0 v 1 1 8 A ug 2 00 3 Can the curvaton paradigm accommodate a low inflation scale ?

The cosmological curvature perturbation may be generated when some ‘curvaton’ field, different from the inflaton, oscillates in a background of unperturbed radiation. In its simplest form the curvaton paradigm requires the Hubble parameter during inflation to be bigger than 10 GeV, but this bound may be evaded if the curvaton field (or an associated tachyon) is strongly coupled to a field which acquires a large value at the end of inflation. As a result the curvaton paradigm might be useful in improving the viability of low-scale inflation models, in which the supersymmetrybreaking mechanism is the same as the one which operates in the vacuum. Introduction In some of the most interesting inflation models, the inflationary potential comes from the same SUSY-breaking mechanism that operates in the vacuum, giving a Hubble parameter is of order the gravitino mass [1]. Inventing some terminology, one may call these models non-hybrid modular inflation [2, 3], hybrid modular inflation [4, 5], and just-beyond-MSSM inflation invoking gauge-mediated [6], gravity-mediated [7] or gaugino-mediated [8] SUSY breaking. In gravity-mediated SUSY breaking the gravitino mass is of order TeV, and in the other schemes it is some orders of magnitude lower except for anomaly-mediated [9] where it is up to 100TeV. An inflationary Hubble parameter of order the gravitino mass is therefore low compared with the maximum value of order 10GeV allowed by the CMB anisotropy [10], and low compared with the value in other sensible-looking models of inflation [1]. The primordial curvature perturbation originates presumably during inflation, as the perturbation of some light scalar field. The question is, which field does the job? The usual answer is; the inflaton [10]. Unfortunately, this ‘inflaton paradigm’ tends to make life difficult for the low-scale models [1]. In its original form, non-hybrid modular inflation predicts a curvature perturbation that is far too small, and so do the just-beyond-MSSM models, unless one admits extreme fine-tuning. The original form of hybrid modular The fine-tuning in model of [7] might be removed if the curvature perturbation is generated during preheating, from the decay products of the perturbed Higgs field [8]. This is an alternative to the curvaton mechanism that we are about to discuss.