A model is developed in which the inflaton potential experiences a sudden small change in its second derivative (the effective mass of the inflaton). An exact treatment demonstrates that the resulting density perturbation has a quasiflat power spectrum with a break in its slope (a step in $n_s$). The step in the spectral index is modulated by characteristic oscillations and results in large running of the spectral index localized over a few e-folds of scales. A field-theoretic model giving rise to such behavior of the inflationary potential is based on a fast phase transition experienced by a second scalar field weakly coupled to the inflaton. Such a transition is similar to that which terminates inflation in the hybrid inflationary scenario. This scenario suggests that the observed running of the spectral index in the WMAP data may be caused by a fast second order phase transition which occurred during inflation.

• Received 11 November 2007

DOI:https://doi.org/10.1103/PhysRevD.77.023514