\feynmandiagram [small, baseline=(a.base), horizontal=a to c, layered layout] {
a -- [double fermion, edge label'=$c$] b [semi hollow triangle] -- [double fermion, edge label'=$c$] c,
b -- [anti boson fermion, out=-135, in=-45, loop, min distance=2cm, edge label=$Q$] b
};
=
\feynmandiagram [small, baseline=(a.base), horizontal=a to c, layered layout] {
a -- [double fermion, edge label'=$c$] b [triangle] -- [double fermion, edge label'=$c$] c,
b -- [anti boson fermion, out=-135, in=-45, loop, min distance=2cm, edge label=$Q$] b
};
+
\feynmandiagram [small, baseline=(a.base), horizontal=a to d, layered layout] {
a -- [double fermion, edge label'=$c$] b [triangle] -- [double fermion, edge label=$c$] c [semi hollow triangle] -- [double fermion, edge label'=$c$] d,
b -- [boson fermion, in=-135, out=-45, edge label'=$Q$] c,
b -- [anti boson fermion, out=-135, in=-45, loop, min distance=2cm, edge label'=$Q$] c
};