OPE of the energy-momentum tensor correlator in massless QCD
  We analytically calculate higher order corrections to coefficient functions of the operator product expansion (OPE) for the Euclidean correlator of two energy-momentum tensors in massless QCD. These are the three-loop contribution to the coefficient C_0 in front of the unity operator O_0=1 and the one and two-loop contributions to the coefficient C_1 in front of the gluon "condensate" operator O_1=-1/4 G^{\mu \nu} G_{\mu \nu}. For the correlator of two operators O_1 we present the coefficient C_1 at two-loop level. 
M. F. Zoller, K. G. Chetyrkin
hep-ph hep-th