Anomalous gravitational TTT vertex, temperature inhomogeneity, and pressure anisotropy

The conformal anomaly in curved spacetime generates a nontrivial anomalous vertex, given by the threepoint correlation function TTT of the energy-momentum tensor T-mu nu. We show that a temperature inhomogeneity in a gas of charged massless particles generates, via the TTT vertex, a pressure anisotropy with respect to the axis of the temperature variation. This very particular signature may provide an experimental access to the elusive gravitational coefficient b which determines the anomaly contribution of the Weyl tensor to the trace of the energy-momentum tensor in curved spacetime. We present an estimate of the pressure anisotropy both for fermionic quasiparticles in the solid-state environment of Dirac semimetals as well as for a quark-gluon plasma in relativistic heavy-ion collisions. In both cases, the pressure anisotropy is small compared to the mean thermal pressure. (C) 2020 The Author(s). Published by Elsevier B.V.