Flux compactifications of string theory on twisted tori

Global aspects of Scherk-Schwarz dimensional reduction are discussed and it is shown that it can usually be viewed as arising from a compactification on the compact space obtained by identifying a (possibly non-compact) group manifold G under a discrete subgroup Gamma, followed by a truncation. This allows a generalisation of Scherk-Schwarz reductions to string theory or M-theory as compactifications on G/Gamma, but only in those cases in which there is a suitable discrete subgroup of G. We analyse such compactifications with flux and investigate the gauge symmetry and its spontaneous breaking. We discuss the covariance under O(d, d), where d is the dimension of the group G, and the relation to reductions with duality twists. The compactified theories promote a subgroup of the O(d, d) that would arise from a toroidal reduction to a gauge symmetry, and we discuss the interplay between the gauge symmetry and the O(d, d, Z) T-duality group, suggesting the role that T-duality should play in such compactifications. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim