Curvature perturbations from a massive vector curvaton

We study a ghost-free model of a massive vector curvaton proposed in the literature, where the quick decrease of the vector background expectation value is avoided by a suitable choice of kinetic and mass functions. The curvaton perturbations of this model have been so far computed assuming that these functions are external classical quantities, and it was found that some special time evolution of these functions leads to scale-invariant and statistically isotropic perturbations of the vector curvaton. However, external functions should be understood as originating from the expectation value of some additional field. Since these functions need to present a nontrivial evolution during inflation, the field cannot be trivially integrated out, and in particular, its perturbations need to be included in the computation. We do so in a minimal implementation of the mechanism, where the additional field is identified with the inflaton. We show that, except for a narrow window of model parameters, the interaction with this field generally causes the curvature perturbations to violate statistical isotropy beyond the observational limit.