In this form it is clear that we can perform separate global flavour transfor- mations on ψ$_{L}$ and ψ$_{R}$ . Since the quarks are in an SU(2) isospin multiplet, we write these transformations as SU (2)$_{L}$ ⊗ U (1)$_{L}$ and SU (2)$_{R}$ ⊗ U (1)$_{R}$ respectively. The Lagrangian is unchanged by these transformations: it is chirally invariant. (The analysis still holds if we turn on the gauge field A$_{µ}$ .) However, this is a symmetry of the Lagrangian which is broken sponta- neously by the choice of vacuum. The most obvious manifestation of this is the absence of a parity-doubled spectrum. If the chiral symmetry was respected, there would be a positive parity hadron for every negative parity hadron. This is not what is seen in nature. For example, the proton has isospin 1 $_{2}$, spin 1 2 and a positive parity. Its mass is 938 MeV. Its parity part- ner, catchingly called the N(1535) S$_{11}$ , has a mass of 1535 MeV [24]. We conclude that chiral symmetry is broken at a quantum level, and the easi- est way to account for this is by assuming that QCD spontaneously forms a quark condensate. In other words, the QCD vacuum gives a non-zero vacuum expectation value to the scalar operator What is the physical interpretation of this VEV? The up and down quarks are very light, and so it costs little energy to create a quark-antiquark pair. However, the binding energy released by a bound quark-antiquark pair is large. All (1.33) is saying is that the process of forming a quark-antiquark pair and then binding them together is exothermic: we get out more energy then we put in. The vacuum expectation value (1.33) means that we can no longer perform independent gauge transformations on the left and right handed 19 〈 0 | ¯ Q$_{L}$Q$_{R}$ + ¯ Q$_{R}$Q$_{L}$ | 0 〉 = 0 (1.33)