Axion Emission by Magnetic-Field Induced Conversion of Longitudinal Plasmons

Magnetic fields mix axions with photons, allowing for the cyclotron process e → ea by virtue of an intermediate plasmon even if axions do not couple to electrons at tree level. The axion and longitudinal-plasmon dispersion relations always cross for a certain wave-number, leading to a resonant enhancement of this process. Even then, however, it cannot quite compete with the usual nucleon processes in a supernova core. The well-known axion window 10 eV ∼ ma < ∼ 10 −2 eV remains open and axions may still constitute the cosmic dark matter.