Probing light pseudoscalars with light propagation, resonance and spontaneouspolarization

Abstract

Radiation propagating over cosmological distances can probe light weakly interactingpseudoscalar (or scalar) particles. The existence of a spin-0 field changes the dynamicalsymmetries of electrodynamics. It predicts spontaneous generation of polarization ofelectromagnetic waves due to mode mixing in the presence of background magnetic field.We illustrate this by calculations of propagation in a uniform medium, as well as in aslowly varying background medium, and finally with resonant mixing. Highly complicatedcorrelations between different Stokes parameters are predicted, depending onthe parameter regimes. The polarization of propagating waves shows interestingand complex dependence on frequency, the distance of propagation, couplingconstants and parameters of the background medium such as the plasma density andthe magnetic field strength. For the first time we study the resonant mixing ofelectromagnetic waves with the scalar field, which occurs when the background plasmafrequency becomes equal to the mass of the scalar field at some point along thepath. Dynamical effects are found to be considerably enhanced in this case. Wealso formulate the condition under which the adiabatic approximation can beused consistently, and find caveats about comparing different frequency regimes.