arXiv subjects

Axel Gallén

Publications and source records attributed to Axel Gallén.

5 recordsLinked to original sources

arXiv2026

Dark photons in exotic Higgs boson decays at FCC-ee

A prospective search for dark photons decaying into collimated, displaced muon pairs is presented for the electron-positron stage of the Future Circular Collider, FCC-ee. Assuming the full luminosity of the Higgs run at 240 GeV, the study probes a broad range of dark photon masses and couplings. The signature comprises two jets from a Z boson decay and two muon pairs from dark photons produced in Higgs decays. A kinematics-based selection achieves zero background while retaining high signal efficiency. Expected limits are set on the Higgs boson branching ratio to dark photons as a function of the dark photon mass and coupling.

hep-ex↗

arXiv2025

Early Career Researcher Input to the European Strategy for Particle Physics Update: White Paper

This document, written by early career researchers (ECRs) in particle physics, aims to represent the perspectives of the European ECR community and serves as input for the 2025--2026 update of the European Strategy for Particle Physics. With input from a community-wide survey, it highlights key challenges faced by ECRs -- career stability, funding access and long-term research opportunities -- while proposing policy recommendations and targeted initiatives. It underscores the importance of practices fostering diverse, equitable, inclusive and healthy workplaces, as well as of stronger ECR communities, and highlights how effective communication and interdisciplinary collaborations reinforce the societal relevance of particle physics and promote continued support for large-scale and long-term projects. Finally, the future of both collider and beyond-collider experiments is addressed, emphasising the critical role of ECRs in shaping future projects. The ECR contribution is formed of two parts: the ten-page executive summary submitted as input to the European Strategy for Particle Physics Update and, as backup document, this extended white paper providing additional context.

hep-ex↗

arXiv2025

Searching for long-lived dark scalars at the FCC-ee

This paper investigates the search for long-lived dark scalars from exotic Higgs boson decays at the Future Circular Collider in its $e^+e^-$ stage, FCC-ee, considering an integrated luminosity of 10.8 $\text{ab}^{-1}$ collected during the ZH run at a center-of-mass energy $\sqrt{s}=240$ GeV. The work considers $Zh$ events where the $Z$ boson decays leptonically and the Higgs boson $h$ decays into two long-lived dark scalars $s$ which further decay into bottom anti-bottom quark pairs. The analysis is performed using a parametrized simulation of the IDEA detector concept and targets dark scalar decays in the tracking volume, resulting in multiple displaced vertices in the final state. The sensitivity towards long-lived dark scalars at FCC-ee is estimated using an event selection requiring two opposite-charge, same-flavor leptons compatible with the $Z$ boson, and at least two displaced vertices in the final state. The selection is seen to efficiently remove the Standard Model background, while retaining sensitivity for dark scalar masses between $m_s=20$ GeV and $m_s=60$ GeV and mean proper lifetimes $cτ$ between approximately 10 mm and 10 m The results show that the search strategy has potential to probe Higgs to dark scalar branching ratios as low as $10^{-4}$ for a mean proper lifetime $cτ\approx 1$ m. The results provide the first sensitivity estimate for exotic Higgs decays at FCC-ee with the IDEA detector concept, using the common FCC framework.

hep-ex↗

arXiv2024

Sparks in the Dark

This study presents a novel method for the definition of signal regions in searches for new physics at collider experiments, specifically those conducted at CERNs Large Hadron Collider. By leveraging multi-dimensional histograms with precise arithmetic and utilizing the SparkDensityTree library, it is possible to identify high-density regions within the available phase space, potentially improving sensitivity to very small signals. Inspired by an ongoing search for dark mesons at the ATLAS experiment, CMS open data is used for this proof-of-concept intentionally targeting an already excluded signal. Several signal regions are defined based on density estimates of signal and background. These preliminary regions align well with the physical properties of the signal while effectively rejecting background events. While not explored in this work, this method is also scalable, which makes it ideal for large datasets such as those expected at the high-luminosity upgrade of the LHC. Finally, this method is flexible and can be easily extended, promising a boost to the signal region definition process for new physics searches at colliders.

hep-ex↗

arXiv2024

Baler -- Machine Learning Based Compression of Scientific Data

Storing and sharing increasingly large datasets is a challenge across scientific research and industry. In this paper, we document the development and applications of Baler - a Machine Learning based data compression tool for use across scientific disciplines and industry. Here, we present Baler's performance for the compression of High Energy Physics (HEP) data, as well as its application to Computational Fluid Dynamics (CFD) toy data as a proof-of-principle. We also present suggestions for cross-disciplinary guidelines to enable feasibility studies for machine learning based compression for scientific data.

physics.comp-ph↗