One-loop phenomenology in brane models
Particular examples of one-loop string effects are explored in the context of brane-based realizations of the Standard Model. We firstly examine the consequences of a phenomenon known as Kinetic Mixing, which couples hidden U(1) gauge factors to visible U(l)'s. The effect is shown to occur in nonsupersymmetric string set-ups between D-branes and D-branes, where it acts either to give millicharges (of e.g. hyper charge) 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 these nonsupersymmetric brane configurations has a generic upper bound of M(_s)≤ 10(^8) GeV. Turning to models based on intersecting branes, Yukawa interactions at one-loop on intersecting D6 branes are calculated. The non-renormalization theorem is demonstrated in supersymmetric configurations, and it is shown how Yukawa ß-functions may be extracted. In addition to the usual logarithmic running, power-law dependence on the infra-red cut-off (associated with Kaluza-Klein modes) is found. The results presented may be used to evaluate coupling renormalization in nonsupersymmetric cases. Much of the discussion is applicable to one-loop calculations on intersecting branes in general.