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Yongchao Lü

Publications and source records attributed to Yongchao Lü.

15 recordsLinked to original sources
arXiv2026

Localization and Abelianization of Strings on Group Manifolds: The Simply Connected Case

We study the localization formula for the partition function of the Wess--Zumino--Witten (WZW) model on the torus for compact, connected, and simply connected simple Lie groups. We identify a missing factor in the original localization treatment of Murthy and Witten and show that it agrees with the result obtained from the Hamiltonian formulation. We trace its origin to the abelianization of the Wess--Zumino amplitude, which reduces to a flat Kalb--Ramond B--field holonomy in a Narain lattice CFT associated with the maximal torus. This establishes a direct relation between the localization result and the lattice description of the corresponding abelian theory. We further analyze the point--particle limit of the torus partition function, in which the WZW model reduces to quantum mechanics on the group manifold, and reproduce Frenkel's heat--kernel trace formula.

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arXiv2026

Localization and Abelianization of Strings on Group Manifolds: The Non-simply Connected Case

We study the partition functions of Wess--Zumino--Witten (WZW) models with compact connected simple Lie group manifolds that are not simply connected. Starting from the modular-invariant partition function of Felder--Gawędzki--Kupiainen (FGK), we derive a localization formula, which can also be obtained directly by supersymmetric localization of the corresponding supersymmetric WZW model. Our results reveal a variety of topological effects associated with the nontrivial topology of the target group. In particular, the Wess--Zumino amplitude is governed by FGK cocycles, while the fermion Pfaffians exhibit global anomalies associated with the holonomies of Pfaffian line bundles, captured by relative Rochlin invariants. Together, these results provide a unified description of the topological contributions to the semiclassical localization of WZW models with non-simply connected target groups.

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arXiv2025

Macdonald Identities and Exact Formulas for Superconformal Indices in Super Yang-Mills Theories

We present exact evaluations of superconformal indices for 4d N =1 and N =2 pure Super Yang-Mills theories with arbitrary simple gauge group G. Our approach applies the Macdonald identities for untwisted affine Lie algebras to the integral formulas of the indices, yielding uniform closed formulas valid for all G, expressed both as q-series and as eta-quotients, related through specialized Macdonald identities. Using similar techniques, we also derive exact expressions for half Schur indices with Neumann boundary conditions and uncover a bilinear structure of the full Schur index. Within the framework of holomorphic-topological twists, we further explore connections to the category of line operators, the K-theoretical Coulomb branch, Schur quantization, IR formulas for the BPS spectrum, and class S constructions.

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arXiv2025

Complex crystallographic reflection groups and Seiberg-Witten integrable systems: rank 1 case

We consider generalisations of the elliptic Calogero--Moser systems associated to complex crystallographic groups in accordance to [1]. In our previous work [2], we proposed these systems as candidates for Seiberg--Witten integrable systems of certain SCFTs. Here we examine that proposal for complex crystallographic groups of rank one. Geometrically, this means considering elliptic curves $T^2$ with $\mathbb{Z}_m$-symmetries, $m=2,3,4,6$, and Poisson deformations of the orbifolds $(T^2\times\mathbb{C})/\mathbb{Z}_m$. The $m=2$ case was studied in [2], while $m=3,4,6$ correspond to Seiberg--Witten integrable systems for the rank 1 Minahan--Nemeshansky SCFTs of type $E_{6,7,8}$. This allows us to describe the corresponding elliptic fibrations and the Seiberg--Witten differential in a compact elegant form. This approach also produces quantum spectral curves for these SCFTs, which are given by Fuchsian ODEs with special properties.

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arXiv2024

Classical and quantum curves of 5d Seiberg's theories and their 4d limit

In this work, we examine the classical and quantum Seiberg-Witten curves of 5d N = 1 SCFTs and their 4d limits. The 5d theories we consider are Seiberg's theories of type $E_{6,7,8}$, which serve as the UV completions of 5d SU(2) gauge theories with 5, 6, or 7 flavors. Their classical curves can be constructed using the five-brane web construction [1]. We also use it to re-derive their quantum curves [2], by employing a q-analogue of the Frobenius method in the style of [3]. This allows us to compare the reduction of these 5d curves with the 4d curves, i.e. Seiberg-Witten curves of the Minahan-Nemeschansky theories and their quantization, which have been identified in [4] with the spectral curves of rank-1 complex crystallographic elliptic Calogero-Moser systems.

