ANOMALY CANCELLATION IN 6 DIMENSIONS

It is shown that anomaly cancellation conditions are sufficient to determine the two most important topological numbers relevant for Calabi-Yau (CY) compactification to six dimensions. This reflects the fact that K3 is the only nontrivial CY manifold in two complex dimensions. The Green-Schwarz counterterms are explicitly constructed and sum rules for charges of additional enhanced U (1) factors are derived and the results with all possible Abelian orbifold constructions of K3 are compared. This includes asymmetric orbifolds as well, showing that it is possible to regain a geometrical interpretation for this class of models. Finally, some models with a broken E7 gauge group which will be useful for more phenomenological applications are discussed herein.