Twisting inflation to sub-Planckian axion decay constants

We study pseudoscalar inflation in the Einstein–Cartan–Palatini formulation of gravity, where torsion plays a dynamical role. We introduce pseudoscalar couplings to two topological invariants: the Pontryagin density and the Nieh–Yan term. While the Pontryagin coupling is strongly constrained by instabilities, the Nieh–Yan interaction modifies the kinetic sector, effectively enhancing the axion decay constant. This allows inflation to proceed on steeper potentials than usual. The resulting torsion background is parity-violating and gives rise to a chiral gravitational wave spectrum, while leaving the scalar sector largely unchanged. We demonstrate that natural inflation remains excluded; however, a squared hilltop potential coupled to the Nieh–Yan term can accommodate Planck 2018 observations with sub-Planckian decay constants.