arXiv subjects

Luca Romano

Publications and source records attributed to Luca Romano.

At least 19 recordsLinked to original sources

arXiv2026

From Galilei to Euclidean Carroll and the Alice Particle: The Times They Are a-Changin'

In generalized, also known as $p$-brane Galilei limits, the speed of light $c$ becomes infinite in the directions transverse to a $(p+1)$-dimensional Lorentzian worldvolume. In this paper, we explain that allowing the worldvolume to be Euclidean, and thus time to be transversal, $p$-brane Galilei limits turn into Carrollian ones, that we refer to as "Euclidean $p$-brane Carroll limits", in which $c$ goes to zero in the $p+1$ worldvolume directions. This leads to a unified approach to taking Galilean and Carrollian limits, whose consequences we explore for $p=0$. We show that the spacetime symmetries that arise from the Euclidean 0-brane Carroll limit can be centrally extended to what we will call the Alice algebra, similar to how the Bargmann algebra centrally extends the Galilei symmetries. This gives rise to the novel notion of an Alice particle, and we obtain the Bargmann and Alice particle actions from a unified limit of the action of a relativistic massive particle or tachyon, suitably coupled to a one-form gauge potential. In the presence of a cosmological constant, we find that the Bargmann and Alice particles undergo stable motion for negative and positive cosmological constant, respectively. Finally, we show that the Bargmann and Alice particle actions can be obtained from null reduction of a massless particle action in a relativistic spacetime with one and two times. Our results indicate that in 10 dimensions, the Alice particle is a decoupling limit of a D$0{}^*$-brane in Hull's type IIA${}^*$ theory, similar to how the Bargmann particle is related to a D$0$-brane in type IIA string theory.

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arXiv2026

Hierarchical Retrieval Augmented Generation for Adversarial Technique Annotation in Cyber Threat Intelligence Text

Mapping Cyber Threat Intelligence (CTI) text to MITRE ATT\&CK technique IDs is a critical task for understanding adversary behaviors and automating threat defense. While recent Retrieval-Augmented Generation (RAG) approaches have demonstrated promising capabilities in this domain, they fundamentally rely on a flat retrieval paradigm. By treating all techniques uniformly, these methods overlook the inherent taxonomy of the ATT\&CK framework, where techniques are structurally organized under high-level tactics. In this paper, we propose H-TechniqueRAG, a novel hierarchical RAG framework that injects this tactic-technique taxonomy as a strong inductive bias to achieve highly efficient and accurate annotation. Our approach introduces a two-stage hierarchical retrieval mechanism: it first identifies the macro-level tactics (the adversary's technical goals) and subsequently narrows the search to techniques within those tactics, effectively reducing the candidate search space by 77.5\%. To further bridge the gap between retrieval and generation, we design a tactic-aware reranking module and a hierarchy-constrained context organization strategy that mitigates LLM context overload and improves reasoning precision. Comprehensive experiments across three diverse CTI datasets demonstrate that H-TechniqueRAG not only outperforms the state-of-the-art TechniqueRAG by 3.8\% in F1 score, but also achieves a 62.4\% reduction in inference latency and a 60\% decrease in LLM API calls. Further analysis reveals that our hierarchical structural priors equip the model with superior cross-domain generalization and provide security analysts with highly interpretable, step-by-step decision paths.

cs.CL↗

arXiv2025

Grounded Visual Factualization: Factual Anchor-Based Finetuning for Enhancing MLLM Factual Consistency

Visual hallucination, where Multimodal Large Language Models fabricate details inconsistent with image content, critically undermines their reliability. Existing fine-tuning methods offer limited improvement, failing to deeply intervene in factual reasoning. This paper introduces Grounded Visual Factualization (GVF) Finetuning, a novel approach to systematically enhance MLLM visual factual consistency. GVF integrates explicit factual signals via three core mechanisms: Factual Anchor Data Augmentation, enriching training data with structured factual anchors and counter-factual prompts; Fact-Aware Instruction Tuning, embedding these cues into explicit instructions; and a Factual Consistency Loss function, specifically penalizing factual inaccuracies. Evaluated on LLaVA-1.5-13B, GVF Finetuning significantly outperforms standard fine-tuning on the VHTest benchmark for both Open-Ended Question (OEQ) and Yes/No Question (YNQ) formats. Crucially, GVF maintains or even slightly improves performance on general multimodal benchmarks like MME and POPE, demonstrating effective mitigation of visual hallucinations without compromising general understanding and reasoning abilities.

