From Axions to Other WISPs

Abstract

It is a tantalizing question whether there is new physics below the Standard Model. That is to ask, whether there are new very light particles – apart from the known ones with sub-eV mass, the photon and the neutrinos – which are very weakly coupled to the Standard Model. In fact, embeddings of the latter into more unified theories, in particular into string theory, suggest their possible existence in a so-called hidden sector of the theory. Prominent examples of inhabitants of the latter are the axion and axion-like particles, arising as pseudo NambuGoldstone bosons associated with the breakdown of global anomalous U(1) symmetries. They occur generically in realistic string compactifications, as we will review below. Extra, hidden U(1) gauge bosons are also frequently encountered in string embeddings of the Standard Model, as we will summarize below. There is no reason why some of these hidden U(1) gauge bosons can not be massless or very light, in which case they also belong to the class of very weakly interacting sub-eV particles (WISPs). Further candidates for WISPs are very light hidden sector particles which are charged under the hidden U(1)s. In this contribution, we will take a top-down point of view: we will illustrate how axions and other WISPs arise in the course of compactification of the extra dimensions of string theory. For the bottom-up point of view, i.e. for arguments and phenomenological as well as cosmological hints which point to the possible existence of WISPs, see the contributions of Joerg Jaeckel and Javier Redondo in these proceedings. Axions from string compactifications.– The low-energy effective actions describing the dynamics of the massless bosonic excitations of the heterotic and type II string theories in 9+1 dimensions are summarized in Table 1. As we will see, after compactification of six of the spatial dimensions, pseudo-scalar fields a will generically arise which have a coupling a trG∧G