The strong CP problem
Old problems (like old soldiers) never die – they just fade away
15.1 The problem
It is often listed among the attractive features of QCD that it ‘naturally’ conserves baryon number, flavour, parity and CP. This had been the consensus for some time - until it was pointed out  that it is not quite true.1
Let us go back to the QCD Lagrangian given in Eq. (8.9) and note that we can add a term constructed out of the field strength tensor defined in Eq. (8.10): henceforth to be denoted by . It is gauge invariant and ...