Dynamics of axion-neutral pseudoscalar mixing

Axions mix with neutral pions after the QCD phase transition through their common coupling to the radiation bath via a Chern-Simons term, as a consequence of the Uo1THORN anomaly. The nonequilibrium effective action that describes this mixing phenomenon is obtained to second order in the coupling of neutral pions and axions to photons. We show that a misaligned axion condensate induces a neutral pion condensate after the QCD phase transition. We argue that even a very small axion-photon coupling will seed the chiral phase transition. The dynamics of the pion condensate displays long and short timescales and decays on the longer timescale exhibiting a phenomenon akin to the purification in a kaon beam. On the intermediate timescales the macroscopic pion condensate is proportional to a condensate of the Abelian Chern-Simons term induced by the axion. We argue that the coupling to the common bath also induces kinetic mixing. We obtain the axion and pion populations, and these exhibit thermalization with the bath. The mutual coupling to the bath induces long-lived axion-neutral pion coherence independent of initial conditions. The framework of the effective action and many of the consequences are more broadly general and applicable to scalar or pseudoscalar particles mixing in a medium.

Automated summary

After the QCD phase transition, axions mix with neutral pions through their common coupling to the radiation bath via a Chern-Simons term. The nonequilibrium effective action, describing this mixing phenomenon, is obtained to second order in the coupling of neutral pions and axions to photons. A misaligned axion condensate is shown to induce a neutral pion condensate after the QCD phase transition. Even a very small axion-photon coupling is argued to seed the chiral phase transition. The dynamics of the pion condensate displays long and short timescales, with decay on the longer timescale resembling purification in a kaon beam. Additionally, the coupling to the common bath induces kinetic mixing. The populations of axions and pions exhibit thermalization with the bath, and the coupling to the bath induces long-lived coherence between axions and neutral pions independent of initial conditions. The framework of the effective action and many of the consequences are more broadly applicable to scalar or pseudoscalar particles mixing in a medium.