Recently there has been a lot of activity in understanding irrelevant deformations of QFTs in 2d. One such well studied deformation is the so called TTbar deformation. In the talk I'm going to give an introduction to TTbar deformation of QFT2. I will show why TTbar deformation is well defined despite the fact that this deformation triggers a flow up the RG trajectory in the space of theories. I'm going to prove the celebrated factorization property of the TTbar operator and finally derive the spectrum of the deformed theory. If time permits, I'm going to talk about the modular properties of the deformed partition function and the constraints imposed on the spectrum of the deformed theory from modular bootstrap.
I'm mostly going to focus the following papers:
hep-th/0401146, 1608.05499, 1608.05534, 1801.06895, 1808.02492