We provide a systematic treatment of chemical equilibrium in the presence of a specific type of time dependent background. The type of time dependent background we consider appears, for example, in recently proposed axion/Majoron leptogenesis models [A. Kusenko, K. Schmitz, and T. T. Yanagida, Phys. Rev. Lett. 115, 011302 (2015) and M. Ibe and K. Kaneta, Phys. Rev. D 92, 035019 (2015)]. In describing the chemical equilibrium we use quantities which are invariant under redefinition of fermion phases (we refer to this redefinition as a change of basis for short), and therefore it is a basis invariant treatment. The change of the anomaly terms due to the change of the path integral measure [K. Fujikawa, Phys. Rev. Lett. 42, 1195 (1979) and K. Fujikawa, Phys. Rev. D 29, 285 (1984)] under a basis change is taken into account. We find it is useful to go back and forth between different bases, and there are insights which can be more easily obtained in one basis rather than another. A toy model is provided to illustrate the ideas. For the axion leptogenesis model [A. Kusenko, K. Schmitz, and T. T. Yanagida, Phys. Rev. Lett. 115, 011302 (2015)], our result suggests that at $T>{10}^{13}\mathrm{GeV}$, when sphaleron processes decouple and ${\mathrm{\Gamma }}_{B+L}\ll H<{\mathrm{\Gamma }}_L$ (where $H$ is the Hubble parameter at temperature $T$ and ${\mathrm{\Gamma }}_L$ is the $\mathrm{\Delta }L=2$ lepton number violating interaction rate), the amount of $B-L$ created is controlled by the smallness of the sphaleron interaction rate, ${\mathrm{\Gamma }}_{B+L}$. Therefore it is not as efficient as described. In addition, we notice an interesting modification of gauge boson dispersion relations at subleading order.

In this paper, change of basis does not mean change of Lorentz frame. All calculations in this paper are performed in the center-of-momentum frame of the thermal plasma, i.e. the Lorentz frame in which the average momentum of particles is zero.