The Yang--Baxter Symmetry in Field Theory

TL;DR

This review discusses the role of Yang--Baxter symmetry in two-dimensional integrable quantum field theories, highlighting the algebraic structures, conserved charges, and recent advances in their systematic construction via Bethe Ansatz.

Contribution

It provides a comprehensive overview of the Yang--Baxter algebra in integrable QFTs, including recent generalizations and explicit eigenvalue computations.

Findings

Integrable QFTs possess infinitely many non-commuting conserved charges.

Quantum monodromy operators generate a representation of q-deformed affine Lie algebras.

Explicit Bethe Ansatz solutions yield eigenvalues of transfer matrices and conserved charges.

Abstract

This is a review on infinite non-abelian symmetries in two-dimensional field theories. We show how any integrable QFT enjoys the existence of infinitely many {\bf conserved} charges. These charges {\bf do not commute} between them and satisfy a Yang--Baxter algebra. They are generated by quantum monodromy operators and provide a representation of $q-$deformed affine Lie algebras $U_q({\hat\G})$. We review the work by de Vega, Eichenherr and Maillet on the bootstrap construction of the quantum monodromy operators in classically scale invariant theories where the classical monodromy matrix is conserved. Then, the recent generalization to the sine--Gordon (or massive Thirring) model, where such operators do not possess a classical analogue is given (This provides a representation of $S{\hat U}(2)_q$). It is then reported on the recent work by Destri and de Vega, where both commuting and non-commuting integrals of motion are systematically obtained by Bethe Ansatz in the light-cone approach. The eigenvalues of the six--vertex alternating transfer matrix $\tau(\l)$ are explicitly computed on a generic physical state through algebraic Bethe ansatz. In the thermodynamic limit $\tau(\l)$ turns out to be a two--valued periodic function. One determination generates the local abelian charges, including energy and momentum, while the other yields the abelian subalgebra of the (non--local) YB algebra. In particular, the bootstrap results coincide with the ratio between the two determinations of the lattice transfer matrix.[Based on a Lecture at the Clausthal Symposium].