Structure of Resonance in Preheating after Inflation

We consider preheating in the theory $1/4 \lambda \phi^4 + 1/2 g^2\phi^2\chi^2$, where the classical oscillating inflaton field $\phi$ decays into $\chi$-particles and $\phi$-particles. The parametric resonance which leads to particle production in this conformally invariant theory is described by the Lame equation. It significantly differs from the resonance in the theory with a quadratic potential. The structure of the resonance depends in a rather nontrivial way on the parameter $g^2/\lambda$. We construct the stability/instability chart in this theory for arbitrary $g^2/\lambda$. We give simple analytic solutions describing the resonance in the limiting cases $g^2/\lambda\ll 1$ and $g^2/\lambda \gg 1$, and in the theory with $g^2=3\lambda$, and with $g^2 =\lambda$. From the point of view of parametric resonance for $\chi$, the theories with $g^2=3\lambda$ and with $g^2 =\lambda$ have the same structure, respectively, as the theory $1/4 \lambda \phi^4$, and the theory $\lambda /(4 N) (\phi^2_i)^2$ of an N-component scalar field $\phi_i$ in the limit $N \to \infty$. We show that in some of the conformally invariant theories such as the simplest model $1/4 \lambda\phi^4$, the resonance can be terminated by the backreaction of produced particles long before $<\chi^2>$ or $<\phi^2 >$ become of the order $\phi^2$. We analyze the changes in the theory of reheating in this model which appear if the inflaton field has a small mass.