Running Spectral Index as a Probe of Physics at High Scales

Abstract

The WMAP results on the scalar spectral index n and its running with scale, though preliminary, open a very interesting window to physics at very high energies. We address the problem of finding inflaton potentials well motivated by particle physics which can accomodate WMAP data. We make a model independent analysis of a large class of models: those with flat tree-level potential lifted by radiative corrections, which cause the slow rolling of the inflaton and the running of n. This includes typical hybrid inflation models. In the small-coupling regime the predictions for the size and running of n are remarkably neat, e.g. -dn/dln k=(n-1)^2 << 1, and n does not cross n=1, contrary to WMAP indications. On the other hand, n can run significantly if the couplings are stronger but at the price of having a small number of e-folds, Ne. We also examine the effect of mass thresholds crossed during inflation. Finally, we show that the presence of non-renormalizable operators for the inflaton, suppressed by a mass scale above the inflationary range, is able to give both dn/dln k ~ O(-0.05) and Ne ~ 50.