Inflation with vector fields revisited: non-Gaussianities

A bstract We revisit the resulting bispectrum of inflation with kinetic-coupled vector fields by organizing the dynamics in terms of h , which measures the vector kinetic contribution relative to that of the scalar field. We derive a low-energy effective field theory (EFT) and evaluate the bispectrum of the curvature perturbation for the large- h regime. For h ≫ 1, the entropic perturbation becomes heavy and can be integrated out; the remaining curvature mode has an imaginary sound speed and undergoes transient growth before horizon crossing. In contrast to the h ≪ 1 regime, where transfer from the vector sector persists outside the horizon and produces a local-type contribution enhanced as h²NK³ , we find that the bispectrum signals are controlled by the matching parameter x ≃ 1 . 63 Ah , where A < 1 is a numerical factor remaining dependent on the UV-to-EFT matching prescription. As a result, in addition to the known flattened-enhanced signals scaling as x 3 , a flattened-enhanced signal scaling as x 2 and a pronounced local projection scaling as x are present. The bispectrum therefore distinguishes vector-supported inflationary dynamics even for an exactly isotropic background.