On heterotic orbifolds,M-theory and type I' brane engineering

Horava--Witten M theory -- heterotic string duality poses special problemsfor the twisted sectors of heterotic orbifolds. In [1] we explained how in Mtheory the twisted states couple to gauge fields apparently living on M9 branesat both ends of the eleventh dimension at the same time. The resolutioninvolves 7D gauge fields which live on fixed planes of the (T^4/Z_N) x(S^1/Z_2) x R^{5,1} orbifold and lock onto the 10D gauge fields along theintersection planes. The physics of such intersection planes does not followdirectly from the M theory but there are stringent kinematic constraints due toduality and local consistency, which allowed us to deduce the local fields andthe boundary conditions at each intersection. In this paper we explain variousphenomena at the intersection planes in terms of duality between HW and type I'superstring theories. The orbifold fixed planes are dual to stacks of D6branes, the M9 planes are dual to O8 orientifold planes accompanied by D8branes, and the intersections are dual to brane junctions. We engineer severaljunction types which lead to distinct patterns of 7D/10D gauge field locking,7D symmetry breaking and/or local 6D fields. Another aspect of braneengineering is putting the junctions together; sometimes, the combined effectis rather spectacular from the HW point of view and the quantum numbers of sometwisted states have to `bounce' off both ends of the eleventh dimension beforetheir heterotic identity becomes clear. Some models involve D6/O8 junctionswhere the string coupling diverges towards the orientifold plane. We use theheterotic-HW-I' duality to predict what should happen at such junctions.Comment: 118 pages, uses phyzzx, color printer advice