# Why is the coupling of the matter field with the inflaton field neglected at the inflationary epoch?

The simplest model of inflation involves a hypothetical, yet undiscovered, single scalar field $\phi$ called \emph{inflaton}. The action of this scalar field minimally coupled to gravity is given by $$S[\phi,g]=\int d^4x\sqrt{-g}[\frac{1}{2}\mathcal{R}+\frac{1}{2}g_{\mu\nu}\partial^\mu\phi \partial^\nu\phi-V(\phi)-\mathcal{L}_{\phi-\text{matter}}]$$ $$=S_{EH}+S_{\phi}+S_{\phi-matter}$$ Why is it that in the discussion of inflation, one often neglects the coupling of the matter field with $\phi$ i.e., $S_{\phi-matter}$?