arXiv subjects

Stefan Groot Nibbelink

Publications and source records attributed to Stefan Groot Nibbelink.

At least 19 recordsLinked to original sources

arXiv2026

PRAISE: A Reproducible Workflow for Non-Programmer Scientists to Implement Published AI Models

Artificial Intelligence (AI) models are increasingly developed, published and used across scientific fields. Researchers, students and industry professionals who want to apply these models to their own research questions often lack the programming and data science expertise to do so. Existing guidelines cover single steps, but not the route from a published model to working predictions on one's own data. Here we present the hands-on workflow PRAISE, a Practical, Reproducible AI-model Implementation for Science and Education, that helps non-programmers implement published AI models step by step. Because implementations rarely work the first time, PRAISE anticipates common failures and provides a fallback route for each. As a proof of principle, three BSc chemistry students and their tutor applied PRAISE without prior programming or data science knowledge. They reproduced, validated and extended a state-of-the-art model that predicts enzyme-substrate interactions. This shows that with PRAISE, implementing an AI model from the literature becomes a learnable skill, one that turns non-programmer scientists from users into evaluators and improvers of AI models.

cs.AI↗

arXiv2026

Classification of order-two T-duality orbifolds at the SO(12) free fermionic point

Asymmetric orbifolds provide concrete examples of non-geometric constructions dubbed T-folds. All Z2 point groups of six dimensional asymmetric order-two orbifolds are identified. The order-two T-fold configurations on the SO(12) lattice are classified at the fermionic point of type II string theories for all Z2 point groups. The spectra of the models on these configurations are presented parametrically, in terms of certain generalised GSO phases. The minimal effective Hodge numbers were found to be (h_{11},h_{12})=(1,1) for six T-fold configurations. Asymmetric generalisations of the mirror symmetry map are conjectured. The orientifoldable configurations using the basic worldsheet parity were identified within the classification. Twisted sectors corresponding to pure asymmetric twists may contain Rarita-Schwinger multiplets with spin-3/2 states. Throughout the paper, generalised GSO projections are chosen to preserve the maximal amount of supersymmetry possible. Relaxing this, the order-two point groups were identified for which non--supersymmetric T-folds can be constructed. Since some of them may enhance to N=1 or, even, N=2 supergravities, we argue that appearance of spin-3/2 states necessarily implies that the spectrum has reorder itself in a supersymmetric fashion and hence that the vacuum energy vanishes.

hep-th↗

arXiv2024

GLSM resolutions of torsional heterotic Z2xZ2 orbifolds

Heterotic toriodal Z2xZ2 orbifolds may possess discrete torsion between the two defining orbifold twists in the form of additional cocycle factors in their one-loop partition functions. Using Gauged Linear Sigma Models (GLSMs) the consequences of discrete torsion can be uncovered when the orbifold is smoothed out by switching on appropriate blowup modes. Here blowup modes with twisted oscillator excitations are chosen to reproduce bundles that are close to the standard embedding without torsion. The orbifold resolutions with discrete torsion are distinguished from resolutions without torsion, since they require NS5-branes at their exceptional cycles.

hep-th↗

arXiv2024

Free fermionic webs of heterotic T-folds

Moduli stabilisation is key to obtaining phenomenologically viable string models. Non-geometric compactifications, like T-duality orbifolds (T-folds), are capable of freezing many moduli. However, in this Letter we emphasise that T-folds, admitting free fermionic descriptions, can be associated with a large number of different T-folds with varying number of moduli, since the fermion pairings for bosonisation are far from unique. Consequently, in one description a fermionic construction might appear to be asymmetric, and hence non-geometric, while in another it admits a symmetric orbifold description. We introduce the notion of intrinsically asymmetric T-folds for fermionic constructions that do not admit any symmetric orbifold description after bosonisation. Finally, we argue that fermion symmetries induce mappings in the bosonised description that extend the T-duality group.

hep-th↗

arXiv2021

A Worldsheet Perspective on Heterotic T-Duality Orbifolds

Asymmetric heterotic orbifolds are discussed from the worldsheet perspective. Starting from Buscher's gauging of a theory of D compact bosons the duality covariant description of Tseytlin is obtained after a non-Lorentz invariant gauge fixing. A left-over of the gauge symmetry can be used to removed the doubled constant zero modes so that D physical target space coordinate remain. This can be thought of as the worldsheet realization of the strong constraint of double field theory. The extension of this description to the heterotic theory is straightforward as all results are written in terms of the invariant and the generalized metrics. An explicit method is outline how to obtain a generalized metric which is invariant under T-duality orbifold actions. It is explicitly shown how shift orbifolds lead to redefinitions of the Narain moduli. Finally, a number of higher dimensional T-folds are constructed including a novel asymmetric Z6 orbifold.

