arXiv subjects

Spyros Argyropoulos

Publications and source records attributed to Spyros Argyropoulos.

6 recordsLinked to original sources

arXiv2026

MadSpin2: automated spin-entangled decays of heavy resonances via the spin-density matrix formalism

The simulation of processes that contain multiple particles from the decay of unstable resonances becomes computationally challenging both at leading order and beyond. This paper presents MADSPIN2, a new version of MADSPIN, a dedicated tool tackling this problem, primarily for events generated with the MG5AMC event generator. While still based on the Frixione-Laenen-Motylinski-Webber formalism, the new version leverages spin-density matrices to lift most of the limitations of the previous implementation. It supports loop-induced processes, N -body decays and samples defined by amplitude-level interference between distinct hard-process amplitudes. It also introduces the multiple-pole approximation and a resonance-helicity decomposition, including off-diagonal helicity-interference contributions. These developments substantially broaden the phenomenological scope of MADSPIN, enabling applications that were previously inaccessible within the framework, several of which are demonstrated in this work.

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arXiv2024

Density dependent displaced vertex signatures as a novel probe of light dark sector scalars at the LHC

Dynamical theories of dark energy predict new degrees of freedom with particular environmental sensitivity to avoid constraints on fifth forces. We show that the similar, yet complementary multi-purpose detector setup of the ATLAS and CMS experiments provides a unique opportunity to place sensitivity on such scenarios in a narrow, yet relevant parameter range. Furthermore, our investigation gives rise to a novel phenomenological signature that the LHC experiments can pursue to exploit their complementary detector design from a BSM perspective.

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arXiv2024

Novel collider signatures in the type-I 2HDM+$a$ model

The 2HDM+$a$ model is one of the main models used in the interpretations of dark matter searches at the LHC. So far, all the 2HDM+$a$ benchmarks considered by the ATLAS and CMS experiments are limited to a type-II Yukawa sector, in which the Higgs bosons $A$, $H$, and $H^\pm$ are all constrained to be mass-degenerate and heavier than around $600 \, {\rm GeV}$. In this work, we present the first detailed study of 2HDM+$a$ models with a type-I Yukawa sector, which, for moderate values of $\tanβ$, lift the constraints from flavour physics, allowing the extra Higgs bosons to be even lighter than the $125 \, {\rm GeV}$ Higgs boson discovered at the LHC. We discuss several benchmarks where the $A$, $H$, and $H^\pm$ states are not necessarily mass-degenerate and the signatures that arise in these models, some of which have not yet been explored at the LHC. We present the dominant channels in the studied benchmarks and the expected sensitivity in Run 2 data using truth-level analyses and discuss potential improvements in the experimental searches for Run 3.

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arXiv2022

Collider Searches for Dark Matter through the Higgs Lens

Despite the fact that dark matter constitutes one of the cornerstones of the standard cosmological paradigm, its existence has so far only been inferred from astronomical observations and its microscopic nature remains elusive. Theoretical arguments suggest that dark matter might be connected to the symmetry-breaking mechanism of the electroweak interactions or of other symmetries extending the Standard Model of particle physics. The resulting Higgs bosons, including the $125 \, {\rm GeV}$ spin-0 particle discovered recently at the Large Hadron Collider therefore represent a unique tool to search for dark matter candidates at collider experiments. This article reviews some of the relevant theoretical models as well as the results from the searches for dark matter in signatures that involve a Higgs-like particle at the Large Hadron Collider.

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arXiv2022

Benchmarking LHC searches for light 2HDM+$\boldsymbol{a}$ pseudoscalars

Using two suitable benchmark scenarios that satisfy the experimental constraints on the total decay width of the $125 \, {\rm GeV}$ Higgs boson, we determine the bounds on light CP-odd spin-0 states in the 2HDM+$a$ model that arise from existing LHC searches. Our work represents the first thorough study that considers the parameter space with $m_a \lesssim 100 \, {\rm GeV}$ and should prove useful for 2HDM+$a$ interpretations of future ATLAS, CMS and LHCb searches for pseudoscalars with masses below the electroweak scale.

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arXiv2014

Effects of color reconnection on $t\bar{t}$ final states at the LHC

The modeling of color reconnection has become one of the dominant sources of systematic uncertainty in the top mass determination at hadron colliders. The uncertainty on the top mass due to color reconnection is conventionally estimated by taking the difference in the predictions of a model with and a model without color reconnection. We show that this procedure underestimates the uncertainty when applied to the existing models in {\sc Pythia}~8. We introduce two new classes of color reconnection models, each containing several variants, which encompass a variety of scenarios that could be realized in nature and we study how they affect the reconstruction of the top mass. After tuning the new models to existing LHC data, the remaining spread of predictions is used to derive a more realistic uncertainty for the top mass, which is found to be around 500 MeV. We also propose how future LHC measurements with $t\bar{t}$ events can be used to further constrain these models and reduce the associated modeling uncertainty.

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