(|udu\rangle-|duu\rangle)$ is antisymmetric (with respect to two first quarks only, for simplicity).
If we take into account other variables than just flavour, then we should exchange all variables of two quarks in order to "interchange the quarks".