After a brief introduction to axions in string theory, discussing their quality and the origin of their couplings, I will review recent progress in understanding their properties in different string theories from type IIB to heterotic string theories. I will show how the discovery of a light ALP — that is, above the QCD line — is incompatible with unification and heterotic string theory....
In the post-inflationary axion scenario, axions are produced by axion strings. A crucial parameter for the prediction of the axion dark matter mass is the length per unit volume of axion strings at the time of the QCD transition. This can be established only by numerical simulations, which are limited in volume and time, and must be extrapolated to physical values. After a brief history of...
