IF Colloquium No.27 – Introduction to Axion-Like Particles

ALPs are light pseudo-scalar particles arising from many beyond standard model theories of particle physics including String/M theory. The discussion on how the strong CP problem became the first motivation for the QCD axion is our starting point. A few common realizations of the QCD axion will also be discussed. The existence of ALPs from String/M theory is tightly related to the compactification process of the extradimension such as Calabi-Yau manifolds. Combining with the instanton effects giving the mass, ALPs from String/M theory generically comes in abundance and their masses could span many orders of magnitude. This gives rise to the popular phrase “String Axiverse” which provides a vast playing ground for particle physics phenomenologists. A variety of probes and experiments, ranging from cosmology, astrophysics and collider physics, will finally be discussed.