\feynmandiagram[horizontal=a to b, layered layout] {
a[dot,label=left:$g_0$] 
-- [double charged boson, half right, looseness=2.0, edge label'=$b$]
b[dot,label=right:$g_0$],
a -- [double fermion, half right, looseness=1.0, edge label=$c_\sigma$] b
-- [double charged scalar, half right, looseness=1.0, edge label=$f_\sigma$] a
-- [double gluon, half left, looseness=2.0, edge label=$a$] b
};