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Changrim Ahn

Publications and source records attributed to Changrim Ahn.

At least 19 recordsLinked to original sources
arXiv2026

New RG flows between non-Unitary CFTs from exact massless scattering theories

We construct complete solutions of the massless S-matrix bootstrap equations describing a new infinite family of integrable RG flows from the higher-fusion-level minimal models M(3,2n+3;n) to the non-unitary Virasoro minimal models M(2,2n+3), including a flow from M(3,5) to the Yang--Lee CFT M(2,5) when n=1. We provide strong evidence for this identification by matching conformal perturbation theory around the UV fixed points with the small-volume expansion of the corresponding thermodynamic Bethe ansatz equations. We further show that a family of non-invertible Verlinde defect lines is preserved along the flows.

hep-th↗
arXiv2026

UV completion of 2D Ising CFT:a golden E_8 massless $S$-matrix

We present a full classification of UV complete QFTs that RG flow to the 2D Ising CFT by solving the bootstrap equations for massless right--left S-matrices. For the Ising model with E_8 spectrum, we find exactly four completions, arising from higher-T\bar T-type deformations, including a previously unknown ``golden flow'' whose UV fixed point is a diagonal su(2) coset CFT (c=25/14) along Δ_{\rm rel}=2/7. A universal Fibonacci/E_8 structure governs the R--L adjacency matrices and the Y-system periods, so that the E_8 symmetry persists across all RG scales.

hep-th↗
arXiv2025

Massless S-matrix Bootstraps and RG Flows

We find complete solutions of S-matrix bootstrap equations for the scatterings of massless particles in (1+1) dimensions with A_n symmetries. We show that only three types (minimal, diagonal, and saturated cases) of these S-matrices can generate RG flows that lead to UV-complete theories, and other RG flows predicted in [11] are inconsistent with the bootstrap equations. Using these S-matrices, we derived the TBA equations and corresponding Y-systems that generate the RG flows between the IR and UV CFTs.

hep-th↗
arXiv2024

New Integrable RG flows with Parafermions

We consider irrelevant deformations of massless RSOS scattering theories by an infinite number of higher TTbar_{s+1} operators which introduce extra non-trivial CDD factors between left-movers and right-movers. It is shown that the resulting theories can be UV complete after bypassing typical Hagedorn-like singularities if the coefficients of the deformations are fine-tuned. In this way, we have discovered that only two new UV complete QFTs are associated with a M_p (p=3,4,...) minimal CFT based on the integrable structure of the RSOS scattering theory. One is the massless Z_{p-1} parafermionic sinh-Gordon models (PShG) with a self-dual coupling constant. This correspondence is confirmed by showing that the scale-dependent vacuum energies computed by the thermodynamic Bethe ansatz based on the S-matrices match those from the quantization conditions for the PShG models using the reflection amplitudes. The other UV QFT is reached from M_p by following the roaming trajectory of the Z_{p-1} parafermionic minimal series.

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arXiv2024

Hagedorn singularity in exact U_q(su(2)) S-matrix theories with arbitrary spins

Generalizing the quantum sine-Gordon and sausage models, we construct exact S-matrices for higher spin representations with quantum U_q(su(2)) symmetry, which satisfy unitarity, crossing-symmetry, and the Yang-Baxter equations with minimality assumption, i.e. without any unnecessary CDD factor. The deformation parameter q is related to a coupling constant. Based on these S-matrices, we derive the thermodynamic Bethe ansatz equations for q a root of unity in terms of a universal kernel where the nodes are connected by graphs of non-Dynkin type. We solve these equations numerically to find out Hagedorn-like singularities in the free energies at some critical scales and find a universality in the critical exponents, all near 0.5 for different values of the spin and the coupling constant.

hep-th↗
arXiv2022

On the classification of UV completions of integrable $T \bar{T}$ deformations of CFT

