Bispectrum from a time-derivative cubic scalar interaction
= Bispectrum from a time-derivative cubic scalar interaction
For a massless scalar in <de Sitter spacetime> with interaction $S_{\rm int}=(\lambda/3!)\int dt\,d^3x\,a^3\dot\phi^2\phi$, the tree-level late-time <primordial bispectrum> is
$$
B=\frac{\lambda H^4}{12k_1^3k_2^3k_3^3K^2}
\sum_{\rm cyclic}k_j^2k_l^2(K+k_i),
\qquad K=k_1+k_2+k_3.
$$
It scales as $B(\mu k_1,\mu k_2,\mu k_3)=\mu^{-6}B(k_1,k_2,k_3)$.