C-PERIODIC AND G-PERIODIC QCD AT FINITE TEMPERATURE

When shifted a distance L in a spatial direction a C-periodic field is replaced by its charge conjugate. C-periodic boundary conditions break the Z(3) center symmetry of SU(3) pure gauge theory explicitly. As a consequence, the Polyakov loop, as the order parameter of the deconfinement transition to the gluon plasma phase, does not vanish in a finite C-periodic volume (while it is zero when periodic boundary conditions are used). When quarks are included one can use G-periodic boundary conditions, where G denotes G-parity. G-periodic boundary conditions break SU(2)R x SU(2)L x U(1)R = L chiral symmetry explicitly down to SU(2)R = L x Z(2)R = L. Then, even in the massless limit the chiral order parameter [PSI-PSIBAR], which determines the transition to the quark-gluon plasma phase, does not vanish in a finite volume. The effect of C-periodic and G-periodic boundary conditions on the dynamics of the Goldstone bosons is discussed in the framework of low-energy chiral lagrangians.