It is well understood that 2d conformal field theory (CFT) deformed by an irrelevant $T\bar{T}$ perturbation of dimension $4$ has universal properties. In particular, for the most interesting cases, the theory develops a singularity in the ultra-violet (UV), signifying a shortest possible distance, with a Hagedorn transition in applications to string theory. We show that by adding an infinite number of higher $[T\bar{T}]_{s>1}$ irrelevant operators with tuned couplings, this singularity can be resolved and the theory becomes UV complete with UV central charge $c_{\rm UV}$. We propose an approach to classifying the possible UV completions that are integrable. For the Ising model with $c_{\rm IR} = \tfrac{1}{2}$ we find 3 UV completions based on a single massless Majorana fermion description with $c_{\rm UV} = \tfrac{7}{10}$ and $ \tfrac{3}{2}$, which both have ${\cal N}=1$ supersymmetry and were previously known, and we argue that these are the only solutions to our classification problem based on this spectrum of particles. We find 3 additional ones with a spectrum of 8 massless particles related to the Lie group $E_8$ appropriate to a magnetic perturbation with $c_{\rm UV} = \tfrac{21}{22} , \tfrac{15}{2}$ and $\tfrac{31}{2}$. We argue that it is likely there are more cases for this $E_8$ spectrum. We also study simpler cases based on ${\rm su(3)}$ and ${\rm su(4)}$ where we can propose complete classifications. For ${\rm su(3)}$ the infra-red (IR) theory is the 3-state Potts model with $c_{\rm IR} = \tfrac{4}{5}$ and we find 3 completions with $\tfrac{4}{5} < c_{\rm UV} \leq \tfrac{16}{5}$. For the ${\rm su(4)}$ case, which has $3$ particles and $c_{\rm IR} =1$, and we find $11$ UV completions with $1 < c_{\rm UV} \leq 5$, most of which were previously unknown.

hep-th↗
arXiv2022

Spectrum of the Hypereclectic Spin Chain and Pólya Counting

In earlier work we proposed a generating function that encodes the Jordan block spectrum of the integrable Hypereclectic spin chain, related to the one-loop dilatation operator of the dynamical fishnet quantum field theory. We significantly improve the expressions for these generating functions, rendering them much more explicit and elegant. In particular, we treat the case of the full spin chain without imposing any cyclicity constraints on the states, as well as the case of cyclic states. The latter involves the Pólya enumeration theorem in conjunction with q-binomial coefficients.

hep-th↗
arXiv2021

Combinatorial Solution of the Eclectic Spin Chain

The one-loop dilatation operator in the holomorphic 3-scalar sector of the dynamical fishnet theory is studied. Due to the non-unitary nature of the underlying field theory this operator, dubbed the eclectic spin chain Hamiltonian, is non-diagonalisable. The corresponding spectrum of Jordan blocks leads to logarithms in the two-point functions, which is characteristic of logarithmic conformal field theories. It was previously conjectured that for certain filling conditions and generic couplings the spectrum of the eclectic model is equivalent to the spectrum of a simpler model, the hypereclectic spin chain. We provide further evidence for this conjecture, and introduce a generating function which fully characterises the Jordan block spectrum of the simplified model. This function is found by purely combinatorial means and is simply related to the q-binomial coefficient.

hep-th↗
arXiv2020

The Integrable (Hyper)eclectic Spin Chain

We refine the recently introduced notion of eclectic spin chains by including a maximal number of deformation parameters. These models are integrable, nearest-neighbor n-state spin chains with exceedingly simple non-hermitian Hamiltonians. They turn out to be non-diagonalizable in the multiparticle sector (n>2), where their "spectrum" consists of an intricate collection of Jordan blocks of arbitrary size and multiplicity. We show how and why the quantum inverse scattering method, sought to be universally applicable to integrable nearest-neighbor spin chains, essentially fails to reproduce the details of this spectrum. We then provide, for n=3, detailed evidence by a variety of analytical and numerical techniques that the spectrum is not "random", but instead shows surprisingly subtle and regular patterns that moreover exhibit universality for generic deformation parameters. We also introduce a new model, the hypereclectic spin chain, where all parameters are zero except for one. Despite the extreme simplicity of its Hamiltonian, it still seems to reproduce the above "generic" spectra as a subset of an even more intricate overall spectrum. Our models are inspired by parts of the one-loop dilatation operator of a strongly twisted, double-scaled deformation of N=4 Super Yang-Mills Theory.

hep-th↗
arXiv2018

Giant magnon-like solution in Sch_5 x S^5

In this paper we have found a classical giant magnon-like solution with both infinite and finite angular momentum moving in Sch_5 x S^5 with B-field, which is believed to be dual to dipole-deformed N=4 super Yang-Mills theory. This string state propagates as a point particle in non-trivial subspace of the Sch_5 space but shows a giant magnon-like property in the S^2 subspace. We derive the energy-momentum dispersion relations and their finite-size correction for the case of finite but large angular momentum.

