The axion is a hypothetical particle that first appeared as an explanation to the CP problem of Quantum Chromodynamics, however, its potential as a dark matter particle was soon discovered. Nonetheless, decades after it was first postulated, there is no experimental evidence of the axion despite several searches for this minuscule particle. The difficulty in this process was already foreseen, given that the axion couples very weakly to the Standard Model, in addition to being very stable. But all is not lost, the silver lining in the so far fruitless search for this "invisible" boson is that several constraints on the axion’s mass, as well as its coupling constants have been set, opening the door to new challenges. This paper starts with an introduction to the axion and its origin, describing its role as a dark matter particle. Secondly, it highlights some of the constraints imposed by experiments. Lastly, two papers are reviewed in more detail, which describe two very different, but equally important experiments.
arXiv:1705.02290, arXiv:2010.00169
