Formation and decay of oscillons after inflation in the presence of an external coupling. Part I. Lattice simulations

We investigate the formation and decay of oscillons during the post-inflationary reheating epoch from inflaton oscillations around asymptotically flat potentials V(φ) in the presence of an external coupling of the form 1/2 g2φ2χ2. It is well-known that in the absence of such an external coupling, the attractive self-interaction term in the potential leads to the formation of copious amounts of long-lived oscillons both for symmetric and asymmetric plateau potentials. We perform a detailed numerical analysis to study the formation of oscillons in the α-attractor E- and T-model potentials using the publicly available lattice simulation code CosmoLattice. We observe the formation of nonlinear oscillon-like structures with the average equation of state ⟨wφ⟩ ≃ 0 for a range of values of the inflaton self-coupling λ and the external coupling g2. Our results demonstrate that oscillons form even in the presence of an external coupling and we determine the upper bound on g2 which facilitates oscillon formation. We also find that eventually, these oscillons decay into the scalar inflaton radiation as well as into the quanta of the offspring field χ. Thus, we establish the possibility that reheating could have proceeded through the channel of oscillon decay, along with the usual decay of the oscillating inflaton condensate into χ particles. For a given value of the self-coupling λ, we notice that the lifetime of a population of oscillons decreases with an increase in the strength of the external coupling, following an (approximately) inverse power-law dependence on g2.