In models with extra dimensions, the usual (3 + 1)-dimensional space-time xµ ≡ (x0, x1, x2, x3) is extended to include additional spatial dimensions parameterized by coordinates x4, x5, …, x3+N. Here N is the number of extra dimensions. String theory arguments would suggest that in principle N can be as large as 6 or 7. In this chapter, we are interested in extra-dimensional (ED) models where all particles of the Standard Model (SM) are allowed to propagate in the bulk, i.e. along any of the x3+i (i = 1, …, N) directions [100]. In order to avoid a blatant contradiction with the observed reality, the extra dimensions in such models must be extremely small: smaller than the smallest scale which has been currently resolved by experiment. Therefore, the extra dimensions are assumed to be suitably compactified on some manifold of sufficiently small size (see Fig. 15.1).

Depending on the type of metric in the bulk, the ED models fall into one of the following two categories: flat, also known as ‘universal’ extra dimensions (UED) models, discussed in Section 15.1, or warped ED models, discussed in Section 15.2. As it turns out, the collider signals of the ED models are strikingly similar to the signatures of supersymmetry (SUSY) discussed in Chapter 13. Section 15.3 outlines some general methods for distinguishing an ED model from SUSY at high-energy colliders.