Semiclassical analysis of axion-assisted and axion-driven pair production

Abstract

We study the pair production of fermions in a time dependent axion background with and without an electric background. We construct the adiabatic mode functions which incorporate the gauge field and the axion velocity dependence of the dispersion relation. The semiclassical approach using this adiabatic basis shows two types of pair production. One is axion-assisted pair production: the presence of the axion velocity gives enhancement and interference effects on the pair production driven by the electric field. The other is axion-driven pair production: the time variation of the axion velocity causes the pair production even though the electric field is absent.

Transient phenomena in the axion assisted Schwinger effect

“…Ref. [27] confirms this result in the c m = 0 basis. Indeed, one can eliminate c m by the chiral rotation ψ → e −iθmγ 5 ψ.…”

“…Moreover, it was recently pointed out that a non-vanishing axion velocity has the intriguing property of boosting the Schwinger pair production of charged fermions in an electric field [26]. In the axion-assisted Schwinger effect [26] a sufficiently large constant axion velocity can exponentially enhance the Schwinger production rate for finite fermion momentum in the final state whereas the axion-driven pair production [27] relies a non-vanishing acceleration of the axion field.…”

“…labeling terms as vacuum contribution, time-dependent Bogoliubov coefficients or higher-dimensional operators, while the physical observables are unaffected by these different basis choicesas they should. We secondly use this opportunity to compare different approaches in the existing literature, in particular [26,27] and [31]. In this context, we highlight the importance of choosing a regularization scheme which respects the symmetries of the theory and the benefits of making use of the Adler-Bell-Jackiw anomaly equation [32,33] as a necessary verification of this.…”

“…The key point is that the above equation explicitly involves Ω ± , and thus it is more straightforward to see the importance of Ω ± on particle production with, e.g. the WKB method [27]. However, this basis has its own subtlety, related to the regularization and renormalization, and demonstrating this point is our main goal in section 3.…”

“…[26] we take the Hamiltonian basis (c 5 = 0), while ref. [27] takes the fermion current basis (c m = 0). 4 Our goal here is to see that the fermion energy density operator is diagonalized only in the Hamiltonian basis, and not in the fermion current basis, which is crucial for our study of the anomaly equation in section 3.2.…”