  The effect of self-interactions of heavy scalar fields during inflation on
the primordial non-Gaussianity is studied. We take a specific constant-turn
quasi-single field inflation as an example. We derive an effective theory with
emphasis on non-linear self-interactions of heavy fields and calculate the
corresponding non-Gaussianity, which is of equilateral type and can be as
relevant as those computed previously in the literature. We also derive the
non-Gaussianity by directly using the in-in formalism, and verify the
equivalence of these two approaches.
