CP Violation from Five-Dimensional QED

It is shown that QED in (1+4)-dimensional space-time, with the fifth dimension compactified on a circle, is, in general, a CP violating theory. Depending on the fermionic boundary conditions, CP violation may be either explicit (through the Scherk-Schwarz mechanism) or spontaneous (via the Hosotani mechanism). The fifth component of the gauge field acquires (at the one-loop level) a nonzero vacuum expectation value which, in the presence of two fermionic fields, leads to spontaneous CP violation when the boundary conditions are CP symmetric. Phenomenological consequences are illustrated by a calculation of the electric dipole moment for the fermionic zero modes.