We investigate preheating in a higher-dimensional generalized Kaluza-Klein theory with a quadratic inflaton potential V(φ) = 1/2m2φ2 including metric perturbations explicitly. The system we consider is the multi-field model where there exists a dilaton field σ which corresponds to the scale of compactifications and another scalar field χ coupled to inflaton with the interaction 1/2g2φ2χ2 + g-2φ3χ. In the case of g̃ = 0, we find that the perturbation of dilaton does not undergo parametric amplification while the χ field fluctuation can be enhanced in the usual manner by parametric resonance. In the presence of the g-2φ3χ coupling, the dilaton fluctuation in sub-Hubble scales is modestly amplified by the growth of metric perturbations for the large coupling g̃. In super-Hubble scales, the enhancement of the dilaton fluctuation as well as metric perturbations is weak, taking into account the backreaction effect of created χ particles. We argue that not only is it possible to predict the ordinary inflationary spectrum in large scales but extra dimensions can be held static during preheating in our scenario.
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