Invisible axions and large-radius compactifications - art. no. 105023

We study some of the novel effects that arise when the QCD axion is placed in the "bulk" of large extra spacetime dimensions. First. we find that the mass of the axion can become independent of the energy scale associated wit h the breaking of the Peccei-Quinn symmetry. This implies that the mass of the axion can be adjusted independently of its couplings to ordinary matter , a feature which is not possible in four dimensions and which may contribu te to axion invisibility. Second, we discuss the new phenomenon of laborato ry axion oscillations (analogous to neutrino oscillations), and show that t hese oscillations cause laboratory axions to "decohere" extremely rapidly a s a result of Kaluza-Klein mixing. This decoherence may also be a contribut ing factor to axion invisibility. Third, we discuss the role of Kaluza-Klei n axions in axion-mediated processes and decays, and propose several experi mental tests of the higher-dimensional nature of the axion. Finally, we sho w that under certain circumstances the presence of an infinite tower of Kal uza-Klein axion modes can significantly accelerate the dissipation of the e nergy associated with cosmological relic axion oscillations, thereby enabli ng the Peccei-Quinn symmetry-breaking scale to exceed the usual four-dimens ional relic oscillation bounds. Together, these ideas therefore provide new ways of obtaining an "invisible" axion within the context of higher-dimens ional theories with large-radius compactifications.