Hence , the axion is actually a pseudo-Nambu -- Goldstone boson .
However , the non-trivial QCD vacuum effects ( e.g. , instantons ) spoil the Peccei -- Quinn symmetry explicitly and provide a small mass for the axion .
The axion is actually a pseudo-Nambu -- Goldstone boson .
However , the non-trivial QCD vacuum effects ( e.g. , instantons ) spoil the Peccei -- Quinn symmetry explicitly and provide a small mass for the axion .