AxionCount · Theory, Phenomenology, and Cosmology of the Multiple Axion Scenario

AxionCount aims to explore the theoretical and phenomenological implications of the existence of multiple axions in nature. Axions are hypothetical particles that could resolve two major enigmas in fundamental physics: the origin of dark matter and the absence of CP violation in the strong sector. Given their potential significance, extensive theoretical efforts and strong experimental programs have been launched to investigate their existence. However, several well-motivated theories suggest that the axion may not exist in isolation but alongside other scalar companions. Despite this, much current research focuses on the single axion paradigm. AxionCount's primary goal is to explore how the axion experimental program could address the questions: How many axions exist? How would the presence of multiple species alter the axion quest?The project will develop new tools to study the implications of multiple axions. On the theoretical side, AxionCount will investigate axion oscillations, construct the Effective Field Theory for multiple axions, and explore models that predict the existence of several axions. On the phenomenological side, the project will assess how experimental constraints are modified by the presence of multiple axions. Special emphasis will be placed on how experiments such as CAST and IAXO at CERN, the host institution, can distinguish a multiple axion signal from that of a single axion. Finally, from a cosmological perspective, AxionCount will analyze the evolution of multiple axions in the early universe and calculate the axionic dark matter abundance.Overall, AxionCount aims to rethink the parameter space of multiple axions, expanding experimental opportunities and fostering synergies not only between theory and experiment, but also among different experimental efforts, as their findings may be interconnected within the multiple axion framework.