cs.CL↗

arXiv2025

Higher-Order Newton-Cartan Gravity

We study the non-relativistic Newton-Cartan limit of higher-order gravity theories in arbitrary dimensions. We first study it at the level of the action by introducing an additional 1-form gauge field and coupling it appropriately to the gravity sector. We extend this procedure to any theory whose Lagrangian is a function of the Ricci scalar, quadratic Ricci tensor and quadratic Riemann tensor. We also study the limit of the equations of motion for two models, Einstein-Gauss-Bonnet gravity and quadratic Ricci scalar theory. We prove that, in the first case, it is possible to obtain the Poisson equation by introducing a scalar field and imposing an appropriate constraint. In the latter case, we show that it is possible to get the Poisson equation from the limit of the equations of motion as long as the on-shell constraint used in the two-derivative theory is supplemented with a further condition. We give the expressions of the two higher-order corrected Poisson equations in terms of curvatures of Newton-Cartan geometry. In both cases, we derive the full set of non-relativistic equations and study their boost transformations. The two sets of equations of motion define zero-torsion Gauss-Bonnet Newton-Cartan gravity and zero-torsion quadratic Ricci scalar Newton-Cartan gravity.

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arXiv2025

Heterotic String Sigma Models: Discrete Light Cone Quantization and Its Current-Current Deformation

We propose a two-dimensional superstring sigma model that defines a self-contained corner of heterotic string theory, whose second quantization is heterotic matrix string theory. This worldsheet theory arises from a BPS decoupling limit that zooms in on the heterotic string, under which the target space geometry becomes non-Lorentzian. This construction generalizes the Gomis-Ooguri formulation of non-relativistic string theory to the heterotic case. We show how such a worldsheet theory provides a first-principles definition of the heterotic string in the discrete light cone quantization via T-duality. By turning on a current-current deformation akin to a $T\bar{T}$ deformation, the conventional heterotic string theory with a Lorentzian target space is recovered. We analyze the gauge and gravitational anomalies with respect to the lowest-order quantum corrections in the sigma model and show how the worldsheet theory is consistent with non-Lorentzian supergravity in the target space.

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arXiv2024

From Relativistic Gravity to the Poisson Equation

We consider the non-relativistic limit of general relativity coupled to a $(p+1)$-form gauge field and a scalar field in arbitrary dimensions and investigate under which conditions this gives rise to a Poisson equation for a Newton potential describing Newton-Cartan gravity outside a massive $p$-dimensional extended object, a so-called $p$-brane. Given our Ansatz, we show that not all the $p$-branes satisfy the required conditions. We study theories whose dynamics is defined by a Lagrangian as well as systems that are defined by a set of equations of motion not related to a Lagrangian. We show that, within the Lagrangian approach, a Poisson equation can be obtained provided that the coupling of the scalar field is fine-tuned such that the non-relativistic Lagrangian is invariant under an emerging local dilatation symmetry. On the other hand, we demonstrate that in the absence of a Lagrangian a Poisson equation can be obtained from a set of equations of motion that is not dilatation invariant. We discuss how our Ansatz could be generalized such as to include more $p$-branes giving rise to a Poisson equation.

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arXiv2023

Non-Relativistic Heterotic String Theory

In this work we consider heterotic-gravity as the low-energy approximation to heterotic string theory. We define a consistent non-relativistic limit of heterotic gravity that includes the Yang-Mills Chern-Simons term. We perform three tests on the heterotic limit: we use it to (i) derive the non-relativistic transformation rules, (ii) show the existence of a finite non-relativistic heterotic action and (iii) obtain the longitudinal non-relativistic T-duality rules. We show that in all these cases the limit procedure is well defined, free of divergences and leads to consistent non-trivial results. We comment on the interpretation of the T-duality rules in terms of a heterotic non-relativistic geometry underlying non-relativistic heterotic string theory.

