Publications

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Dirac spin liquid as an “unnecessary” quantum critical point on square lattice antiferromagnets

Yunchao Zhang, Xue-Yang Song, T. Senthil

SciPost Phys. Core 8, 024 (2025) · published 18 February 2025 | · pdf

Quantum spin liquids are exotic phases of quantum matter especially pertinent to many modern condensed matter systems. Dirac spin liquids (DSLs) are a class of gapless quantum spin liquids that do not have a quasi-particle description and are potentially realized in a wide variety of spin $1/2$ magnetic systems on $2d$ lattices. In particular, the DSL in square lattice spin-$1/2$ magnets is described at low energies by $(2+1)d$ quantum electrodynamics with $N_f=4$ flavors of massless Dirac fermions minimally coupled to an emergent $U(1)$ gauge field. The existence of a relevant, symmetry-allowed monopole perturbation renders the DSL on the square lattice intrinsically unstable. We argue that the DSL describes a stable continuous phase transition within the familiar Neel phase (or within the Valence Bond Solid (VBS) phase). In other words, the DSL is an "unnecessary" quantum critical point within a single phase of matter. Our result offers a novel view of the square lattice DSL in that the critical spin liquid can exist within either the Neel or VBS state itself, and does not require leaving these conventional states.
The Parisi-Sourlas uplift and infinitely many solvable 4d CFTs

Emilio Trevisani

SciPost Phys. 18, 056 (2025) · published 17 February 2025 | · pdf

Parisi-Sourlas (PS) supersymmetry is known to emerge in some models with random field type of disorder. When PS SUSY is present the $d$-dimensional theory allows for a $d-2$-dimensional description. In this paper we investigate the reversed question and we provide new indications that any given CFT$_{d-2}$ can be uplifted to a PS SUSY CFT$_{d}$. We show that any scalar four-point function of a CFT$_{d-2}$ is mapped to a set of 43 four-point functions of the uplifted CFT$_{d}$ which are related to each other by SUSY and satisfy all necessary bootstrap axioms. As a byproduct we find 43 non trivial relations between conformal blocks across dimensions. We test the uplift in generalized free field theory (GFF) and find that PS SUSY is a powerful tool to bootstrap an infinite class of previously unknown GFF observables. Some of this power is shown to persist in perturbation theory around GFF. We explain why all diagonal minimal models admit an uplift and we show exact results for correlators and CFT data of the $4d$ uplift of the Ising model. Despite being strongly coupled $4d$ CFTs, the uplifted minimal models contain infinitely many conserved currents and are expected to be integrable.

1 citation
Higher-matter and Landau-Ginzburg theory of higher-group symmetries

Ruizhi Liu, Ran Luo, Yi-Nan Wang

SciPost Phys. 18, 052 (2025) · published 13 February 2025 | · pdf

Higher-matter is defined by higher-representation of a symmetry algebra, such as the $p$-form symmetries, higher-group symmetries or higher-categorical symmetries. In this paper, we focus on the cases of higher-group symmetries, which are formulated in terms of the strictification of weak higher-groups. We systematically investigate higher-matter charged under 2-group symmetries, defined by automorphism 2-representations. Furthermore, we construct a Lagrangian formulation of such higher-matter fields coupled to 2-group gauge fields in the path space of the spacetime manifold. We interpret such model as the Landau-Ginzburg theory for 2-group symmetries, and discuss the spontaneous symmetry breaking (SSB) of 2-group symmetries under this framework. Examples of discrete and continuous 2-groups are discussed. Interestingly, we find that a non-split 2-group symmetry can admit an SSB to a split 2-group symmetry, where the Postnikov class is trivialized. We also briefly discuss the strictification of weak 3-groups, weak 3-group gauge fields and 3-representations in special cases.

1 citation
Wilson loops and spherical branes

Davide Astesiano, Pieter Bomans, Fridrik Freyr Gautason, Valentina Giangreco M. Puletti, Alexia Nix

SciPost Phys. 18, 050 (2025) · published 12 February 2025 | · pdf

We study $\frac{1}{2}$-BPS Wilson loop operators in maximally supersymmetric Yang-Mills theory on $d$-dimensional spheres. Their vacuum expectation values can be computed at large $N$ through supersymmetric localisation. The holographic duals are given by back-reacted spherical D-branes. For $d≠ 4$, the resulting theories are non-conformal and correspondingly, the dual geometries do not possess an asymptotic AdS region. The main aim of this work is to compute the holographic Wilson loops by evaluating the partition function of a probe fundamental string and M2-brane in the dual geometry, focusing on the next-to-leading order. Along the way, we highlight a variety of issues related to the presence of a non-constant dilaton. In particular, the structure of the divergences of the one-loop partition functions takes a non-universal form in contrast to examples available in the literature. We devise a general framework to treat the divergences, successfully match the sub-leading scaling with $\lambda$ and $N$, and provide a first step towards obtaining the numerical prefactor.
Asymptotic symmetries in the $TsT/T\bar{T}$ correspondence

