Importance of axionlike particles for veryhighenergy astrophysics
Abstract
Several extensions of the Standard Model predict the existence of AxionLike Particles (ALPs), very light spinzero bosons with a twophoton coupling. ALPs can give rise to observable effects in veryhighenergy astrophysics. Above roughly 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 pairs. In the presence of largescale 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, photonALP oscillations are energyindependent, 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 veryhighenergy 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 ARGOYBJ 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.

INFN, Sezione di Pavia, via A. Bassi 6, I – 27100 Pavia, Italy

Dipartimento di Fisica, Università di Udine, Via delle Scienze 208, I – 33100 Udine,
and INAF and INFN, Sezioni di Trieste, Italy

Dipartimento di Fisica, Università dell’Insubria, Via Valleggio 11, I – 22100 Como, Italy

Email: marco.roncadelli@pv.infn.it
1 Introduction
In spite of the great success scored by the Standard Model of particle physics, nowadays no one regards it as the ultimate theory. Indeed, a big effort has been devoted over the last few decades to extend it in order to have a truly unified theory of all interactions including gravity. So many specific attempts have been put forward and a remarkable thing is that several of them generically predict the existence of AxionLike Particles (ALPs), namely very light spinzero bosons with a twophoton coupling. They closely resemble the Axion, which is the PseudoGoldstone boson associated to the PecceiQuinn symmetry invented to naturally solve the strong CP problem. However, two important differences exist mainly because the Axion arises in a very specific context while in dealing with ALPs the aim is to bring out their properties in a modelindependent fashion as much as possible [alprev]. First, only ALPphoton interaction terms are taken into account, and so ALPs are described by the Lagrangian
(1) 
where is the ALP field. Second, the parameters and are to be regarded as unrelated for ALPs – while they are closely related for the Axion – and it is merely assumed that and .
Our aim is to summarize some very recent results that we have obtained concerning the implications of ALPs for veryhighenergy (VHE) blazar observations. We refer to our original paper for a very thorough presentation – which also includes cosmological effects that are discarded here for lack of space – and for a complete list of references [dgr].