  We investigate radiative corrections to the inflaton potential from heavy
fields undergoing a fast-roll phase transition. We find that a logarithmic
one-loop correction to the inflaton potential involving this field can induce a
temporary running of the spectral index. The induced running can be a short
burst of strong running, which may be related to the observed anomalies on
large scales in the cosmic microwave spectrum, or extend over many e-folds,
sustaining an effectively constant running to be searched for in the future. We
implement this in a general class of models, where effects are mediated through
a heavy messenger field sitting in its minimum. Interestingly, within the
present framework it is a generic outcome that a large running implies a small
field model with a vanishing tensor-to-scalar ratio, circumventing the normal
expectation that small field models typically lead to an unobservable small
running of the spectral index. An observable level of tensor modes can also be
accommodated, but, surprisingly, this requires running to be induced by a
curvaton. If upcoming observations are consistent with a small tensor-to-scalar
ratio as predicted by small field models of inflation, then the present study
serves as an explicit example contrary to the general expectation that the
running will be unobservable.
