Carlo Angelantonj

Type-I strings on magnetised orbifolds and brane transmutation

Abstract In the presence of internal magnetic fields, a D9 brane can acquire a D5 (or anti-D5) R-R charge, and can therefore contribute to the corresponding tadpole. In the resulting vacua, supersymmetry is generically broken and tachyonic instabilities are present. However, suitable choices for the magnetic fields, corresponding to self-dual configurations in the internal space, can yield new chiral supersymmetric vacua with gauge groups of reduced rank, where the magnetic energy saturates, partly or fully, the negative tension of the O5+ planes. These models contain Green–Schwarz couplings to untwisted R-R forms not present in conventional orientifolds.

Chiral asymmetry in four-dimensional open-string vacua

Abstract Starting from the type IIB string on the Z orbifold, we construct some chiral open-string vacua with N = 1 supersymmetry in four dimensions. The Chan-Paton group depends on the (quantized) NS-NS antisymmetric tensor. The largest choice, SO (8) × SU (12) × U (1), has an anomalous U (1) factor whose gauge boson acquires a mass of the order of the string scale. The corresponding open-string spectrum comprises only Neumann strings and includes three families of chiral multiplets in the ( 8, 12 ∗ ) + ( 1, 66 ) representation. A comparison is drawn with a heterotic vacuum with non-standard embedding, and some properties of the low-energy effective field theory are discussed.

Type I vacua with brane supersymmetry breaking

Abstract We show how chiral type I models whose tadpole conditions have no supersymmetric solution can be consistently defined introducing antibranes with non-supersymmetric world volumes. At tree level, the resulting stable non-BPS configurations correspond to tachyon-free spectra, where supersymmetry is broken at the string scale on some (anti)branes but is exact in the bulk, and can be further deformed by the addition of brane–antibrane pairs of the same type. As a result, a scalar potential is generated, that can stabilize some radii of the compact space. This setting has the novel virtue of linking supersymmetry breaking to the consistency requirements of an underlying fundamental theory.

Comments on open-string orbifolds with a non-vanishing Bab

Abstract We study the effect of a non-vanishing flux for the NS–NS antisymmetric tensor in open-string orbifolds. As in toroidal models, the total dimension of the Chan–Paton gauge group is reduced proportionally to the rank of Bab, both on D9- and on D5-branes, while the Mobius amplitude involves some signs that, in the Z 2 orbifold, allow one to connect continuously U(n) groups to Sp(n)⊗Sp(n) groups on each set of D-branes. In this case, non-universal couplings between twisted scalars and gauge vectors arise, as demanded by the generalised Green–Schwarz mechanism. We also comment on the role of the NS–NS antisymmetric tensor in a recently proposed type I scenario, where supersymmetry is broken on the D-branes, while it is preserved in the bulk.

Comments on Gepner models and type I vacua in string theory

We construct open descendants of Gepner models, concentrating mainly on the six-dimensional case, where they give type I vacua with rich patterns of Chan-Paton symmetry breaking and various numbers of tensor multiplets, including zero. We also relate the models in D < 10 without open sectors, recently found by other authors, to the generalized Klein-bottle projections allowed by the crosscap constraint.

K3×T2/Z2 orientifolds with fluxes, open string moduli and critical points

Abstract We extend the four-dimensional gauged supergravity analysis of type IIB vacua on K3×T 2 / Z 2 to the case where also D3 and D7 moduli, belonging to N =2 vector multiplets, are turned on. In this case, the overall special geometry does not correspond to a symmetric space, unless D3 or D7 moduli are switched off. In the presence of non-vanishing fluxes, we discuss supersymmetric critical points which correspond to Minkowski vacua, finding agreement with previous analysis. Finally, we point out that care is needed in the choice of the symplectic holomorphic sections of special geometry which enter the computation of the scalar potential.

New D = 4 gauged supergravities from N=4 orientifolds with fluxes

We consider classes of T 6 -orientifolds, where the orientifold projection contains an inversion I 9?p on 9?p coordinates, transverse to a Dp-brane. In absence of fluxes, the massless sector of these models corresponds to diverse forms of = 4 supergravity, with six bulk vector multiplets coupled to = 4 Yang-Mills theory on the branes. They all differ in the choice of the duality symmetry corresponding to different embeddings of SU(1,1) ? SO(6,6+n) in Sp(24+2n,), the latter being the full group of duality rotations. Hence, these lagrangians are not related by local field redefinitions. When fluxes are turned on one can construct new gaugings of = 4 supergravity, where the twelve bulk vectors gauge some nilpotent algebra which, in turn, depends on the choice of fluxes.

Non-tachyonic open descendants of the 0B string theory

Abstract We use the crosscap constraint to construct open descendants of the 0B string compactified on T 6 / Z 3 and on T 4 / Z 2 free of tachyons both in the closed and in the open unoriented sectors. In four dimensions the construction results in a Chan-Paton gauge group U(8)⊗U(12)⊗U(12) with three generations of chiral fermions in the representations ( 8 ,1, 12 )⊕(8,12,1)⊕(1, 66 ,1)⊕(1,1,66) .

Discrete deformations in type I vacua

Abstract We study supersymmetric orientifolds where the world-sheet parity transformation is combined with a conjugation of some compact complex coordinates. We investigate their T-duality relation to standard orientifolds and discuss the origin of continuous and discrete moduli. In contrast to standard orientifolds, the antisymmetric tensor describes a continuous deformation, while the off-diagonal part of the metric is frozen to quantized values and is responsible for the rank reduction of the gauge group. We also give a geometrical interpretation of some recently constructed six-dimensional permutational orientifolds.

Non-supersymmetric type I strings with zero vacuum energy

Abstract We study open descendants of non-supersymmetric type IIB asymmetric (freely acting) obifolds with zero cosmological constant. A generic feature of these models is that supersymmetry remains unbroken on the brane at all mass levels, while it is broken in the bulk in a way that preserves Fermi-Bose degeneracy in both the massless and massive (closed string) spectrum. This property remains valid in the heterotic dual of the type 11 model but only for the massless excitations. A possible application of these constructions concerns scenarios of low-energy supersymmetry breaking with large dimensions.