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$2+2=4$

Motivated by the observation that $2+2=4$, we consider four-dimensional $\mathcal{N}=2$ superconformal field theories on $S^2\timesΣ$, turning on a suitable rigid supergravity background. On the one hand, reduction of a four-dimensional theory ${T}$ on a Riemann surface $Σ$ leads to a family $\mathscr{F}[{T}, Σ]$ of two-dimensional $(2,2)$ unitary SCFTs, a two-dimensional analog of the four-dimensional theories of class $\mathscr{S}$. On the other hand, reduction on $S^2$ yields a non-unitary two-dimensional CFT $\mathscr{C}[{T}]$ whose chiral algebra is the same as the one associated to ${T}$ by the standard SCFT/VOA correspondence. This construction upgrades the vertex operator algebra to a full-fledged two-dimensional CFT. What's more, it leads to a novel 2d/2d correspondence, a "$2+2 = 4$" analog of the "$4+2=6$" AGT correspondence: the $S^2$ partition function of $\mathscr{F}[{T}; Σ]$ is computed by correlation functions of $\mathscr{C}[{T}]$ on $Σ$. The elliptic genus of $\mathscr{F}[{T}; Σ]$ is instead computed by a topological QFT $\mathscr{E}[T]$ on $Σ$. A central question is whether one can give a purely two-dimensional presentation of the family $\mat