Importance of axion-like particles for very-high-energy astrophysics

TL;DR

This paper explores how axion-like particles (ALPs) could explain increased transparency of the universe to very-high-energy photons from blazars, suggesting potential for their detection with current and future gamma-ray telescopes.

Contribution

It provides a systematic analysis of ALPs' role in enhancing photon survival probabilities at very-high energies, linking particle physics with astrophysical observations.

Findings

Photon-ALP oscillations increase photon survival at high energies.

Current telescopes can test ALP effects in astrophysical data.

ALPs could be discovered with laboratory experiments like GammeV and ALPS.

Abstract

Several extensions of the Standard Model predict the existence of Axion-Like Particles (ALPs), very light spin-zero bosons with a two-photon coupling. ALPs can give rise to observable effects in very-high-energy astrophysics. Above roughly 100 GeV the horizon of the observable Universe progressively shrinks as the energy increases, due to scattering of beam photons off background photons in the optical and infrared bands, which produces e+e- pairs. In the presence of large-scale magnetic fields photons emitted by a blazar can oscillate into ALPs on the way to us and back into photons before reaching the Earth. Since ALPs do not interact with background photons, the effective mean free path of beam photons increases, enhancing the photon survival probability. While the absorption probability increases with energy, photon-ALP oscillations are energy-independent, and so the survival probability increases with energy compared to standard expectations. We have performed a systematic analysis of this effect, interpreting the present data on very-high-energy photons from blazars. Our predictions can be tested with presently operating Cherenkov Telescopes like H.E.S.S., MAGIC, VERITAS and CANGAROO III as well as with detectors like ARGO-YBJ and MILAGRO and with the planned Cherenkov Telescope Array and the HAWC-ray observatory. ALPs with the right properties to produce the above effects can possibly be discovered by the GammeV experiment at FERMILAB and surely by the planned photon regeneration experiment ALPS at DESY.