  Taking a two interacting scalar toy model with interaction term
$g\phi\chi^2$, we study the production of $\chi$ particles coming from the
decay of an asymptotic and highly occupied beam of $\phi$ particles. We perform
a nonperturbative 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 nonperturbative 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 $p^2
m_\chi^4/(2g^2)$. We also provide an analytical expression for the spontaneous
decay rate at the earliest time.
