Tadpole counterterm
= Tadpole counterterm
{title2=$\mathcal L_{\rm ct}\supset-\delta t\,\phi$}
A linear scalar <counterterm> fixes the one-point function or the expansion point of the scalar potential. An unbroken <parity symmetry in quantum field theory> forbids this term when the scalar is a <pseudoscalar>. A vanishing one-loop <tadpole diagram> alone would not prove the all-orders restriction without the symmetry argument.