Detection of a New Light Boson by Cherenkov Telescopes?

Recent observations by Imaging Atmospheric Cherenkov Telescopes (IACTs) strongly suggest a transparency of the Universe to very-high-energy (VHE) photons larger than expected from current models of the Extragalactic Background Light. It has been shown that such transparency can be naturally explained by the DARMA scenario. Its basic ingredient is a very light Axion-Like Particle (ALP) predicted by many extensions of the Standard Model of elementary particles and allows for the occurrence of photon-ALP oscillations in cosmic magnetic fields. We discuss the implications of the DARMA scenario for the VHE gamma-ray spectra of blazars by contemplating all such blazars at once, which allows us to correlate the emitted Γem and observed Γobs spectral indexes. We show that the predicted observed spectral index ΓobsDARMA actually fits observations even starting from the same nominal value Γem≅2.4 for all VHE blazars. Moreover, ΓobsDARMA becomes independent of redshift for 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. Our result also offers an important observational test for models of dark energy wherein quintessence is coupled to the photon through an effective dimension-five operator.