Magnon inflation : slow roll with steep potentials

We find multi-scalar effective field theories (EFTs) that can achieve a slow infl ationary roll despite having a scalar potential that does not satisfy G(ab) partial derivative(a) V partial derivative(b) V << V (2) / M-P(2) (where G(ab) is the target-space metric). They evade the usual slow-roll conditions on V because their kinetic energies are dominated by single-derivative terms rather than the usual two-derivative terms. Single derivatives dominate during slow roll and so do not require a breakdown of the usual derivative expansion that underpins calculational control in much of cosmology. The presence of such terms requires some sort of UV Lorentz-symmetry breaking during inflation (besides the usual cosmological breaking). Chromo-natural inflation provides one particular example of a UV theory that can generate the multi-field single-derivative terms we consider, and we argue that the EFT we find indeed captures the slow-roll conditions for its background evolution. We also show that our EFT can be understood as a multi-field generalization of the single-field Cuscuton models. The multi-field case introduces a new feature, however: the scalar kinetic terms de fine a target-space 2-form, F-ab, whose antisymmetry gives new ways for slow roll to be achieved.