Four-point tree amplitude in phi cubed theory
= Four-point tree amplitude in phi cubed theory
The four-point <tree-level Feynman diagram> has two cubic vertices joined by one propagator. The three ways to divide four labeled external legs into vertex pairs give the $s$, $t$, and $u$ channels. For interaction $+\lambda\phi^3/3!$, the amplitude is $\mathcal M=-\lambda^2[(s-m^2)^{-1}+(t-m^2)^{-1}+(u-m^2)^{-1}]$, with the <Feynman i-epsilon prescription> understood. All channels interfere in its modulus squared.