Scalar field mixing induced by common intermediate states*

We study the dynamics of particle mixing induced by their coupling to a common intermediate state or decay channel, which is of broad fundamental interest within the context of CP violation and/or baryogenesis. Field mixing may also be a consequence of "portals", connecting standard model degrees of freedom to hypothetical ones via mediator particles beyond the standard model. Using effective field theory (EFT), we showed that the field mixing is described by a set of Langevin equations, where off-diagonal elements in the self-energy leads to field mixing. Off-diagonal noise-correlations lead to a long-time correlation between fields. We discussed non-degenerate and nearly degenerate field mixing and found that particle mixing and quantum beats are strongly enhanced when the masses of the fields are approximately the same. This may point to a new way to search for axion-like particles using synthetic axions through quantum beats in the decay products (photons). A Quantum master equation is derived to complement EFT methods, revealing oscillations of the number density and coherence between fields.