A three three-brane universe: new phenomenology for the new millennium?

We consider an extension of the Randall-Sundrum model with three parallel 3 -branes in a 5-dimensional spacetime. This new construction, apart from pro viding a solution to the Planck hierarchy problem, has the advantage that t he SM fields are confined on a positive tension brane. The study of the phe nomenology of this model reveals an anomalous first KK state which is gener ally much lighter than the remaining tower and also much more strongly coup led to matter. Bounds on the parameter space of the model can be placed by comparison of specific processes with the SM background as well as by the l atest Cavendish experiments. The model suggests a further exotic possibilit y if one drops the requirement of solving the hierarchy problem. In this ca se gravity may result from the exchange of the ordinary graviton plus an ul tralight KK state and modifications of gravity may occur at both small and extremely large scales. (C) 2000 Elsevier Science B.V. All rights reserved.