The effect of photon-axion-like particle conversions in galaxy clusters   on very high energy {\gamma}-ray spectra
  Very high energy (VHE, energy >~ 100 GeV) {\gamma}-rays originating from extragalactic sources interact with low energy photons of background radiation fields and produce electron-positron pairs. Alternatively, in the presence of ambient magnetic fields, they can convert into hypothetical axion-like particles (ALPs), pseudo-scalar spin-0 bosons, predicted by extensions of the standard model. These particles propagate unimpeded over cosmological distances. Here, the effect of photon-ALP oscillations in magnetic fields of galaxy clusters and the Milky Way on VHE {\gamma}-ray spectra is studied. It is shown that this mechanism can lead to a substantial enhancement of the VHE flux and a spectral hardening, thus effectively reducing the opacity of the Universe to VHE {\gamma}-rays. 
Manuel Meyer and Dieter Horns and Luca Maccione and Alessandro Mirizzi   and Daniele Montanino and Marco Roncadelli
astro-ph.HE hep-ph