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We study freely acting orbifolds of type IIB string theory on T5 that spontaneously break supersymmetry from N = 8 to N = 6, 4, 2 or 0 in five dimensions. We focus on orbifolds that are a ℤp quotient by a T-duality acting on T4 and a shift on the remaining S1. Modular invariant partition functions are constructed and detailed examples of both symmetric and asymmetric orbifolds are presented, including new examples of five-dimensional non-supersymmetric string theories with no tachyons. The orbifolds we consider arise at special points in the moduli space of string theory compactifications with a duality twist. The supergravity limit of these are Scherk-Schwarz reductions which generate gauged supergravities with positive definite classical potentials on the moduli space in five dimensions. Both symmetric and asymmetric freely acting orbifolds give a landscape of Minkowski vacua. For gauged supergravities to belong to this landscape, we find a number of constraints and conditions. Firstly, the scalar potential should lead to a massive spectrum with masses that obey quantization conditions arising from a string theory orbifold, which we discuss in detail. Secondly, we find constraints on the massless sector, e.g. in the examples of orbifolds preserving sixteen supersymmetries in five dimensions that we consider, only an odd number of vector multiplets arises. Lastly, we present new examples of candidate asymmetric orbifolds with modular invariant partition functions, but with non-integral coefficients in the q q ¯ -expansion in the twisted sectors. [ABSTRACT FROM AUTHOR]

We study freely acting orbifolds of type IIB string theory on T5 that spontaneously break supersymmetry from N = 8 to N = 6, 4, 2 or 0 in five dimensions. We focus on orbifolds that are a ℤp quotient by a T-duality acting on T4 and a shift on the remaining S1. Modular invariant partition functions are constructed and detailed examples of both symmetric and asymmetric orbifolds are presented, including new examples of five-dimensional non-supersymmetric string theories with no tachyons. The orbifolds we consider arise at special points in the moduli space of string theory compactifications with a duality twist. The supergravity limit of these are Scherk-Schwarz reductions which generate gauged supergravities with positive definite classical potentials on the moduli space in five dimensions. Both symmetric and asymmetric freely acting orbifolds give a landscape of Minkowski vacua. For gauged supergravities to belong to this landscape, we find a number of constraints and conditions. Firstly, the scalar potential should lead to a massive spectrum with masses that obey quantization conditions arising from a string theory orbifold, which we discuss in detail. Secondly, we find constraints on the massless sector, e.g. in the examples of orbifolds preserving sixteen supersymmetries in five dimensions that we consider, only an odd number of vector multiplets arises. Lastly, we present new examples of candidate asymmetric orbifolds with modular invariant partition functions, but with non-integral coefficients in the q q ¯ -expansion in the twisted sectors. [ABSTRACT FROM AUTHOR]

The (4,0) supermultiplet in six dimensions contains a fourth-rank tensor gauge field with the symmetries of the Riemann tensor and is superconformal, with 32+32 supersymmetries. Dimensional reduction on a circle gives the five dimensions N=8 supergravity multiplet, with the fourth-rank tensor reducing to the graviton. If there is an interacting (4,0) theory, it should reduce to the full N=8 supergravity theory and so would give a conformal theory of gravity that would reduce to conventional gravity with the usual two-derivative action at low energies. This paper revisits the conjecture that a non-lagrangian interacting (4,0) superconformal theory arises from a strong coupling limit of five-dimensional supergravity (suitably embedded in M -theory) describing M -theory at energies beyond the Planck scale. A key test for this is identified: M -theory toroidally compactified to five dimensions should have certain BPS states carrying a singlet central charge. These 1/2 BPS states are not related to any of the standard BPS states by dualities and do not correspond to non-singular soliton solutions—they appear to correspond to singular solutions related to gravitational instantons. Such states are needed to provide the Kaluza–Klein modes for the compactified six-dimensional theory. If there are no such states, then the conjecture is false, while the presence of such states would be strong indication that the conjecture could be true. [ABSTRACT FROM AUTHOR]