Numerical study of multiparticle production in $\lambda\phi^4$ theory

We study multiparticle production in the unbroken $(3+1)$-dimensional $\lambda\phi^4$ theory using the semiclassical method of singular solutions. We show that the probabilities of these processes are exponentially suppressed in terms of a small coupling constant $\lambda \ll 1$ if the multiplicity of the final state is large: $n \gg 1$. At $n \ll \lambda^{-1}$ the probabilities agree with well-known perturbative results. At $n \gg \lambda^{-1}$ they are dominated by loop effects and decrease exponentially with $n$, as we show for the first time.