A falsely fat curvaton with an observable running of the spectral tilt
  In slow roll inflation, the running of the spectral tilt is generically
proportional to the square of the deviation from scale invariance, $\alpha_s
\propto (n_s-1)^2$, and it 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.