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arXiv2022

Generalized Newton-Cartan Geometries for Particles and Strings

We discuss the generalized Newton-Cartan geometries that can serve as gravitational background fields for particles and strings. In order to enable us to define affine connections that are invariant under all the symmetries of the structure group, we describe torsionful geometries with independent torsion tensors. A characteristic feature of the non-Lorentzian geometries we consider is that some of the torsion tensors are so-called `intrinsic torsion' tensors that cannot be absorbed in any of the spin connections. Setting some components of these intrinsic torsion tensors to zero leads to constraints on the geometry. For both particles and strings, we discuss various such constraints that can be imposed consistently with the structure group symmetries. In this way, we reproduce several results in the literature.

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arXiv2022

The Supersymmetric Neveu-Schwarz Branes of Non-Relativistic String Theory

We construct the basic Neveu-Schwarz (NS) brane solutions of non-relativistic string theory using longitudinal T-duality as a solution generating technique. Extending the NS background fields to a supergravity multiplet, we verify that all solutions we find are half-supersymmetric. The two perturbative solutions we find both have an interpretation as the background geometry outside a string-like object. Correspondingly, we refer to these non-Lorentzian backgrounds as winding string and unwound string solution. Whereas the winding string is part of the on-shell spectrum of non-relativistic string theory, the unwound string only makes sense off-shell where it mediates the instantaneous gravitational force. Seen from the nine-dimensional point of view, we find that the winding string solution is sourced by a non-relativistic massive particle and that the unwound string solution is sourced by a massless Galilean particle of zero colour and spin. We explain how these two string solutions fit into a discrete lightcone quantization of string theory. We shortly discuss the basic NS five-brane and Kaluza-Klein monopole solutions and show that they are both half-supersymmetric.

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arXiv2021

A Non-Relativistic Limit of NS-NS Gravity

We discuss a particular non-relativistic limit of NS-NS gravity that can be taken at the level of the action and equations of motion, without imposing any geometric constraints by hand. This relies on the fact that terms that diverge in the limit and that come from the Vielbein in the Einstein-Hilbert term and from the kinetic term of the Kalb-Ramond two-form field cancel against each other. This cancelling of divergences is the target space analogue of a similar cancellation that takes place at the level of the string sigma model between the Vielbein in the kinetic term and the Kalb-Ramond field in the Wess-Zumino term. The limit of the equations of motion leads to one equation more than the limit of the action, due to the emergence of a local target space scale invariance in the limit. Some of the equations of motion can be solved by scale invariant geometric constraints. These constraints define a so-called Dilatation invariant String Newton-Cartan geometry.

hep-th↗

arXiv2020

Non-Relativistic Supersymmetry on Curved Three-Manifolds

We construct explicit examples of non-relativistic supersymmetric field theories on curved Newton-Cartan three-manifolds. These results are obtained by performing a null reduction of four-dimensional supersymmetric field theories on Lorentzian manifolds and the Killing spinor equations that their supersymmetry parameters obey. This gives rise to a set of algebraic and differential Killing spinor equations that are obeyed by the supersymmetry parameters of the resulting three-dimensional non-relativistic field theories. We derive necessary and sufficient conditions that determine whether a Newton-Cartan background admits non-trivial solutions of these Killing spinor equations. Two classes of examples of Newton-Cartan backgrounds that obey these conditions are discussed. The first class is characterised by an integrable foliation, corresponding to so-called twistless torsional geometries, and includes manifolds whose spatial slices are isomorphic to the Poincar\'e disc. The second class of examples has a non-integrable foliation structure and corresponds to contact manifolds.

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arXiv2020

Lie Algebra Expansion and Integrability in Superstring Sigma-Models

Lie algebra expansion is a technique to generate new Lie algebras from a given one. In this paper, we apply the method of Lie algebra expansion to superstring $\sigma$-models with a $\mathbb{Z}_4$ coset target space. By applying the Lie algebra expansion to the isometry algebra, we obtain different $\sigma$-models, where the number of dynamical fields can change. We reproduce and extend in a systematic way actions of some known string regimes (flat space, BMN and non-relativistic in AdS$_5 \times$S$^5$). We define a criterion for the algebra truncation such that the equations of motion of the expanded action of the new $\sigma$-model are equivalent to the vanishing curvature condition of the Lax connection obtained by expanding the Lax connection of the initial model.

