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