Phenomenological aspects of axion-like particles in cosmology and astrophysics

Abstract

Cosmology and particle physics are closer today than ever before, with several searches un derway at the interface between cosmology, particle physics, and field theory. The mystery of dark matter (DM) is one of the greatest common unsolved problems between these fields. It is established now based on many astrophysical and cosmological observations that only a small fraction of the total matter content of the universe is made of baryonic matter, while the vast majority is constituted by dark matter. However, the nature of such a component is still unknown. One theoretically well-motivated approach to understanding the nature of dark matter would be through looking for light pseudo-scalar candidates for dark matter such as axions and axion-like particles (ALPs). Axions are hypothetical elementary particles resulting from the Peccei-Quinn (PQ) solution to the strong CP (charge-parity) problem in quantum chromodynamics (QCD). Furthermore, many theoretically well-motivated ex tensions to the standard model of particle physics (SMPP) predicted the existence of more pseudo-scalar particles similar to the QCD axion and called ALPs. Axions and ALPs are characterized by their coupling with two photons. While the coupling parameter for axions is related to the axion mass, there is no direct relation between the coupling parameter and the mass of ALPs. Nevertheless, it is expected that ALPs share the same phenomenology of axions. In the past years, axions and ALPs regained popularity and slowly became one of the most appealing candidates that possibly contribute to the dark matter density of the universe. In this thesis, we start by illustrating the current status of axions and ALPs as dark matter candidates. One exciting aspect of axions and ALPs is that they can interact with pho tons very weakly. Therefore, we focus on studying the phenomenology of axions and ALPs interactions with photons to constrain some of their properties. In this context, we consider a homogeneous cosmic ALP background (CAB) analogous to the cosmic microwave background (CMB) and motivated by many string theory models of the early universe. The coupling between the CAB ALPs traveling in cosmic magnetic fields and photons allows ALPs to oscillate into photons and vice versa. Using the M87 jet environment, we test the CAB model that is put forward to explain the soft X-ray excess iv in the Coma cluster due to CAB ALPs conversion into photons. Then we demonstrate the potential of the active galactic nuclei (AGNs) jet environment to probe low-mass ALP models and to potentially exclude the model proposed to explain the Coma cluster soft X-ray excess. Further, we adopt a scenario in which ALPs may form a Bose-Einstein condensate (BEC) and, through their gravitational attraction and self-interactions, they can thermalize to spa tially localized clumps. The coupling between ALPs and photons allows the spontaneous decay of ALPs into pairs of photons. For ALP condensates with very high occupation num bers, the stimulated decay of ALPs into photons is also possible, and thus the photon occupation number can receive Bose enhancement and grows exponentially. We study the evolution of the ALPs field due to their stimulated decays in the presence of an electro magnetic background, which exhibits an exponential increase in the photon occupation number by taking into account the role of the cosmic plasma in modifying the photon growth profile. In particular, we focus on quantifying the effect of the cosmic plasma on the stimulated decay of ALPs as this may have consequences on the detectability of the radio emissions produced from this process by the forthcoming radio telescopes such as the Square Kilometer Array (SKA) telescopes with the intention of detecting the cold dark matter (CDM) ALPs. Finally, finding evidence for the presence of axions or axion-like particles would point to new physics beyond the standard model (BSM). This should have implications in developing our understanding of the nature of dark matter and the physics of the early universe evolution.