New bi-harmonic superspace formulation of $4D, \mathcal{N}=4$ SYM theory

We develop a novel bi-harmonic $\mathcal{N}=4$ superspace formulation of the $\mathcal{N}=4$ supersymmetric Yang-Mills theory (SYM) in four dimensions. In this approach, the $\mathcal{N}=4$ SYM superfield constraints are solved in terms of on-shell $\mathcal {N}=2$ harmonic superfields. Such an approach provides a convenient tool of constructing the manifestly $\mathcal{N}=4$ supersymmetric invariants and further rewriting them in $\mathcal{N}= 2$ harmonic superspace. In particular, we present $\mathcal{N}=4$ superfield form of the leading term in the $\mathcal{N}=4$ SYM effective action which was known previously in $\mathcal{N}=2$ superspace formulation.