Taking a two interacting scalar toy model with interaction term $gphichi^2$, we study the production of $chi$-particles coming from the decay of an asymptotic and highly occupied beam of $phi$-particles. We perform a non-perturbative analysis coming from parametric resonant instabilities and investigate the possibility that massive $chi$-particles are produced from decays of massless $phi$-particles from the beam. Although this process is not present in a perturbative analysis, our non perturbative approach allows it to happen under certain conditions. For a momentum $p$ of the beam particles and a mass $m_chi$ of the produced ones, we find that the decay is allowed if the energy density of the beam exceeds the instability threshold $p^2mc^4/(2g^2)$. We also provide an analytical expression for the spontaneous decay rate at the earliest time.
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