Is a step in the primordial spectral index favored by CMB data ?

Abstract

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 \cite{minu} 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 $k_0\lesssim 0.003{\mathrm{Mpc}}^{-1}$ 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.

Several new results, figures and enhanced discussion. Main conclusions unchanged. Matches the final version to be published in JCAP

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