A Nieh-Yan-like topological invariant in General Relativity

Abstract: In the present work we will introduce and prove a topological invariant term in General Relativity involving the torsion tensor that has never been showed before. Such a term is a slight modification of the Nieh-Yan four-form and likewise it stems from a Chern-Simons three-form. We provide the proof in both holonomic and orthogonal basis and show that its integral reduces to a boundary term that vanishes with the right conditions. As all topological invariant objects the new term does not affect the Einstein field equations in pure gravity, but when matter fields couple to the gravitational field, the torsion tensor arises and its contribution changes the "rules of the game". Therefore it is of great importance to study how those pieces irrelevant in bare gravity modify the interaction with fields of the Standard Model.