arXiv subjects

C. Angelantonj

Publications and source records attributed to C. Angelantonj.

16 recordsLinked to original sources

arXiv2024

Rigid Vacua with Brane Supersymmetry Breaking

We construct new string vacua featuring Brane Supersymmetry Breaking, based on $T^4/\mathbb{Z}_N$ orientifolds with $\text{O}5_+$ planes and $\overline{\text{D}5}$ branes. Differently from the original construction, in these vacua the cancellation of twisted R-R charges of D-branes and orientifold planes is achieved in a non-trivial way, and results in rigid configurations with few open-string moduli, which highly restrict the possible deformations of the model. The breaking of supersymmetry and, consequently, the non-vanishing untwisted and twisted NS-NS tadpoles, generates a rich potential both for the dilaton and the blown-up moduli. We also uncover the stringy origin of the $J$ form entering the gauge kinetic functions in the low-energy effective action, and display its relation to the NS-NS tadpoles for the scalars in the tensor multiplets. As a result, the $J$ form can be consistently identified also when supersymmetry is broken, thus solving an embarrassing puzzle related to its very existence. We also discuss the unitarity constraints for one-dimensional defects in these vacua, where the Ka$\check{\text{c}}$-Moody algebra for D9 and $\overline{\text{D}5}$ can be realised both in the left-moving and right-moving sectors.

hep-th↗

arXiv2010

Spinor-Vector Duality in Heterotic String Orbifolds

The three generation heterotic-string models in the free fermionic formulation are among the most realistic string vacua constructed to date, which motivated their detailed investigation. The classification of free fermion heterotic string vacua has revealed a duality under the exchange of spinor and vector representations of the SO(10) GUT symmetry over the space of models. We demonstrate the existence of the spinor-vector duality using orbifold techniques, and elaborate on the relation of these vacua to free fermionic models.

hep-th↗

arXiv2009

Stringy Instanton Effects in Models with Rigid Magnetised D-branes

We compute instantonic effects in globally consistent T^6/Z2xZ2 orientifold models with discrete torsion and magnetised D-branes. We consider fractional branes and instantons wrapping the same rigid cycles. We clarify and analyse in detail the low-energy effective action on D-branes in these models. We provide explicit examples where instantons induce linear terms in the charged fields, or non-perturbative mass terms are generated. We also find examples where the gauge theory on fractional branes has conformal symmetry at one-loop, broken by instantonic mass terms at a hierarchically small energy scale.

hep-th↗

arXiv2008

Resolution of Hagedorn singularity in superstrings with gravito-magnetic fluxes

We consider closed type II and orientifold backgrounds where supersymmetry is spontaneously broken by asymmetric geometrical fluxes. We show that these can be used to describe thermal ensembles with chemical potentials associated to "gravito-magnetic" fluxes. The thermal free energy is computed at the one-loop string level, and it is shown to be free of the usual Hagedorn-like instabilities for a certain choice of the chemical potentials. In the closed string gravitational sector, as well as in the open string matter sector of the proposed orientifold construction, the free energy turns out to have "Temperature duality" symmetry, ${\cal F}(T/T_H)={T^2\over T_H^2} {\cal F}(T_H/T)$, which requires interchanging the space-time spinor representations $S\leftrightarrow C$. For small temperatures, $T\to 0$, the anti-spinor $C$ decouples from the spectrum while for large temperatures, $T\to \infty$, the spinor $S$ decouples. In both limits the free energy vanishes, as we recover a conventional type II superstring theory. At the self dual point $T=T_H$, the thermal spectra of $S$ and $C$ are identical. Moreover, there are extra massless scalars in the adjoint representation of an SO(4) non-abelian gauge symmetry in the closed-string sector, and open-string massless states charged simultaneously under both the Chan-Paton and the closed-string SO(4) gauge group.

