Anomalies and chiral symmetry in QCD

Understand

I review some aspects of the interplay between anomalies and chiral symmetry.

• The quantum anomaly that breaks the U(1) axial symmetry of massless QCD leaves behind a flavor-singlet discrete chiral invariance.
• When the mass is turned on this residual symmetry has a close connection with the strong CP violating parameter theta.
• One result is that a first order transition is usually expected when the strong CP violating angle passes through pi.