Spectrum of scalar ("adiabatic") perturbations in a model of double inflation

We study numerically a model of double inflation with two massive scalar fields and no interaction potential. Such models display a non-scale-invariant spectrum when the heavy mass $m_h>>m_l$, the light mass. The spectrum of adiabatic perturbations is studied numerically for $10\le p\equiv \frac{m_h}{m_l}\le 25$ for which there is no pronounced intermediate stage between the two inflations. In that case we find that the contribution of the light scalar field to the spectrum $k^{\frac{3}{2}}\Phi \mathrm{}\left(k\right)\mathrm{}$, where $\Phi \mathrm{}\left(k\right)\mathrm{}$ is a rms gravitational potential, is dominant on all scales where the transition from the upper to the lower "plateau" takes place. Normalization to COBE DMR yields $m_h\sim 3\times {10}^{-6\mathrm{}}$ Mp. When the "step" is located around $10h_{100}^{-1\mathrm{}}$ Mpc, we get acceptable biasing parameters $2.30\le b\le 2.72$. However, the model has sensibly less power on intermediate scales than standard CDM, where the latter is known to give acceptable results. This will severely constrain the parameters of the model.