SciPost Phys. 16, 129 (2024) ·
published 22 May 2024

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The solution of Bethe Ansatz equations for XXZ spin chain with the parameter q being a root of unity is infamously subtle. In this work, we develop the rational Qsystem for this case, which offers a systematic way to find all physical solutions of the Bethe Ansatz equations at root of unity. The construction contains two parts. In the first part, we impose additional constraints to the rational Qsystem. These constraints eliminate the socalled FabriciusMcCoy (FM) string solutions, yielding all primitive solutions. In the second part, we give a simple procedure to construct the descendant tower of any given primitive state. The primitive solutions together with their descendant towers constitute the complete Hilbert space. We test our proposal by extensive numerical checks and apply it to compute the torus partition function of the 6vertex model at root of unity.
SciPost Phys. 16, 113 (2024) ·
published 26 April 2024

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Bethe Ansatz equations for spins Heisenberg spin chain with s≥1 are significantly more difficult to analyze than the spin$\tfrac{1}{2}$ case, due to the presence of repeated roots. As a result, it is challenging to derive extra conditions for the Bethe roots to be physical and study the related completeness problem. In this paper, we propose the rational Qsystem for the XXX$_s$ spin chain. Solutions of the proposed Qsystem give all and only physical solutions of the Bethe Ansatz equations required by completeness. This is checked numerically and proved rigorously. The rational Qsystem is equivalent to the requirement that the solution and the corresponding dual solution of the TQrelation are both polynomials, which we prove rigorously. Based on this analysis, we propose the extra conditions for solutions of the XXX$_s$ Bethe Ansatz equations to be physical.
SciPost Phys. 14, 034 (2023) ·
published 15 March 2023

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The rational $Q$system is an efficient method to solve Bethe ansatz equations for quantum integrable spin chains. We construct the rational $Q$systems for generic Bethe ansatz equations described by an $A_{\ell1}$ quiver, which include models with multiple momentum carrying nodes, generic inhomogeneities, generic diagonal twists and $q$deformation. The rational $Q$system thus constructed is specified by two partitions. Under Bethe/Gauge correspondence, the rational $Q$system is in a onetoone correspondence with a 3d $\mathcal{N}=4$ quiver gauge theory of the type ${T}_{{\rho}}^{{\sigma}}[\mathrm{SU}(n)]$, which is also specified by the same partitions. This shows that the rational $Q$system is a natural language for the Bethe/Gauge correspondence, because known features of the ${T}_{{\rho}}^{{\sigma}}[\mathrm{SU}(n)]$ theories readily translate. For instance, we show that the Higgs and Coulomb branch Higgsing correspond to modifying one of the partitions in the rational $Q$system while keeping the other untouched. Similarly, mirror symmetry is realized in terms of the rational $Q$system by simply swapping the two partitions  exactly as for ${T}_{{\rho}}^{{\sigma}}[\mathrm{SU}(n)]$. We exemplify the computational efficiency of the rational $Q$system by evaluating topologically twisted indices for 3d $\mathcal{N}=4$ $\mathrm{U}(n)$ SQCD theories with $n=1,\ldots,5$.
SciPost Phys. 12, 191 (2022) ·
published 10 June 2022

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$\mathrm{T}\overline{\mathrm{T}}$ deformation was originally proposed as an irrelevant solvable deformation for 2d relativistic quantum field theories (QFTs). The same family of deformations can also be defined for integrable quantum spin chains which was first studied in the context of integrability in AdS/CFT. In this paper, we construct such deformations for yet another type of models, which describe a collection of particles moving in 1d and interacting in an integrable manner. The prototype of such models is the LiebLiniger model. This shows that such deformations can be defined for a very wide range of systems. We study the finite volume spectrum and thermodynamics of the $\mathrm{T}\overline{\mathrm{T}}$deformed LiebLiniger model. We find that for one sign of the deformation parameter $(\lambda<0)$, the deformed spectrum becomes complex when the volume of the system is smaller than certain critical value, signifying the break down of UV physics. For the other sign $(\lambda>0)$, there exists an upper bound for the temperature, similar to the Hagedorn behavior of the $\mathrm{T}\overline{\mathrm{T}}$ deformed QFTs. Both behaviors can be attributed to the fact that $\mathrm{T}\overline{\mathrm{T}}$ deformation changes the size the particles. We show that for $\lambda>0$, the deformation increases the spaces between particles which effectively increases the volume of the system. For $\lambda<0$, $\mathrm{T}\overline{\mathrm{T}}$ deformation fattens point particles to finite size hard rods. This is similar to the observation that the action of $\mathrm{T}\overline{\mathrm{T}}$deformed free boson is the NambuGoto action, which describes bosonic strings  also an extended object with finite size.
SciPost Phys. 12, 055 (2022) ·
published 9 February 2022

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We study correlation functions of Dbranes and a supergravity mode in AdS, which are dual to structure constants of two subdeterminant operators with large charge and a BPS singletrace operator. Our approach is inspired by the large charge expansion of CFT and resolves puzzles and confusions in the literature on the holographic computation of correlation functions of heavy operators. In particular, we point out two important effects which are often missed in the literature; the first one is an average over classical configurations of the heavy state, which physically amounts to projecting the state to an eigenstate of quantum numbers. The second one is the contribution from wave functions of the heavy state. To demonstrate the power of the method, we first analyze the threepoint functions in $\mathcal{N}=4$ super YangMills and reproduce the results in field theory from holography, including the cases for which the previous holographic computation gives incorrect answers. We then apply it to ABJM theory and make solid predictions at strong coupling. Finally we comment on possible applications to states dual to black holes and fuzzballs.
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