TT deformation — an introduction

The talk is an introduction to the TT deformation — the deformation of a QFT by the irrelevant operator constructed from the energy-momentum tensor. Its origin and basic definitions will be introduced. Despite being irrelevant (and so hard to deal with in QFT), this deformation is integrable in the sense that some physical quantities can be computed exactly, including the deformed energy spectrum and the S-matrix. The property of this deformation to create objects of finite length from point-like objects will be demonstrated by turning the free boson Lagrangian to the Nambu-Goto Lagrangian. Similar phenomena happen in other systems; for example, a gas of particles of finite length (hard rod model) emerges from a gas of point-like particles. Also the lattice counterpart of the TT deformation will be discussed. Both deformations share the same algebraic features, and some exact results of QFT can be obtained from the thermodynamic limit of the lattice model of statistical physics.