Fragileness of exact I-ball/oscillon

An I-ball/oscillon is a solitonlike oscillating configuration of a real scalar field, and defined as a minimum energy state for a given adiabatic invariant. It lasts for a long time because of the approximate conservation of their adiabatic invariant. In this paper, we examine the stability of a special type of I-ball/oscillon, the "exact" I-ball/oscillon, whose adiabatic invariant is exactly conserved. We show that the exact I-ball/oscillon is generally stable in classical field theory, but one with a certain adiabatic invariant is unstable against small perturbations because fluctuations around the exact I-ball/oscillon profile have instability bands. Accordingly, the exact I-ball/oscillon breaks up in the presence of the fluctuations of corresponding instability modes. We also confirm the fragileness of the exact I-ball/oscillon by the classical lattice simulation.