hep-th↗
arXiv2017

Some semiclassical structure constants for AdS_4 x CP^3

We compute structure constants in three-point functions of three string states in AdS_4 x CP^3 in the framework of the semiclassical approach. We consider HHL correlation functions where two of the states are "heavy" string states of finite-size giant magnons carrying one or two angular momenta and the other one corresponds to such "light" states as dilaton operators with non-zero momentum, primary scalar operators, and singlet scalar operators with higher string levels.

hep-th↗
arXiv2017

NLIE for the Sausage model

The sausage model, first proposed by Fateev, Onofri, and Zamolodchikov, is a deformation of the O(3) sigma model preserving integrability. The target space is deformed from the sphere to "sausage" shape by a deformation parameter ν. This model is defined by a factorizable S-matrix which is obtained by deforming that of the O(3) sigma model by a parameter λ. Clues for the deformed sigma model are provided by various UV and IR information through the thermodynamic Bethe ansatz (TBA) analysis based on the S-matrix. Application of TBA to the sausage model is, however, limited to the case of 1/λinteger where the coupled integral equations can be truncated to a finite number. In this paper, we propose a finite set of nonliear integral equations (NLIEs), which are applicable to generic value of λ. Our derivation is based on T-Q relations extracted from the truncated TBA equations. For consistency check, we compute next-leading order corrections of the vacuum energy and extract the S-matrix information in the IR limit. We also solved the NLIE both analytically and numerically in the UV limit to get the effective central charge and compared with that of the zero-mode dynamics to obtain exact relation between νand λ. This paper is a tribute to the memory of Prof. Petr Kulish.

hep-th↗
arXiv2017

Finite-size effect of η-deformed AdS_5 x S^5 at strong coupling

We compute Luscher corrections for a giant magnon in the η-deformed (AdS_5\times S^5)_η using the su(2|2)_q-invariant S-matrix at strong coupling and compare with the finite-size effect of the corresponding string state, derived previously. We find that these two results match and confirm that the su(2|2)_q-invariant S-matrix is describing world-sheet excitations of the η-deformed background.

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arXiv2015

Bethe states for the two-site Bose-Hubbard model: a binomial approach

We calculate explicitly the Bethe vectors states by the algebraic Bethe ansatz method with the $gl(2)$-invariant $R$-matrix for the two-site Bose-Hubbard model. Using a binomial expansion of the n-th power of a sum of two operators we get and solve a recursion equation. We calculate the scalar product and the norm of the Bethe vectors states. The form factors of the imbalance current operator are also computed.

math-ph↗
arXiv2014

A HHL 3-point correlation function in the η-deformed AdS_5 x S^5

We derive the 3-point correlation function between two giant magnons heavy string states and the light dilaton operator with zero momentum in the η-deformed AdS_5 x S^5 valid for any J_1 and ηin the semiclassical limit. We show that this result satisfies a consistency relation between the 3-point correlation function and the conformal dimension of the giant magnon. We also provide a leading finite J_1 correction explicitly.

hep-th↗
arXiv2014

Finite-size giant magnons on η-deformed AdS_5 x S^5

We consider strings moving in the R_t x S^3_ηsubspace of the η-deformed AdS_5 x S^5 and obtain a class of solutions depending on several parameters. They are characterized by the string energy and two angular momenta. Finite-size dyonic giant magnon belongs to this class of solutions. Further on, we restrict ourselves to the case of giant magnon with one nonzero angular momentum, and obtain the leading finite-size correction to the dispersion relation.

hep-th↗
arXiv2014

On the Renormalization Group Flow in Two Dimensional Superconformal Models

We extend the results on the RG flow in the next to leading order to the case of the supersymmetric minimal models SM_p for p>> 1. We explain how to compute the NS and Ramond fields conformal blocks in the leading order in 1/p and follow the renormalization scheme propsed in [1]. As a result we obtained the anomalous dimensions of certain NS and Ramond fields. It turns out that the linear combination expressing the infrared limit of these fields in term of the IR theory SM_{p-2} is exactly the same as those of the nonsupersymmetric minimal theory.

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