Axions across boundaries between Particle Physics, Astrophysics, Cosmology and forefront Detection Technologies

The purpose of this workshop is to bring together scientists with different backgrounds and expertise to discuss open problems, recent developments and future directions in axion physics, a field that is notoriously replete with interdisciplinary connections. The aim is to foster a fruitful cross breeding between different theoretical areas, with a focus on certain open issues in axion particle physics, astrophysics and cosmology. Quantitative assessments of the axion contribution to Cold Dark Matter (CDM) involve top-notch lattice simulations of non- perturbative QCD effects, as well as of the cosmic evolution of axionic topological defects. Astrophysical observations provide strong bounds on axion properties because stellar evolution would be affected by the existence of axions and, intriguingly, some excesses in star energy losses have been reported. Cosmological scenarios in which the PQ symmetry is broken before inflation foresee axions imprints in the CMB, while in post-inflationary scenarios axion miniclusters, with overdensities several orders of magnitude larger than the local density of CDM, are expected to form, and a reliable assessment of their properties is of utmost importance. From the experimental side, a blossoming of potentially game-changing ideas, with an exciting crossover from experimental particle physics to materials science and cutting-edge technologies is inspiring new methods for axion searches. Novel techniques have been put forth that, besides exploiting the axion- photon coupling, aim to reveal axions via their couplings to nucleons and electrons. The interaction between the experimental and theoretical communities will foster the merging of `how to search’ with `where to search’ into optimized strategies to hunt for the axion.