Instantons, analytic continuation, and $\mathcal{P}\mathcal{T}$-symmetric field theory

Ordinary Hermitian $\lambda {\phi }^4$ theory is known to exist in $d<4$ dimensions when $\lambda >0$. For negative values of the coupling, it has been suggested that a physical meaningful definition of the interacting theory can be given in terms of $\mathcal{P}\mathcal{T}$-symmetric field theory. In this work, we critically re-examine the relation between analytically continued Hermitian field theory with quartic interaction, and $\mathcal{P}\mathcal{T}$-symmetric field theory, including $O(N)$ models. We find that in general $\mathcal{P}\mathcal{T}$-symmetric field theory does not correspond to the analytic continuation of the Hermitian theory, except at high temperature where the instanton contribution present in the analytically continued theory can be neglected.