\feynmandiagram[inline=(a.base), horizontal=a to b] {
a[empty dot] -- [double charged scalar, half left, bend left, edge label=$f_\sigma$] b[crossed dot] -- [double charged boson, half left, bend left, edge label=$b$] a,
};
\stackrel{\mathrm{(NCA)}}{=}
\feynmandiagram[inline=(a.base), horizontal=a to b] {
a[empty dot] -- [double charged scalar, half left, bend left, edge label=$f_\sigma$] b[empty dot] -- [double charged boson, half left, bend left, edge label=$b$] a,
};