hep-th↗

arXiv2007

Metastable String Vacua

We argue that tachyon-free type I string vacua with supersymmetry breaking in the open sector at the string scale can be interpreted, via S and T-duality arguments, as metastable vacua of supersymmetric type I superstring. The dynamics of the process can be partially captured via nucleation of brane-antibrane pairs out of the non-supersymmetric vacuum and subsequent tachyon condensation.

hep-th↗

arXiv2006

An Alternative for Moduli Stabilisation

The one-loop vacuum energy is explicitly computed for a class of perturbative string vacua where supersymmetry is spontaneously broken by a T-duality invariant asymmetric Scherk-Schwarz deformation. The low-lying spectrum is tachyon-free for any value of the compactification radii and thus no Hagedorn-like phase-transition takes place. Indeed, the induced effective potential is free of divergence, and has a global anti de Sitter minimum where geometric moduli are naturally stabilised.

hep-th↗

arXiv2005

Scherk-Schwarz Breaking and Intersecting Branes

We study the effect of Scherk-Schwarz deformations on intersecting branes. Non-chiral fermions in any representation of the Chan-Paton gauge group generically acquire a tree-level mass dependent on the compactification radius and the brane wrapping numbers. This offers an elegant solution to one of the long-standing problems in intersecting-brane-world models where the ubiquitous presence of massless non-chiral fermions is a clear embarrassment for any attempt to describe the Standard Model of Particle Physics.

hep-th↗

arXiv2004

K3 x T^2/Z_2 orientifolds with fluxes, open string moduli and critical points

We extend the four-dimensional gauged supergravity analysis of type IIB vacua on $K3\times T^2/Z_2$ 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.

hep-th↗

arXiv2003

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 N=4 supergravity, with six bulk vector multiplets coupled to N=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)\times SO(6,6+n) in Sp(24+2n,R), 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 N=4 supergravity, where the twelve bulk vectors gauge some nilpotent algebra which, in turn, depends on the choice of fluxes.

hep-th↗

arXiv2002

Rotating D-branes and O-planes

We review orientifold constructions in the presence of magnetic backgrounds both in the open and closed sectors. Generically, the resulting orientifold models have a nice geometric description in terms of rotated D-branes and/or O-planes. In the case of multiple magnetic backgrounds, some amount of supersymmetry is recovered if the magnetic fields are suitably chosen and part of the original D-branes and/or O-planes are transmuted into new ones.

hep-th↗

arXiv2002

Orientifolds of String Theory Melvin backgrounds

We study the dynamics of type I strings on Melvin backgrounds, with a single or multiple twisted two-planes. We construct two inequivalent types of orientifold models that correspond to (non-compact) irrational versions of Scherk-Schwarz type breaking of supersymmetry. In the first class of vacua, D-branes and O-planes are no longer localized in space-time but are smeared along the compact Melvin coordinate with a characteristic profile. On the other hand, the second class of orientifolds involves O-planes and D-branes that are both rotated by an angle proportional to the twist. In case of ``multiple Melvin spaces'', some amount of supersymmetry is recovered if the planes are twisted appropriately and part of the original O-planes are transmuted into new ones. The corresponding boundary and crosscap states are determined.

hep-th↗

arXiv2000

Type-I vacua and brane transmutation

In a classic paper, Fradkin and Tseytlin showed how magnetic deformations can be introduced in open strings. In this contribution we review some recent work on type-I vacua with magnetised branes and describe the role of additional discrete deformations, related to quantised values of the NS-NS antisymmetric tensor B_ab.

hep-th↗

arXiv2000

Type-I strings on magnetised orbifolds and brane transmutation

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.

hep-th↗

arXiv1999

Type I vacua with brane supersymmetry breaking

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.

hep-th↗

arXiv1999

Non-Supersymmetric Type I Strings with Zero Vacuum Energy

We study open descendants of non-supersymmetric type IIB asymmetric (freely acting) orbifolds 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 II model but only for the massless excitations. A possible application of these constructions concerns scenarios of low-energy supersymmetry breaking with large dimensions.

hep-th↗

arXiv1996

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.

hep-th↗