Stability of Poincar\'e gauge theory with cubic order invariants

Abstract

We analyse the stability of the vector and axial sectors of Poincar\'e gauge theory around general backgrounds in the presence of cubic order invariants defined from the curvature and torsion tensors, showing how the latter can in fact cancel out well-known instabilities arising from the quadratic curvature invariants of the theory and accordingly help in the construction of healthy models with both curvature and torsion. For this task, we introduce the most general parity preserving cubic Lagrangian with mixing terms of the curvature and torsion tensors, and find the relations of its coefficients to avoid a pathological behaviour from the vector and axial modes of torsion. As a result, on top of the gravitational constant of General Relativity and the mass parameters of torsion, our action contains 23 additional coupling constants controlling the dynamics of this field. As in the quadratic Poincar\'e gauge theory, we show that a further restriction on the cubic part of the action allows the existence of Reissner-Nordstr\"om-like black hole solutions with dynamical torsion.