Stellar cooling constraints on new light particles and how to evade them

Our star, the Sun, is an exceptional probe of fundamental physics. QCD axions and axion-like particles (often referred to as axions) can be produced in its core and can leave without further interactions, resulting in an energy-loss channel. Since this can imply a dramatic change in solar evolution, stringent constraints can be obtained on the existence of novel particles. Similar bounds can also be obtained from similar astrophysical observables. However, some recent studies show that there are mechanisms that can suppress axion production in stellar plasmas, e.g., if the axion mass depends on density, thereby suggesting that for considering axion production in low-density environments, such as those on Earth, astrophysical constraints may no longer apply due to the much higher densities in stellar interiors. The main goal of the thesis is to reassess the robustness of this class of bounds with a focus on the Sun. The thesis focuses on axion production via Compton scattering and electron-nucleus Bremsstrahlung, the two dominant processes in stellar environments. The results show that even when considering the production of axion at Earth-like densities (around 10 g/cm^3), which are also found in a significant part of the Sun’s interior, the constraints derived from the Sun remain much stronger than those currently obtained from laboratory experiments. This highlights the exceptional sensitivity of stellar systems to weakly interacting particles and reinforces the importance of astrophysical observations in probing new physics.