  We study decoherence effects and phase corrections in heavy
neutrino-antineutrino oscillations (NNOs), based on quantum field theory with
external wave packets. Decoherence damps the oscillation pattern, making it
harder to resolve experimentally. Additionally, it enhances lepton number
violation (LNV) for processes in symmetry-protected low-scale seesaw models by
reducing the destructive interference between mass eigenstates. We discuss a
novel time-independent shift in the phase and derive formulae for calculating
decoherence effects and the phase shift in the relevant regimes, which are the
no dispersion regime and transverse dispersion regime. We find that the phase
shift can be neglected in the parameter region under consideration since it is
small apart from parameter regions with large damping. In the oscillation
formulae, decoherence can be included by an effective damping parameter. We
discuss this parameter and present averaged results, which apply to simulations
of NNOs in the dilepton-dijet channel at the HL-LHC. We show that including
decoherence effects can dramatically change the theoretical prediction for the
ratio of LNV over LNC events.