Zhengyuan Du, Wen-Xin Lai, Kangning Liu, Wei Song

SciPost Phys. 18, 049 (2025) · published 11 February 2025 | · pdf

Starting from holography for IIB string theory on AdS$_3× \mathcal N$ with NS-NS flux, the TsT/$T\bar T$ correspondence is a conjecture that a TsT transformation on the string theory side is holographically dual to the single-trace version of the $T\bar T$ deformation on the field theory side. More precisely, the long string sector of string theory on the TsT-transformed background corresponds to the symmetric product theory whose seed theory is the $T\bar T$-deformed CFT$_2$. In this paper, we study the asymptotic symmetry of the string theory in the bulk. We find a state-dependent, non-local field redefinition under which the worldsheet equations of motion, stress tensor, as well as the symplectic form of string theory after the TsT transformation are mapped to those before the TsT transformation. The asymptotic symmetry in the auxiliary AdS basis is generated by two commuting Virasoro generators, while in the TsT transformed basis is non-linear and non-local. Our result from string theory analysis is compatible with that of the $T\bar T$ deformed CFT$_2$.

2 citations
Difference equations: From Berry connections to the Coulomb branch

Andrea E. V. Ferrari, Daniel Zhang

SciPost Phys. 18, 045 (2025) · published 5 February 2025 | · pdf

In recent work, we demonstrated that a spectral variety for the Berry connection of a 2d $\mathcal{N}=(2,2)$ GLSM with Kähler vacuum moduli space $X$ and Abelian flavour symmetry is the support of a sheaf induced by a certain action on the equivariant quantum cohomology of $X$. This action could be quantised to first-order matrix difference equations obeyed by brane amplitudes, and by taking the conformal limit, vortex partition functions. In this article, we elucidate how some of these results may be recovered from a 3d perspective, by placing the 2d theory at a boundary and gauging the flavour symmetry via a bulk A-twisted 3d $\mathcal{N}=4$ gauge theory (a sandwich construction). We interpret the above action as that of the bulk Coulomb branch algebra on boundary twisted chiral operators. This relates our work to recent constructions of actions of Coulomb branch algebras on quantum equivariant cohomology, providing a novel correspondence between these actions and spectral data of generalised periodic monopoles. The effective IR description of the 2d theory in terms of a twisted superpotential allows for explicit computations of these actions, which we demonstrate for Abelian GLSMs.
ABJ anomaly as a U(1) symmetry and Noether’s theorem

Valentin Benedetti, Horacio Casini, Javier M. Magan

SciPost Phys. 18, 041 (2025) · published 3 February 2025 | · pdf

The Adler-Bell-Jackiw anomaly determines the violation of chiral symmetry when massless fermions are coupled to an abelian gauge field. In its seminal paper, Adler noticed that a modified chiral $U(1)$ symmetry could still be defined, at the expense of being generated by a non-gauge-invariant conserved current. We show this internal $U(1)$ symmetry has the special feature that it transforms the Haag duality violating sectors (or non local operator classes). This provides a simple unifying perspective on the origin of anomaly quantization, anomaly matching, applicability of Goldstone theorem, and the absence of a Noether current. We comment on recent literature where this symmetry is considered to be either absent or non-invertible. We end by recalling the DHR reconstruction theorem, which states $0$-form symmetries cannot be non-invertible for $d>2$, and argue for a higher form-symmetry reconstruction theorem.

1 citation
Learning the simplicity of scattering amplitudes

Clifford Cheung, Aurélien Dersy, Matthew D. Schwartz

SciPost Phys. 18, 040 (2025) · published 3 February 2025 | · pdf

The simplification and reorganization of complex expressions lies at the core of scientific progress, particularly in theoretical high-energy physics. This work explores the application of machine learning to a particular facet of this challenge: the task of simplifying scattering amplitudes expressed in terms of spinor-helicity variables. We demonstrate that an encoder-decoder transformer architecture achieves impressive simplification capabilities for expressions composed of handfuls of terms. Lengthier expressions are implemented in an additional embedding network, trained using contrastive learning, which isolates subexpressions that are more likely to simplify. The resulting framework is capable of reducing expressions with hundreds of terms—a regular occurrence in quantum field theory calculations—to vastly simpler equivalent expressions. Starting from lengthy input expressions, our networks can generate the Parke-Taylor formula for five-point gluon scattering, as well as new compact expressions for five-point amplitudes involving scalars and gravitons. An interactive demonstration can be found at https://spinorhelicity.streamlit.app.

1 citation
Introduction to string theory

Carlo Maccaferri, Fabio Marino, Beniamino Valsesia

SciPost Phys. Lect. Notes 90 (2025) · published 29 January 2025 | · pdf

These are lecture notes of the introductory String Theory course held by one of the authors for the master program of Theoretical Physics at Turin University. The world-sheet approach to String Theory is pedagogically introduced in the framework of the bosonic string and of the superstring.
Gravitational higher-form symmetries and the origin of hidden symmetries in Kaluza-Klein compactifications

Carmen Gómez-Fayrén, Tomás Ortín, Matteo Zatti

SciPost Phys. Core 8, 010 (2025) · published 29 January 2025 | · pdf

We show that, in presence of isometries and non-trivial topology, the Einstein–Hilbert action is invariant under certain transformations of the metric which are not diffeomorphisms. These transformations are similar to the higher-form symmetries of field theories with $p$-form fields. In the context of toroidal Kaluza–Klein compactifications, we show that these symmetries give rise to some of the "hidden symmetries" (dualities) of the dimensionally-reduced theories.

1 citation

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