Expectation value of composite field $T{\bar T}$ in two-dimensional quantum field theory

@inproceedings{Zamolodchikov2004ExpectationVO,
  title={Expectation value of composite field \$T\{\bar T\}\$ in two-dimensional quantum field theory},
  author={Alexander B. Zamolodchikov},
  year={2004}
}
I show that the expectation value of the composite field $T{\bar T}$, built from the components of the energy-momentum tensor, is expressed exactly through the expectation value of the energy-momentum tensor itself. The relation is derived in two-dimensional quantum field theory under broad assumptions, and does not require integrability.