Quasitopological Gravity with Matter: Modified Double-Copy Approach

We extend the recently proposed modified double-copy formalism to quasitopological gravity (QTG) coupled to matter. For spherically symmetric configurations, the QTG field equations in $D-$dimensional curved spacetime with a broad class of matter sources are mapped to equations for an auxiliary nonlinear gauge field in a flat $(D+1)$-dimensional spacetime. The nonlinear electrodynamics governing this auxiliary field is determined entirely by the generating function $h(p)$ that specifies the QTG model, while the corresponding current is determined by the matter stress-energy tensor. Restricting the auxiliary solution to a $D$-dimensional hyperplane and applying the modified double-copy prescription yields the Kerr--Schild metric solving the QTG equations. We show that Maxwell and nonlinear electrodynamics, as well as a broad class of spherically symmetric Yang--Mills fields, provide physical matter sources compatible with this construction. In the absence of null currents, the resulting solutions satisfy a generalized Birkhoff theorem and are static, whereas null charged currents naturally generate Vaidya-type solutions. In the Einstein limit, $h(p)=p$, the auxiliary nonlinear electrodynamics reduces to Maxwell theory.