Torsion induced current-scalaron coupling in Einstein–Cartan gravity

Abstract We investigate the matter current couplings with the scalar degrees of freedom originated from the torsion in Einstein–Cartan (EC) gravity. It has been shown in previous studies that the presence of the operators consisting of torsion components up to dimension four can naturally induce a (pseudo-)scalar degree of freedom, the scalaron. In this work, we consider the couplings between torsion and matter currents in this framework, and show that they can lead to couplings between these currents and the scalaron in the equivalent metric theory. We consider both gauge-invariant and gauge-dependent currents, showing general results and several concrete examples. These results are useful for the discussion of particle production processes after inflation in the EC framework, such as reheating and baryogenesis, and show the connection to the QCD θ term.