Macdonald Index Research Articles

Macdonald indices for four-dimensional N=3 theories

We brute-force evaluate the vacuum character for $\mathcal N=2$ vertex operator algebras labelled by crystallographic complex reflection groups $G(k,1,1)=\mathbb Z_k$, $k=3,4,6$, and $G(3,1,2)$. For $\mathbb Z_{3,4}$ and $G(3,1,2)$ these vacuum characters have been conjectured to respectively reproduce the Macdonald limit of the superconformal index for rank one and rank two S-fold $\mathcal N=3$ theories in four dimensions. For the $\mathbb Z_3$ case, and in the limit where the Macdonald index reduces to the Schur index, we find agreement with predictions from the literature.

Schur sector of Argyres-Douglas theory and $W$-algebra

We study the Schur index, the Zhu’s C_2C2 algebra, and the Macdonald index of a four dimensional \mathcal{N}=2𝒩=2 Argyres-Douglas (AD) theories from the structure of the associated two dimensional WW-algebra. The Schur index is derived from the vacuum character of the corresponding WW-algebra and can be rewritten in a very simple form, which can be easily used to verify properties like level-rank dualities, collapsing levels, and S-duality conjectures. The Zhu’s C_2C2 algebra can be regarded as a ring associated with the Schur sector, and a surprising connection between certain Zhu’s C_2C2 algebra and the Jacobi algebra of a hypersurface singularity is discovered. Finally, the Macdonald index is computed from the Kazhdan filtration of the WW-algebra.

Closed form fermionic expressions for the Macdonald index

We interpret aspects of the Schur indices, that were identified with characters of highest weight modules in Virasoro (p, p′) = (2, 2k + 3) minimal models for k = 1, 2, . . . , in terms of paths that first appeared in exact solutions in statistical mechanics. From that, we propose closed-form fermionic sum expressions, that is, q, t-series with manifestly non-negative coefficients, for two infinite-series of Macdonald indices of (A1, A2k ) Argyres- Douglas theories that correspond to t-refinements of Virasoro (p, p′) = (2, 2k + 3) minimal model characters, and two rank-2 Macdonald indices that correspond to t-refinements of {mathcal{W}}_3 non-unitary minimal model characters. Our proposals match with computations from 4d mathcal{N} = 2 gauge theories via the TQFT picture, based on the work of J Song [75].

Testing Macdonald index as a refined character of chiral algebra

We test in (An−1, Am−1) Argyres-Douglas theories with gcd(n, m) = 1 the proposal of Song’s in [1] that the Macdonald index gives a refined character of the dual chiral algebra. In particular, we extend the analysis to higher rank theories and Macdonald indices with surface operator, via the TQFT picture and Gaiotto-Rastelli-Razamat’s Higgsing method. We establish the prescription for refined characters in higher rank minimal models from the dual (An−1, Am−1) theories in the large m limit, and then provide evidence for Song’s proposal to hold (at least) in some simple modules (including the vacuum module) at finite m. We also discuss some observed mismatch in our approach for surface operators with large vortex number.

Vanishing OPE coefficients in 4d mathcal{N}=2 SCFTs

We compute the superconformal characters of various short multiplets in 4d mathcal{N}=2 superconformal algebra, from which selection rules for operator products are obtained. Combining with the superconformal index, we show that a particular short multiplet appearing in the n-fold product of stress-tensor multiplet is absent in the (A1, A2n) Argyres-Douglas (AD) theory. This implies that certain operator product expansion (OPE) coefficients involving this multiplet vanish whenever the central charge c is identical to that of the AD theory. Similarly, by considering the n-th power of the current multiplet, we show that a particular short multiplet and OPE coefficients vanish for a class of AD theories with ADE flavor symmetry. We also consider the generalized AD theory of type (Ak−1, An−1) for coprime k, n and compute its Macdonald index using the associated W -algebra under a mild assumption. This allows us to show that a number of short multiplets and OPE coefficients vanish in this theory. We also provide a Mathematica file along with this paper, where we implement the algorithm by Cordova-Dumitrescu-Intriligator to compute the spectrum of 4d mathcal{N}=2 superconformal multiplets as well as their superconformal character.

