The propagation of TeV gamma rays can be strongly modified by B-field induced conversion to axionlike particles (ALPs). We show that, at such high energies, photon dispersion is dominated by background photons-the only example where photon-photon dispersion is of practical relevance. We determine the refractive index for all energies and find that, for fixed energy density, background photons below the pair-production threshold dominate. The cosmic microwave background alone provides an "effective photon mass" of m 2 γ = −(1.01 neV × ω/TeV) 2 for ω 1000 TeV. The extragalactic background light is subdominant, but local radiation fields in the galaxy or the source regions provide significant contributions. Photon-photon dispersion is small enough to leave typical scenarios of photon-ALP oscillations unscathed, but big enough to worry about it case by case.

The helicity of a Dirac neutrino with mass m evolves under the influence of a B-field because it has a magnetic dipole moment proportional to m. Moreover, it was recently shown that a polarized or anisotropic medium engenders the same effect for both Dirac and Majorana neutrinos. Because a B-field polarizes a background medium, it instigates helicity oscillations even for Majorana neutrinos unless the medium is symmetric between matter and antimatter. Motivated by these observations, we review the impact of a B-field and of an anisotropic or polarized medium on helicity oscillations for Dirac and Majorana neutrinos from the common perspective of in-medium dispersion.

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