From VOAs to short star products in SCFT
Mykola Dedushenko
TL;DR
This work builds a precise bridge between the 4d $\mathcal{N}=2$ SCFT Schur sector, encoded by a vertex operator superalgebra $V$, and the 3d $\mathcal{N}=4$ Higgs-branch algebra, via KK reduction on $S^1$ in the $S^3\times S^1$ background. The authors show that in the small-$S^1$ limit, Higgs-branch operators survive as 3d cohomology while spinning Schur operators are lifted by line-wrapping effects, and no new cohomology arises; the 3d protected algebra is ${\cal A}_H = {\rm Zhu}_s(V)/N$, where $N$ is the null space of an $s$-twisted trace $T_s$. The torus correlators of the VOA determine a degenerate trace on ${\rm Zhu}_s(V)$ in the high-temperature limit, whose quotient yields a nondegenerate $T_s$ and, through ERS, a short star-product on ${\cal A}_H$. This framework ties the Beem–Rastelli Higgs-branch perspective to the 4d/2d SCFT–VOA correspondence and lays groundwork for concrete calculations in examples and potential extensions to non-unitary and non-$C_2$-cofinite VOAs.
Abstract
We build a bridge between two algebraic structures in SCFT: a VOA in the Schur sector of 4d $\mathcal{N}=2$ theories and an associative algebra in the Higgs sector of 3d $\mathcal{N}=4$. The natural setting is a 4d $\mathcal{N}=2$ SCFT placed on $S^3\times S^1$: by sending the radius of $S^1$ to zero, we recover the 3d $\mathcal{N}=4$ theory, and the corresponding VOA on the torus degenerates to the associative algebra on the circle. We prove that: 1) the Higgs branch operators remain in the cohomology; 2) all the Schur operators of the non-Higgs type are lifted by line operators wrapped on the $S^1$; 3) no new cohomology classes are added. We show that the algebra in 3d is given by the quotient $\mathcal{A}_H = {\rm Zhu}_{s}(V)/N$, where ${\rm Zhu}_{s}(V)$ is the non-commutative Zhu algebra of the VOA $V$ (for ${s}\in{\rm Aut}(V)$), and $N$ is a certain ideal. This ideal is the null space of the (${s}$-twisted) trace map $T_{s}: {\rm Zhu}_{s}(V) \to \mathbb{C}$ determined by the torus 1-point function in the high temperature (or small complex structure) limit. It therefore equips $\mathcal{A}_H$ with a non-degenerate (twisted) trace, leading to a short star-product according to the recent results of Etingof and Stryker. The map $T_{s}$ is easy to determine for unitary VOAs, but has a much subtler structure for non-unitary and non-$C_2$-cofinite VOAs of our interest. We comment on relation to the Beem-Rastelli conjecture on the Higgs branch and the associated variety. A companion paper will explore further details, examples, and some applications of these ideas.