Low-scale closed strings and their duals

We study large dimensions and low string scale in four-dimensional compacti fications of type II theories of closed strings at weak coupling. We find t hat the fundamental string scale, together with all compact dimensions, can be at the TeV, while the smallness of the string coupling accounts for the weakness of gravitational interactions. This is in contrast to the situati on recently studied in type I theories, where the string scale can be lower ed only at the expense of introducing large transverse dimensions felt by g ravity only. As a result, in these type II strings, there are no strong gra vity effects at the TeV, and the main experimental signature is the product ion of Kaluza-Klein excitations with gauge interactions. In the context of type IIB theories, we find a new possibility providing a first instance of large non-transverse dimensions at weak coupling: two of the internal dimen sions seen by gauge interactions can be at the TeV, with the string scale a nd all other dimensions at intermediate energies of the order of 10(11) GeV , where gravity becomes also strong. Finally, using duality, we provide a p erturbative description for the generic case of large dimensions in the het erotic string. In particular, we show that the two type II theories above d escribe the cases of one and two heterotic large dimensions. A new M-theory derivation of heterotic-type II duality is instrumental for this discussio n. (C) 1999 Elsevier Science B.V. All rights reserved.