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# 9.5: Color Symmetry

So why don’t we see fractional charges in nature? This is an important point! In so-called deep inelastic scattering we see pips inside the nucleon – these have been identified as the quarks. We do not see any direct signature of individual quarks. Furthermore, if quarks are fermions, as they are spin $$1/2$$ particles, what about antisymmetry of their wavefunction? Let us investigate the $$\Delta^{++}$$, see Fig. [fig:D++], which consists of three $$u$$ quarks with identical spin and flavour (isospin) and symmetric spatial wavefunction, $\psi_{\rm total} = \psi_{\rm space} \times \psi_{\rm spin} \times \psi_{\rm flavour}.$ This would be symmetric under interchange, which is unacceptable. Actually there is a simple solution. We “just” assume that there is an additional quantity called colour, and take the colour wave function to be antisymmetric: $\psi_{\rm total} = \psi_{\rm space} \times \psi_{\rm spin} \times \psi_{\rm flavour} \times \psi_{\rm colour}$ We assume that quarks come in three colours. This naturally leads to yet another $$SU(3)$$ symmetry, which is actually related to the gauge symmetry of strong interactions, QCD. So we have shifted the question to: why can’t we see coloured particles?

This is a deep and very interesting problem. The only particles that have been seen are colour neutral (“white”) ones. This leads to the assumption of confinement – We cannot liberate coloured particles at “low” energies and temperatures! The question whether they are free at higher energies is an interesting question, and is currently under experimental consideration.