Bounds on universal extra dimensions - art. no. 035002

We show that the bound from the electroweak data on the size of extra dimen sions accessible to all the standard model fields is rather loose. These "u niversal" extra dimensions could have a compactification scale as low as 30 0 GeV for one extra dimension. This is because the Kaluza-Klein number is c onserved and thus the contributions to the clectroweak observables arise on ly from loops. The main constraint comes from weak-isospin violation effect s. We also compute the contributions to the S parameter and the Zb (b) over bar vertex. The direct bound on the compactification scale is set by CDF a nd DO in the few hundred GeV range, and run II of the Tevatron will either discover extra dimensions or else it could significantly raise the bound on the compactification scale. In the case of two universal extra dimensions, the current lower bound on the compactification scale depends logarithmica lly on the ultraviolet cutoff of the higher dimensional theory, but can be estimated to lie between 400 and 800 GeV. With three or more extra dimensio ns, the cutoff dependence may be too strong to allow an estimate.