hep-th↗

arXiv2019

Nonexistence of locally but not globally supersymmetric orbifolds

Motivated by the smallness of the cosmological constant we investigate whether it is possible to have vanishing one-loop heterotic string partition functions for six-dimensional non-supersymmetric toroidal orbifolds. A straightforward way to realize this presents itself, when each orbifold sector separately preserves some Killing spinors, but none of them survives in all sectors combined. By applying some representation theory to the abstract finite point groups underlying toroidal orbifolds it turns out, that this is never possible. This leads to a nonexistence proof of locally but not globally supersymmetric orbifolds.

hep-th↗

arXiv2017

Tension Between a Vanishing Cosmological Constant and Non-Supersymmetric Heterotic Orbifolds

We investigate under which conditions the cosmological constant vanishes perturbatively at the one-loop level for heterotic strings on non-supersymmetric toroidal orbifolds. To obtain model-independent results, which do not rely on the gauge embedding details, we require that the right-moving fermionic partition function vanishes identically in every orbifold sector. This means that each sector preserves at least one, but not always the same Killing spinor. The existence of such Killing spinors is related to the representation theory of finite groups, i.e. of the point group that underlies the orbifold. However, by going through all inequivalent (Abelian and non-Abelian) point groups of six-dimensional toroidal orbifolds we show that this is never possible: For any non-supersymmetric orbifold there is always (at least) one sector, that does not admit any Killing spinor. The underlying mathematical reason for this no-go result is formulated in a conjecture, which we have tested by going through an even larger number of finite groups. This conjecture could be applied to situations beyond symmetric toroidal orbifolds, like asymmetric orbifolds.

hep-th↗

arXiv2017

T-duality orbifolds of heterotic Narain compactifications

To obtain a unified framework for symmetric and asymmetric heterotic orbifold constructions we provide a systematic study of Narain compactifications orbifolded by finite order T-duality subgroups. We review the generalized vielbein that parametrizes the Narain moduli space (i.e. the metric, the B-field and the Wilson lines) and introduce a convenient basis of generators of the heterotic T-duality group. Using this we generalize the space group description of orbifolds to Narain orbifolds. This yields a unified, crystallographic description of the orbifold twists, shifts as well as Narain moduli. In particular, we derive a character formula that counts the number of unfixed Narain moduli after orbifolding. Moreover, we develop new machinery that may ultimately open up the possibility for a full classification of Narain orbifolds. This is done by generalizing the geometrical concepts of Q-, Z- and affine classes from the theory of crystallography to the Narain case. Finally, we give a variety of examples illustrating various aspects of Narain orbifolds, including novel T-folds.

hep-th↗

arXiv2016

Twisted superspace: Non-renormalization and fermionic symmetries in certain (heterotic string inspired) non-supersymmetric field theories

Inspired by the tachyon-free non-supersymmetric heterotic SO(16)xSO(16) string we consider a special class of non-supersymmetric field theories: Those that can be obtained from supersymmetric field theories by supersymmetry breaking twists. We argue that such theories, like their supersymmetric counter parts, may still possess some fermionic symmetries as left-overs of the super gauge transformations and have special one-loop non-renormalization properties due to holomorphicity. In addition, we extend the supergraph techniques to these theories to calculate some explicit supersymmetry-breaking corrections.

hep-ph↗

arXiv2016

Line bundle embeddings for heterotic theories

In heterotic string theories consistency requires the introduction of a non-trivial vector bundle. This bundle breaks the original ten-dimensional gauge groups $\text{E}_8\times\text{E}_8$ or $\text{SO}(32)$ for the supersymmetric heterotic string theories and $\text{SO}(16) \times \text{SO}(16)$ for the non-supersymmetric tachyon-free theory to smaller subgroups. A vast number of MSSM-like models have been constructed up to now, most of which describe the vector bundle as a sum of line bundles. However, there are several different ways of describing these line bundles and their embedding in the ten-dimensional gauge group. We recall and extend these different descriptions and explain how they can be translated into each other.

hep-th↗

arXiv2016

Calabi-Yau compactifications of non-supersymmetric heterotic string theory

Phenomenological explorations of heterotic strings have conventionally focused primarily on the E8xE8 theory. We consider smooth compactifications of all three ten-dimensional heterotic theories to exhibit the many similarities between the non-supersymmetric SO(16)xSO(16) theory and the related supersymmetric E8xE8 and SO(32) theories. In particular, we exploit these similarities to determine the bosonic and fermionic spectra of Calabi-Yau compactifications with line bundles of the non-supersymmetric string. We use elements of four-dimensional supersymmetric effective field theory to characterize the non-supersymmetric action at leading order and determine the Green-Schwarz induced axion-couplings. Using these methods we construct a non-supersymmetric Standard Model(SM)-like theory. In addition, we show that it is possible to obtain SM-like models from the standard embedding using at least an order four Wilson line. Finally, we make a proposal of the states that live on five branes in the SO(16)xSO(16) theory and find under certain assumptions the surprising result that anomaly factorization only admits at most a single brane solution.

hep-th↗

arXiv2016

(MS)SM-like models on smooth Calabi-Yau manifolds from all three heterotic string theories

We perform model searches on smooth Calabi-Yau compactifications for both the supersymmetric E8xE8 and SO(32) as well as for the non-supersymmetric SO(16)xSO(16) heterotic strings simultaneously. We consider line bundle backgrounds on both favorable CICYs with relatively small h_11 and the Schoen manifold. Using Gram matrices we systematically analyze the combined consequences of the Bianchi identities and the tree-level Donaldson-Uhlenbeck-Yau equations inside the Kahler cone. In order to evaluate the model building potential of the three heterotic theories on the various geometries, we perform computer-aided scans. We have generated a large number of GUT-like models (up to over a few hundred thousand on the various geometries for the three heterotic theories) which become (MS)SM-like upon using a freely acting Wilson line. For all three heterotic theories we present tables and figures summarizing the potentially phenomenologically interesting models which were obtained during our model scans.

hep-th↗

arXiv2015

Infinite number of MSSMs from heterotic line bundles?

