- Conference date: 7–9 October 2009
- Location: Assisi (Italy)
A generic prediction of several extensions of the Standard Model of elementary‐particle interactions is the existence of axion‐like particles (ALPs), namely very light spin‐zero bosons characterized by a two‐photon coupling. While elusive in laboratory experiments, ALPs can give rise to observable astrophysical effects for their relevant parameters in experimentally allowed ranges. We show that the unexpectedly low opacity of the Universe inferred by the Imaging Atmospheric Cherenkov Telescopes since 2006 from blazar observations above 100 GeV can be explained naturally within the De Angelis, Roncadelli & Mansutti—hereafter DARMA—scenario, namely in terms of photon‐ALP oscillations occurring in extragalactic magnetic fields. We work out the implications of the DARMA scenario for the VHE gamma‐ray spectra of blazars by contemplating all of them at once, so that the emitted and observed spectral indices can be correlated. We demonstrate that by assuming the same nominal value for all VHE blazars, the predicted observed spectral index actually fits all observations. Moreover, becomes independent of redshift for sufficiently far‐away sources. Our prediction can be tested with the satellite‐borne Fermi/LAT detector as well as with the ground‐based IACTs H.E.S.S., MAGIC, CANGAROO III, VERITAS and the Extensive Air Shower arrays ARGO‐YBJ and MILAGRO.
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