Brane-antibrane kinetic mixing, millicharged particles and SUSY breaking

It is known that hidden U(1) gauge factors can couple to visible U(1)s through kinetic mixing. This phenomenon is shown to occur in non-supersymmetric string set-ups, between D-branes and (D) over bar -branes. Kinetic mixing, if present, acts either to give millicharges (of, e.g., hypercharge) to would-be hidden sector fermions, or to generate an enhanced communication of supersymmetry breaking that dominates over the usual gravitational suppression. In either case, the conclusion is that the string scale in non-supersymmetric brane configurations has a generic upper bound of M-s less than or similar to 10(8) GeV. (C) 2004 Elsevier B.V. All rights reserved.