\draw (-2,0) node[left] {$\varphi_a$} -- (2,0) node[right] {$\varphi_c$} (0,2) node[above] {$\varphi_b$} -- (0,-2) node[below] {$\varphi_d$}; \draw[->,yshift=5pt] (-1 ...

\draw (-2,0) node[left] {$\varphi_a$} -- (0,0) -- (1.5,1.5) node[above right] {$\varphi_b$} (0,0) -- (1.5,-1.5) node[below right] {$\varphi_c$}; \draw[->,yshift=5pt ...

In particular, radiative corrections (described by one- and multi-loop Feynman diagrams) have to be considered. Although a growing number of physicists are involved in such projects, multi-loop ...