Vector Curvaton with varying Kinetic Function
  A new model realisation of the vector curvaton paradigm is presented and
analysed. The model consists of a single massive Abelian vector field, with a
Maxwell type kinetic term. By assuming that the kinetic function and the mass
of the vector field are appropriately varying during inflation, it is shown
that a scale invariant spectrum of superhorizon perturbations can be generated.
These perturbations can contribute to the curvature perturbation of the
Universe. If the vector field remains light at the end of inflation it is found
that it can generate substantial statistical anisotropy in the spectrum and
bispectrum of the curvature perturbation. In this case the non-Gaussianity in
the curvature perturbation is predominantly anisotropic, which will be a
testable prediction in the near future. If, on the other hand, the vector field
is heavy at the end of inflation then it is demonstrated that particle
production is approximately isotropic and the vector field alone can give rise
to the curvature perturbation, without directly involving any fundamental
scalar field. The parameter space for both possibilities is shown to be
substantial. Finally, toy-models are presented which show that the desired
variation of the mass and kinetic function of the vector field can be
realistically obtained, without unnatural tunings, in the context of
supergravity or superstrings.