VOAs labelled by complex reflection groups and 4d SCFTs

We define and study a class of mathcal{N} = 2 vertex operator algebras {mathcal{W}}_{mathrm{G}} labelled by complex reflection groups. They are extensions of the mathcal{N} = 2 super Virasoro algebra obtained by introducing additional generators, in correspondence with the invariants of the complex reflection group G. If G is a Coxeter group, the mathcal{N} = 2 super Virasoro algebra enhances to the (small) mathcal{N} = 4 superconformal algebra. With the exception of G = ℤ2, which corresponds to just the mathcal{N} = 4 algebra, these are non-deformable VOAs that exist only for a specific negative value of the central charge. We describe a free-field realization of {mathcal{W}}_{mathrm{G}} in terms of rank(G) βγbc ghost systems, generalizing a construction of Adamovic for the mathcal{N} = 4 algebra at c = −9. If G is a Weyl group, {mathcal{W}}_{mathrm{G}} is believed to coincide with the mathcal{N} = 4 VOA that arises from the four-dimensional super Yang-Mills theory whose gauge algebra has Weyl group G. More generally, if G is a crystallographic complex reflection group, {mathcal{W}}_{mathrm{G}} is conjecturally associated to an mathcal{N} = 3 4d superconformal field theory. The free-field realization allows to determine the elusive “R-filtration” of {mathcal{W}}_{mathrm{G}} , and thus to recover the full Macdonald index of the parent 4d theory.

Macdonald index and chiral algebra

For any 4d mathcal{N} = 2 SCFT, there is a subsector described by a 2d chiral algebra. The vacuum character of the chiral algebra reproduces the Schur index of the corresponding 4d theory. The Macdonald index counts the same set of operators as the Schur index, but the former has one more fugacity than the latter. We conjecture a prescription to obtain the Macdonald index from the chiral algebra. The vacuum module admits a filtration, from which we construct an associated graded vector space. From this grading, we conjecture a notion of refined character for the vacuum module of a chiral algebra, which reproduces the Macdonald index. We test this prescription for the Argyres-Douglas theories of type (A1, A2n) and (A1, D2n+1) where the chiral algebras are given by Virasoro and widehat{mathfrak{su}}(2) affine Kac-Moody algebra. When the chiral algebra has more than one family of generators, our prescription requires a knowledge of the generators from the 4d.

Argyres-Douglas theories, the Macdonald index, and an RG inequality

We conjecture closed-form expressions for the Macdonald limits of the superconformal indices of the (A_1, A_{2n-3}) and (A_1, D_{2n}) Argyres-Douglas (AD) theories in terms of certain simple deformations of Macdonald polynomials. As checks of our conjectures, we demonstrate compatibility with two S-dualities, we show symmetry enhancement for special values of n, and we argue that our expressions encode a non-trivial set of renormalization group flows. Moreover, we demonstrate that, for certain values of n, our conjectures imply simple operator relations involving composites built out of the SU(2)_R currents and flavor symmetry moment maps, and we find a consistent picture in which these relations give rise to certain null states in the corresponding chiral algebras. In addition, we show that the Hall-Littlewood limits of our indices are equivalent to the corresponding Higgs branch Hilbert series. We explain this fact by considering the S^1 reductions of our theories and showing that the equivalence follows from an inequality on monopole quantum numbers whose coefficients are fixed by data of the four-dimensional parent theories. Finally, we comment on the implications of our work for more general N=2 superconformal field theories.

Superconformal indices of generalized Argyres-Douglas theories from 2d TQFT

We study superconformal indices of 4d N=2 class S theories with certain irregular punctures called type $I_{k, N}$. This class of theories include generalized Argyres-Douglas theories of type $(A_{k-1}, A_{N-1})$ and more. We conjecture the superconformal indices in certain simplified limits based on the TQFT structure of the class S theories by writing an expression for the wave function corresponding to the puncture $I_{k, N}$. We write the Schur limit of the wave function when $k$ and $N$ are coprime. When $k=2$, we also conjecture a closed-form expression for the Hall-Littlewood index and the Macdonald index for odd $N$. From the index, we argue that certain short-multiplet which can appear in the OPE of the stress-energy tensor is absent in the $(A_1, A_{2n})$ theory. We also discuss the mixed Schur indices for the N=1 class S theories with irregular punctures.