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I have not understood what it means when one writes ##\pi^+=u\bar{d}##, for example. I though it simply meant that the ##\pi^+## meson was composed of one up-quark and one anti-down-quark. However, that doesn't explain what writing ##\pi^0=\frac{1}{\sqrt{2}}(d\bar{d}-u\bar{u})## means. I'd say that writing ##\pi^+=u\bar{d}## means that the ##\pi^+## meson's wavefunction is the tensor product of the wavefunctions of ##u## and ##\bar{d}##. Is that it?

A related question: how can one draw the feynman diagram for something like ##\pi^0\rightarrow A+B##? My problem is that I usually start by writing the quarks which constitute each particle, but in this case the left hand side particle is a composition of two "states". Shall I draw one feynman diagram for ## d\bar{d}## and another one for ##u\bar{u}##?