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arXiv2023

D-type Minimal Conformal Matter: Quantum Curves, Elliptic Garnier Systems, and the 5d Descendants

We study the quantization of the 6d Seiberg-Witten curve for D-type minimal conformal matter theories compactified on a two-torus. The quantized 6d curve turns out to be a difference equation established via introducing codimension two and four surface defects. We show that, in the Nekrasov-Shatashvili limit, the 6d partition function with insertions of codimension two and four defects serve as the eigenfunction and eigenvalues of the difference equation, respectively. We further identify the quantum curve of D-type minimal conformal matters with an elliptic Garnier system recently studied in the integrability community. At last, as a concrete consequence of our elliptic quantum curve, we study its RG flows to obtain various quantum curves of 5d ${\rm Sp}(N)+N_f \mathsf{F},N_f\leq 2N+5$ theories.

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arXiv2021

Inozemtsev System as Seiberg-Witten Integrable system

In this work we establish that the Inozemtsev system is the Seiberg-Witten integrable system encoding the Coulomb branch physics of 4d $\mathcal{N}=2$ USp(2N) gauge theory with four fundamental and (for $N \geq 2$) one antisymmetric tensor hypermultiplets. We describe the transformation from the spectral curves and canonical one-form of the Inozemtsev system in the $N=1$ and $N=2$ cases to the Seiberg-Witten curves and differentials explicitly, along with the explicit matching of the modulus of the elliptic curve of spectral parameters to the gauge coupling of the field theory, and of the couplings of the Inozemtsev system to the field theory mass parameters. This result is a particular instance of a more general correspondence between crystallographic elliptic Calogero-Moser systems with Seiberg-Witten integrable systems, which will be explored in future work.

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arXiv2020

From Exact Results to Gauge Dynamics on $\mathbb{R}^3\times S^1$

We revisit the vacuum structure of the $\mathcal{N}=1$ Intriligator-Seiberg-Shenker model on $\mathbb{R}^3\times S^1$. Guided by the Cardy-like asymptotics of its Romelsberger index, and building on earlier semi-classical results by Poppitz and Ünsal, we argue that previously overlooked non-perturbative effects generate a Higgs-type potential on the classical Coulomb branch of the low-energy effective 3d $\mathcal{N}=2$ theory. In particular, on part of the Coulomb branch we encounter the first instance of a dynamically-generated quintic monopole superpotential.

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arXiv2019

Notes on anomalies, elliptic curves and the BS-D conjecture

We consider anomaly cancellation for $SU(N)\times SU(2)\times U(1)$ gauge theories where the left-handed chiral multiplets are in higher $SU(2)$ representations. In particular, if the left-handed quarks and leptons transform under the triplet representation of $SU(2)$ and if the $U(1)$ gauge group is compact then up to an overall scaling there is only one possible nontrivial assignment for the hypercharges if $N=3$, and two if $N=9$. Otherwise there are infinitely many. We use the Mordell-Weil theorem, Mazur's theorem and the Cremona elliptic curve database which uses Kolyvagin's theorem on the Birch Swinnerton-Dyer conjecture to prove these statements.

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arXiv2018

Geometric constraints on the space of N=2 SCFTs III: enhanced Coulomb branches and central charges

This is the third in a series of three papers on the systematic analysis of rank 1 four dimensional $\mathcal{N}=2$ SCFTs. In the first two papers we developed and carried out a strategy for classifying and constructing physical planar rank-1 Coulomb branch geometries of $\mathcal{N}=2$ SCFTs. Here we describe general features of the Higgs and mixed branch geometries of the moduli space of these SCFTs, and use this, along with their Coulomb branch geometry, to compute their conformal and flavor central charges. We conclude with a summary of the state of the art for rank-1 $\mathcal{N}=2$ SCFTs.