We consider heterotic E8xE8 supergravity compactified on smooth Calabi-Yau manifolds with line bundle gauge backgrounds. Infinite sets of models that satisfy the Bianchi identities and flux quantization conditions can be constructed by letting their background flux quanta grow without bound. Even though we do not have a general proof, we find that all examples are at the boundary of the theory's validity: the Donaldson-Uhlenbeck-Yau equations, which can be thought of as vanishing D-term conditions, cannot be satisfied inside the Kaehler cone unless a growing number of scalar Vacuum Expectation Values (VEVs) is switched on. As they are charged under various line bundles simultaneously, the gauge background gets deformed by these VEVs to a non-Abelian bundle. In general, our physical expectation is that such infinite sets of models should be impossible, since they never seem to occur in exact CFT constructions.

hep-th↗

arXiv2015

Model building with the non-supersymmetric heterotic SO(16)xSO(16) string

In this talk we review recent investigations of the non-supersymmetric heterotic SO(16)xSO(16) string on orbifolds and smooth Calabi-Yaus. Using such supersymmetry preserving backgrounds allows one to re-employ commonly known model building techniques. We will argue that tachyons do not appear on smooth Calabi-Yaus to leading order in alpha' and g_s. Twisted tachyons may arise on singular orbifolds, where some of these approximations break down. However, they get lifted in full blow-up. Finally, we show that model searches is viable by identifying over 12,000 of SM-like models on various orbifold geometries.

hep-th↗

arXiv2014

Non-supersymmetric heterotic model building

We investigate orbifold and smooth Calabi-Yau compactifications of the non-supersymmetric heterotic SO(16)xSO(16) string. We focus on such Calabi-Yau backgrounds in order to recycle commonly employed techniques, like index theorems and cohomology theory, to determine both the fermionic and bosonic 4D spectra. We argue that the N=0 theory never leads to tachyons on smooth Calabi-Yaus in the large volume approximation. As twisted tachyons may arise on certain singular orbifolds, we conjecture that such tachyonic states are lifted in the full blow-up. We perform model searches on selected orbifold geometries. In particular, we construct an explicit example of a Standard Model-like theory with three generations and a single Higgs field.

hep-th↗

arXiv2014

Torus partition functions and spectra of gauged linear sigma models

Worldsheet (0,2) gauged linear sigma models are often used to study supersymmetric heterotic string compactifications with non-trivial vector bundles. We make use of supersymmetric localization techniques to determine their one-loop partition functions. In particular we derive conditions which ensure that the full partition function is modular invariant and we propose a method to determine the massless and massive target space matter spectrum.

hep-th↗

arXiv2014

Renormalization of a Lorentz invariant doubled worldsheet theory

Manifestly T-duality covariant worldsheet string models can be constructed by doubling the coordinate fields. We describe the underlying gauge symmetry of a recently proposed Lorentz invariant doubled worldsheet theory that makes half of the worldsheet degrees of freedom redundant. By shifting the Lagrange multiplier, that enforces the gauge fixing condition, the worldsheet action can be cast into various guises. We investigate the renormalization of this theory using a non-linear background / quantum split by employing a normal coordinate expansion adapted to the gauge-fixed theory. The propagator of the doubled coordinates contains a projection operator encoding that half of them do not propagate. We determine the doubled target space equations of motion by requiring one-loop Weyl invariance. Some of them are generalizations of the conventional sigma model beta-functions, while others seem to be novel to the doubled theory: In particular, a dilaton equation seems related to the strong constraint of double field theory. However, the other target space field equations are not identical to those of double field theory.

hep-th↗

arXiv2013

MSSM-like models on Z(8) toroidal orbifolds

We extend the promising heterotic string searches for MSSM-like models to Z_8 orbifolds. There exist five inequivalent Z_8 toroidal orbifolds distinguished by two types of twists that act on five different torus lattices; one of which cannot be represented as a Lie-algebra root lattice. Contrary to previous investigations, we study the consequences of the different underlying orbifold torus lattices. Therefore, rather than focussing on one particular geometry, we perform systematic model searches on these five Z_8 orbifolds simultaneously, taking all possible inequivalent SU(5) and SO(10) gauge shifts as our starting point. We present cumulative Tables and Figures comparing the chiral SM and vector-like exotic spectra on these geometries.

hep-th↗