In slow roll inflation, the running of the spectral tilt is generically proportional to the square of the deviation from scale invariance, αs (ns-1)2, and is therefore currently undetectable. We present a mechanism able to generate a much larger running within slow roll. The mechanism is based on a curvaton field with a large mass term, and a time evolving normalization. This may happen for instance to the angular direction of a complex field in presence of an evolving radial direction. At the price of a single tuning between the mass term and the rate of change of the normalization, the curvaton can be made effectively light at the CMB scales, giving a spectral tilt in agreement with observations. The lightness is not preserved at later times, resulting in a detectable running of the spectral tilt. This mechanism shows that fields with a large mass term do not necessarily decouple from the inflationary physics, and provides a new tool for model building in inflation.
- cosmological perturbation theory