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arXiv2020

Carroll versus Galilei from a Brane Perspective

We show that our previous work on Galilei and Carroll gravity, apt for particles, can be generalized to Galilei and Carroll gravity theories adapted to p-branes (p = 0, 1, 2, ...). Within this wider brane perspective, we make use of a formal map, given in the literature, between the corresponding p-brane Carroll and Galilei algebras where the index describing the directions longitudinal (transverse) to the Galilei brane is interchanged with the index covering the directions transverse (longitudinal) to the Carroll brane with the understanding that the time coordinate is always among the longitudinal directions. This leads among other things in 3D to a map between Galilei particles and Carroll strings and in 4D to a similar map between Galilei strings and Carroll strings. We show that this formal map extends to the corresponding Lie algebra expansion of the Poincar\'e algebra and, therefore, to several extensions of the Carroll and Galilei algebras including central extensions. We use this formal map to construct several new examples of Carroll gravity actions. Furthermore, we discuss the symmetry between Carroll and Galilei at the level of the p-brane sigma model action and apply this formal symmetry to give several examples of 3D and 4D particles and strings in a curved Carroll background.

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arXiv2019

Non-Relativistic Four Dimensional p-Brane Supersymmetric Theories and Lie Algebra Expansion

We apply the Lie algebra expansion method to the $\mathcal{N}=1$ super-Poincar\'e algerba in four dimensions. We define a set of p-brane projectors that induce a decomposition of the super-Poincar\'e algebra preparatory for the expansion. We show that starting from the $\mathcal{N}=1$ supergravity action in four dimensions it is possible to obtain two non-relativistic supersymmetric theories, one describing strings, the other membranes.

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arXiv2019

Orbits in Non-Supersymmetric Magic Theories

We determine and classify the electric-magnetic duality orbits of fluxes supporting asymptotically flat, extremal black branes in $D=4,5,6$ space-time dimensions in the so-called non-supersymmetric magic Maxwell-Einstein theories, which are consistent truncations of maximal supergravity and which can be related to Jordan algebras (and related Freudenthal triple systems) over the split complex numbers $\mathbb{C}_{s}$ and the split quaternions $\mathbb{H}_{s}$. By studying the stabilizing subalgebras of suitable representatives, realized as bound states of specific weight vectors of the corresponding representation of the electric-magnetic duality symmetry group, we obtain that, as for the case of maximal supergravity, in magic non-supersymmetric Maxwell-Einstein theories there is no splitting of orbits, namely there is only one orbit for each non-maximal rank element of the relevant Jordan algebra (in $D=5$ and $6$) or of the relevant Freudenthal triple system (in $D=4$).

hep-th↗

arXiv2019

Lie Algebra Expansions and Actions for Non-Relativistic Gravity

We show that the general method of Lie algebra expansions can be applied to re-construct several algebras and related actions for non-relativistic gravity that have occurred in the recent literature. We explain the method and illustrate its applications by giving several explicit examples. The method can be generalized to include the construction of actions for ultra-relativistic gravity, i.e. Carroll gravity, and non-relativistic supergravity as well.

hep-th↗

arXiv2017

Newton-Cartan Gravity and Torsion

We compare the gauging of the Bargmann algebra, for the case of arbitrary torsion, with the result that one obtains from a null-reduction of General Relativity. Whereas the two procedures lead to the same result for Newton-Cartan geometry with arbitrary torsion, the null-reduction of the Einstein equations necessarily leads to Newton-Cartan gravity with zero torsion. We show, for three space-time dimensions, how Newton-Cartan gravity with arbitrary torsion can be obtained by starting from a Schroedinger field theory with dynamical exponent z=2 for a complex compensating scalar and next coupling this field theory to a z=2 Schroedinger geometry with arbitrary torsion. The latter theory can be obtained from either a gauging of the Schroedinger algebra, for arbitrary torsion, or from a null-reduction of conformal gravity.

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arXiv2017

Non-Supersymmetric Magic Theories and Ehlers Truncations

We consider the non-supersymmetric "magic" theories based on the split quaternion and the split complex division algebras. We show that these theories arise as "Ehlers" $SL(2,\mathbb{R})$ and $SL(3,\mathbb{R})$ truncations of the maximal supergravity theory, exploiting techniques related to very-extended Kac-Moody algebras. We also generalise the procedure to other $SL(n,\mathbb{R})$ truncations, resulting in additional classes of non-supersymmetric theories, as well as to truncations of non-maximal theories. Finally, we discuss duality orbits of extremal black-hole solutions in some of these non-supersymmetric theories.

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