On Factorizable S-matrices, Generalized TTbar, and the Hagedorn Transition

Understand

We study solutions of the Thermodynamic Bethe Ansatz equations for relativistic theories defined by the factorizable $S$-matrix of an integrable QFT deformed by CDD factors.

• Such $S$-matrices appear under generalized TTbar deformations of integrable QFT by special irrelevant operators.
• The TBA equations, of course, determine the ground state energy $E(R)$ of the finite-size system, with the spatial coordinate compactified on a circle of circumference $R$.
• We limit attention to theories involving just one kind of stable particles, and consider deformations of the trivial (free fermion or boson) $S$-matrix by CDD factors with two elementary poles and regular high energy asymptotics -- the "2CDD model".