  The relaxation dynamics of an open quantum system is determined by the
competition between the coherent Hamiltonian dynamics of a system and the
dissipative dynamics due to interactions with environments. It is therefore of
fundamental interest to understand the transition from the coherent to
incoherent regimes. We find that hitherto unrecognized quasiparticles --
incoherentons -- describe this coherent-to-incoherent transition in eigenmodes
of a Liouvillian superoperator that governs the dynamics of an open quantum
many-body system. Here, an incoherenton is defined as an interchain bound state
in an auxiliary ladder system that represents the density matrix of a system.
The Liouvillian eigenmodes are classified into groups with different decay
rates that reflect the number of incoherentons involved therein. We also
introduce a spectral gap -- quantum coherence gap -- that separates the
different groups of eigenmodes. We demonstrate the existence of incoherentons
in a lattice boson model subject to dephasing, and show that the quantum
coherence gap closes when incoherentons are deconfined, which signals a
dynamical transition from incoherent relaxation with exponential decay to
coherent oscillatory relaxation. Furthermore, we discuss how the decoherence
dynamics of quantum many-body systems can be understood in terms of the
generation, localization, and diffusion of incoherentons.