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arXiv2017

Geometric constraints on the space of N=2 SCFTs I: physical constraints on relevant deformations

We initiate a systematic study of four dimensional $\mathcal{N}=2$ superconformal field theories (SCFTs) based on the analysis of their Coulomb branch geometries. Because these SCFTs are not uniquely characterized by their scale-invariant Coulomb branch geometries we also need information on their deformations. We construct all inequivalent such deformations preserving $\mathcal{N}=2$ supersymmetry and additional physical consistency conditions in the rank 1 case. These not only include all the ones previously predicted by S-duality, but also 16 additional deformations satisfying all the known $\mathcal{N}=2$ low energy consistency conditions. All but two of these additonal deformations have recently been identified with new rank 1 SCFTs; these identifications are briefly reviewed. Some novel ingredients which are important for this study include: a discussion of RG-flows in the presence of a moduli space of vacua; a classification of local $\mathcal{N}=2$ supersymmetry-preserving deformations of unitary $\mathcal{N}=2$ SCFTs; and an analysis of charge normalizations and the Dirac quantization condition on Coulomb branches. This paper is the first in a series of three. The second paper, 1601.00011, gives the details of the explicit construction of the Coulomb branch geometries discussed here, while the third, 1609.04404, discusses the computation of central charges of the associated SCFTs.

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arXiv2017

Geometric constraints on the space of N=2 SCFTs II: Construction of special Kähler geometries and RG flows

This is the second in a series of three papers on systematic analysis of rank 1 Coulomb branch geometries of four dimensional $\mathcal{N}$=2 SCFTs. In the first paper we developed a strategy for classifying physical rank-1 CB geometries of $\mathcal{N}$=2 SCFTs. Here we show how to carry out this strategy computationally to construct the Seiberg-Witten curves and one-forms for all the rank-1 SCFTs. Explicit expressions are given for all cases, with the exception of the $N_f$=4 SU(2) gauge theory and the En SCFTs which were previously constructed. Our classification includes all known rank-1 theories plus a new one with an abelian flavor group, plus nine additional theories whose existence is more speculative. Four of those, reported in our first paper, depend on the assumption of new frozen rank-1 SCFTs. Here we also also show that the assumption of the existence of certain rank-0 $\mathcal{N}$=2 SCFTs leads to five additional consistent rank-1 CB geometries.

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arXiv2017

Seiberg-Witten geometries for Coulomb branch chiral rings which are not freely generated

Coulomb branch chiral rings of $\mathcal N=2$ SCFTs are conjectured to be freely generated. While no counter-example is known, no direct evidence for the conjecture is known either. We initiate a systematic study of SCFTs with Coulomb branch chiral rings satisfying non-trivial relations, restricting our analysis to rank 1. The main result of our study is that (rank-1) SCFTs with non-freely generated CB chiral rings when deformed by relevant deformations, always flow to theories with non-freely generated CB rings. This implies that if they exist, they must thus form a distinct subset under RG flows. We also find many interesting characteristic properties that these putative theories satisfy which may behelpful in proving or disproving their existence using other methods.

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arXiv2016

Expanding the landscape of $\mathcal{N}$=2 rank 1 SCFTs

We refine our previous proposal for systematically classifying 4d rank-1 $\mathcal N=2$ SCFTs by constructing their possible Coulomb branch geometries. Four new recently discussed rank-1 theories, including novel $\mathcal{N}=3$ SCFTs, sit beautifully in our refined classification framework. By arguing for the consistency of their RG flows we can make a strong case for the existence of at least four additional rank-1 SCFTs, nearly doubling the number of known rank-1 SCFTs. The refinement consists of relaxing the assumption that the flavor symmetries of the SCFTs have no discrete factors. This results in an enlarged (but finite) set of possible rank-1 SCFTs. Their existence can be further constrained using consistency of their central charges and RG flows.

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