Massive and massless phases in self-dual $Z_N$ spin models: some exact results from the thermodynamic Bethe ansatz

We propose and investigate the thermodynamic Bethe ansatz equations for the minimal $W_p^N$ models~(associated with the $A_{N-1}$ Lie algebra) perturbed by the least~($Z_N$ invariant) primary field $\Phi_N$. Our results reproduce the expected ultraviolet and infrared regimes. In particular for the positive sign of the perturbation our equations describe the behaviour of the ground state flowing from the $W_p^N$ model to the next $W_{p-1}^N$ fixed point.

Inspired by previous work on the constraints that duality imposes on beta functions of spin models, we propose a consistency condition between those functions and RG flows at different points in coupling constant space. We show that this consistency holds for a non self-dual model which admits an exact RG flow, but that it is violated when the RG flow is only approximate. We discuss the use of this deviation as a test for the ``goodness'' of proposed RG flows in complicated m...

We present the thermodynamic Bethe ansatz as a way to factorize the partition function of a 2d field theory, in particular, a conformal field theory and we compare it with another approach to factorization due to K. Schoutens which consists of diagonalizing matrix recursion relations between the partition functions at consecutive levels. We prove that both are equivalent, taking as examples the SU(2) spinons and the 3-state Potts model. In the latter case we see that there ar...

The $Z_N$-invariant chiral Potts model is considered as a perturbation of a $Z_N$ conformal field theory. In the self-dual case the renormalization group equations become simple, and yield critical exponents and anisotropic scaling which agree with exact results for the super-integrable lattice models. Although the continuum theory is not Lorentz invariant, it respects a novel type of space-time symmetry which allows for the observed spontaneous breaking of translational symm...

We suggest an approach to the problem of finding integral equations for the excited states of an integrable model, starting from the Thermodynamic Bethe Ansatz equations for its ground state. The idea relies on analytic continuation through complex values of the coupling constant, and an analysis of the monodromies that the equations and their solutions undergo. For the scaling Lee-Yang model, we find equations in this way for the one- and two- particle states in the spin-zer...

This review was born as notes for a lecture given at the YRIS school on integrability in Durham, in the summer of 2015. It deals with a beautiful method, developed in the mid-nineties by V.V. Bazhanov, S.L. Lukyanov and A.B. Zamolodchikov and, as such, called BLZ. This method can be interpreted as a field theory version of the quantum inverse scattering (QIS), also known as algebraic Bethe ansatz (ABA). Starting with the case of conformal field theories (CFT) we show how to b...

Systems of integral equations are proposed which generalise those previously encountered in connection with the so-called staircase models. Under the assumption that these equations describe the finite-size effects of relativistic field theories via the Thermodynamic Bethe Ansatz, analytical and numerical evidence is given for the existence of a variety of new roaming renormalisation group trajectories. For each positive integer $k$ and $s=0,\dots, k-1$, there is a one-parame...

The holographic renormalization group (RG) flows in certain self-dual two dimensional QFT's models are studied. They are constructed as holographic duals to specific New Massive 3d Gravity (NMG) models coupled to scalar matter with "partially self-dual" superpotentials. The standard holographic RG constructions allow us to derive the exact form of their $\beta$- functions in terms of the corresponding NMG's domain walls solutions. By imposing invariance of the free energy, th...

I study some classes of RG flows in three dimensions that are classically conformal and have manifest g -> 1/g dualities. The RG flow interpolates between known (four-fermion, Wilson-Fischer, phi_3^6) and new interacting fixed points. These models have two remarkable properties: i) the RG flow can be integrated for arbitrarily large values of the couplings g at each order of the 1/N expansion; ii) the duality symmetries are exact at each order of the 1/N expansion. I integrat...

The Schramm-Loewner evolution (SLE) describes the continuum limit of domain walls at phase transitions in two dimensional statistical systems. We consider here the SLEs in the self-dual Z(N) spin models at the critical point. For N=2 and N=3 these models correspond to the Ising and three-state Potts model. For N>5 the critical self-dual Z(N) spin models are described in the continuum limit by non-minimal conformal field theories with central charge c>=1. By studying the repre...