Casimir energy and radius stabilization in five and six dimensional orbifolds

We compute the one-loop Casimir energy of gravity and matter fields, obeyin g various boundary conditions, in 5-dimensional S-1/Z(2) and 6-dimensional T-2/Z(k) orbifolds. We discuss the role of the Casimir energy in possible r adius stabilization mechanisms and show that the presence of massive as wel l as massless fields can lead to minima with zero cosmological constant. In the 5-d orbifold, we also consider the case where kinetic terms localized at the fixed points are not small. We take into account their contribution to the Casimir energy and show that localized kinetic terms can also provid e a mechanism for radius stabilization. We apply our results to a recently proposed 5-dimensional supersymmetric model of electroweak symmetry breakin g and show that the Casimir energy with the minimal matter content is repul sive. Stabilizing the radius with zero cosmological constant requires, in t his context, adding positive bulk cosmological constant and negative brane- tension counterterms.