Parity and Large Gauge Invariance in Thermal QED_3

Abstract

We settle the ``apparent'' paradox present in thermal QED_3 that the perturbative series is not invariant, as manifested by the temperature dependence of the induced Chern-Simons term, by showing that large (unlike small) transformations and hence their Ward identities, are not perturbative order-preserving. Instead the thermal effective gauge field actions induced by charged fermions in QED_3 can be made invariant under both small and large gauge transformations by suitable regularization of the Dirac operator determinant, at the usual price of parity anomalies. Our result is illustrated by a concrete example.