A sudden small change in the second derivative of the inflaton potential can result in a universal local feature in the spectrum of primordial perturbations generated during inflation .
The exact solution describing this feature ( ( 1 ) ) is characterized by a step in the spectral index modulated by characteristic oscillations and results in a large running of the spectral index localized over a few e-folds of scale .
In this paper we confront this step-like feature with the 5 year WMAP results and demonstrate that it provides a better fit to this data than a featureless initial spectrum .
If such a feature exists at all , then it should lie at sufficiently large scales k _ { 0 } \lesssim 0.003 { Mpc } ^ { -1 } corresponding to l \lesssim 40 .
The sign of the effect is shown to correspond to the negative running of n _ { s } localized near this scale .
This feature could arise as a result of a ‘ mini-waterfall ’ -type fast second order phase transition experienced by an auxiliary heavy field during inflation , in a model similar to hybrid inflation ( though for a different choice of parameters ) .
If this is the case , then the auxiliary field should be positively coupled to the inflaton .