"Small instantons and the axion mass" by Alex Pomarol (IFAE)

I will discuss the sensitivity of the axion mass on UV modifications of QCD. In particular, I will consider contributions from small instantons to the axion mass in the case in which gluons propagate in some extra dimension at high energies. In the case of a d=5 AdS space, this is expected to be similar to QCD interacting with an additional  (strongly-coupled) colored sector. Contrary to expectations, I will show that small instantons are suppressed, and only UV localized instanton-anti-instanton contributions can be sizeable. For flat extra dimensions, I will show that small instantons can be enhanced by power-law corrections due to the presence of the Kaluza-Klein states.