Five-brane Instantons vs Flux-induced Gauging of Isometries

In five-dimensional heterotic M-theory there is necessarily nonzero background flux, which leads to gauging of an isometry of the universal hypermultiplet moduli space. This isometry, however, is poised to be broken by M5-brane instanton effects. We show that, similarly to string theory, the background flux allows only brane instantons that preserve the above isometry. The zero-mode counting for the M5 instantons is related to the number of solutions of the Dirac equation on their worldvolume. We investigate that equation in the presence of generic background flux and also, in a particular case, with nonzero worldvolume flux.Comment: 27 pages; reference adde

Supergravity description of spacetime instantons

We present and discuss BPS instanton solutions that appear in type II string theory compactifications on Calabi-Yau threefolds. From an effective action point of view these arise as finite action solutions of the Euclidean equations of motion in four-dimensional N=2 supergravity coupled to tensor multiplets. As a solution generating technique we make use of the c-map, which produces instanton solutions from either Euclidean black holes or from Taub-NUT like geometries.Comment: 35 pages, some clarifications adde

D=4, N=2 Gauged Supergravity in the Presence of Tensor Multiplets

Using superspace techniques we construct the general theory describing D=4, N=2 supergravity coupled to an arbitrary number of vector and scalar--tensor multiplets. The scalar manifold of the theory is the direct product of a special Kaehler and a reduction of a Quaternionic-Kaehler manifold. We perform the electric gauging of a subgroup of the isometries of such manifold as well as ``magnetic'' deformations of the theory discussing the consistency conditions arising in this process. The resulting scalar potential is the sum of a symplectic invariant part (which in some instances can be recast into the standard form of the gauged N =2 theory) and of a non--invariant part, both giving new deformations. We also show the relation of such theories to flux ompactifications of type II string theories.Comment: LaTeX, 26 pages. v2 Improved discussion in sect. 4 and references adde

An index for the Dirac operator on D3 branes with background fluxes

We study the problem of instanton generated superpotentials in Calabi-Yau orientifold compactifications directly in type IIB string theory. To this end, we derive the Dirac equation on a Euclidean D3 brane in the presence of background fluxes. We propose an index which governs whether the generation of a superpotential in the effective 4d theory by D3 brane instantons is possible. Applying the formalism to various classes of examples, including the K3 x T^2/Z_2 orientifold, in the absence and presence of fluxes, we show that our results are consistent with conclusions attainable via duality from an M-theory analysis.Comment: Fermion coupling to five-form restored, conclusions of the paper unchange

A preliminary analysis of in-depth accident data for powered two-wheelers and bicycles in Europe

Despite progress from scientific and technological advancements, road safety remains a major issue worldwide. Road accident impacts such as fatalities, injuries and property damage consist considerable costs borne not only by involved people but society as well. This study aims to present preliminary findings of in-depth accident analysis for two-wheelers (bicycles and powered two wheelers – PTWs) across six countries in Europe. Data regarding the conditions underlying accident occurrence are presented, including time and date, weather, vehicle and road conditions and rider-related parameters such as age, intoxication and use of protective equipment. In addition, a Two Step Cluster Analysis is implemented in order to explore any possible classification of the analysed cases. It appears that two clusters are formed: the first includes more favourable conditions (“no wind, no drugs, good lighting”) while the second consists of less favourable conditions for road safety (“windy, lighting, unknown DUI condition”). This hints at a meaningful separation of the examination of two-wheeler accidents when the influence of outside factors is considerable. The inclusion of different but representative areas across Europe offers robustness and transferability to the data and respective results

De Sitter vacua from N=2 gauged supergravity

Typical de Sitter (dS) vacua of gauged supergravity correspond to saddle points of the potential and often the unstable mode runs into a singularity. We explore the possibility to obtain dS points where the unstable mode goes on both sides into a supersymmetric smooth vacuum. Within N=2 gauged supergravity coupled to the universal hypermultiplet, we have found a potential which has two supersymmetric minima (one of them can be flat) and these are connected by a de Sitter saddle point. In order to obtain this potential by an Abelian gauging, it was important to include the recently proposed quantum corrections to the universal hypermultiplet sector. Our results apply to four as well as five dimensional gauged supergravity theories.Comment: 25 pages, 1 figure, add refs and corrected typo

Metastable de Sitter vacua in N=2 to N=1 truncated supergravity

We study the possibility of achieving metastable de Sitter vacua in general N=2 to N=1 truncated supergravities without vector multiplets, and compare with the situations arising in N=2 theories with only hypermultiplets and N=1 theories with only chiral multiplets. In N=2 theories based on a quaternionic manifold and a graviphoton gauging, de Sitter vacua are necessarily unstable, as a result of the peculiar properties of the geometry. In N=1 theories based on a Kahler manifold and a superpotential, de Sitter vacua can instead be metastable provided the geometry satisfies some constraint and the superpotential can be freely adjusted. In N=2 to N=1 truncations, the crucial requirement is then that the tachyon of the mother theory be projected out from the daughter theory, so that the original unstable vacuum is projected to a metastable vacuum. We study the circumstances under which this may happen and derive general constraints for metastability on the geometry and the gauging. We then study in full detail the simplest case of quaternionic manifolds of dimension four with at least one isometry, for which there exists a general parametrization, and study two types of truncations defining Kahler submanifolds of dimension two. As an application, we finally discuss the case of the universal hypermultiplet of N=2 superstrings and its truncations to the dilaton chiral multiplet of N=1 superstrings. We argue that de Sitter vacua in such theories are necessarily unstable in weakly coupled situations, while they can in principle be metastable in strongly coupled regimes.Comment: 40 pages, no figure

D-instantons and twistors: some exact results

We present some results on instanton corrections to the hypermultiplet moduli space in Calabi-Yau compactifications of Type II string theories. Previously, using twistor methods, only a class of D-instantons (D2-instantons wrapping A-cycles) was incorporated exactly and the rest was treated only linearly. We go beyond the linear approximation and give a set of holomorphic functions which, through a known procedure, capture the effect of D-instantons at all orders. Moreover, we show that for a sector where all instanton charges have vanishing symplectic invariant scalar product, the hypermultiplet metric can be computed explicitly.Comment: 32 pages, 3 figures, uses JHEP3.cls; some changes in section 3.3.3; corrected formula for the contact potentia