Photo-production of axions in Supernovae

Abstract

Compact stellar objects like supernovae and neutron stars are believed to cool by emitting axions predominantly via axion bremsstrahlung ($NN\to NNa$), pion conversion (${\pi }^{-}{p}^{+}\to Na$) and photo-production ($\gamma N\to Na$). In this paper, we study in detail the photo-production channel, from the unavoidable anomaly induced Wess-Zumino-Witten term $\propto {ϵ}^{\mu \nu \alpha \beta }{F}_{\mu \nu }{\partial }_{\alpha }a{\omega }_{\beta }$ in conjunction with the low energy pion photo-production data. We found that for heavier axions, i.e., $m_a\sim \mathcal{O}(100)$ MeV, photo-production processes can be dominant compared to the usual axion emission processes. In addition, the spectrum of axions emitted in the process is significantly harder than those originating from bremsstrahlung.

Comments: 10 pages, 5 figures, Appendix