Sensitivity of the Cherenkov Telescope Array to the detection of axion-like particles at high gamma-ray opacities

Extensions of the Standard Model of particles commonly predict the existence of axion(-like) particles (ALPs) that could be detected through their coupling to photons in external magnetic fields. This coupling could lead to modifications of $γ$-ray spectra from extragalactic sources. Above a certain energy, the $γ$-ray flux should be exponentially damped due to the interaction with photons of background radiations fields. ALPs, on the other hand, propagate unimpeded over cosmological distances and a reconversion into $γ$-rays could lead to an additional component in the spectra. Here, we present the sensitivity of the proposed Cherenkov Telescope Array (CTA) to detect this spectral hardening. Using the full instrumental response functions of CTA, a combined likelihood analysis of four $γ$-ray sources shows that a significant detection of the ALP signal is possible for couplings $g_{aγ} \gtrsim 2\times10^{-11}\,\mathrm{GeV}^{-1}$ and ALP masses $m_a \lesssim 100\,\mathrm{neV}$. We discuss the dependency of these values on different model assumptions and magnetic-field scenarios and identify the best observation strategy to search for an ALP induced boost of the $γ$-ray flux.

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