arXiv subjects

Simon Knapen

Publications and source records attributed to Simon Knapen.

At least 19 recordsLinked to original sources

arXiv2026

Higgs Couplings at a Future Wakefield Collider

We explore the potential of multiple possible future 10 TeV wakefield colliders to measure electroweak couplings of the Higgs boson. We find that the beam-beam interactions are not an impediment to high precision measurements of the Higgs couplings, provided that the luminosity spectra can be measured or calculated to high accuracy. In addition to $e^+ e^-$ colliders, we also assess the effectiveness of alternatives such as $e^- e^-$ colliders or $\gamma \gamma$ colliders, which by-pass the positron acceleration challenge for wakefield colliders. We find that a 10 $\text{ab}^{-1}$ dataset at a $\gamma\gamma$ collider yields qualitatively similar sensitivity to 10 $\text{ab}^{-1}$ at a muon collider and 1 $\text{ab}^{-1}$ at an $e^+e^-$ wakefield collider.

hep-ph↗

arXiv2026

Bounds on axion-like particles from fully merged photons at the LHC

I show that, in the mass range $0.1\,\text{GeV} < m_a < 6\,\text{GeV}$, the strongest current bound on axion-like particles (ALPs) coupled to photons comes from very high energy $a\gamma$ production at the LHC. In this regime, the two photons from the ALP decay are collimated and reconstructed as a single photon.

hep-ph↗

arXiv2026

Dark Matter Detection Using Phonon Sensing in Amorphous Materials

We present a concept for a tabletop-scale detector with an amorphous target designed to search for dark matter absorption into phonon excitations. In crystalline materials, absorption occurs only at narrow resonances where the dark matter mass matches a zero momentum optical phonon mode, whereas amorphous targets provide a broadband response that can substantially enhance the absorption rate away from these resonances. The predicted backgrounds arise from the relaxation of disorder-induced metastable defects in the amorphous target, as well as from low-energy noise intrinsic to superconducting phonon sensors. A prototype detector with a target mass of only a few $\mu$g could provide broadband sensitivity to dark photon absorption across the 50 meV-200 meV mass range, probing up to two orders of magnitude beyond existing constraints.

hep-ph↗

arXiv2025

Searches for electroweak states at future plasma wakefield colliders

We quantify the discovery potential of future multi-TeV plasma wakefield colliders for new electroweak multiplets. We include beam-beam effects through realistic luminosity spectra, comparing five collider configurations: $e^+e^-$ and $e^-e^-$ machines with round- and flat-beams, and a $\gamma\gamma$ collider. The beam-beam effects qualitatively change search strategies relative to idealized mono-energetic lepton colliders, highlighting the importance of the low-energy part of the luminosity spectrum and additional beam-induced initial-state channels. Our results have implications for accelerator R&D priorities, since key electroweak targets may remain accessible even if efficient positron acceleration and flat-beam delivery prove technically challenging at the multi-TeV scale.

hep-ph↗

arXiv2025

Spin-Dependent Scattering of Sub-GeV Dark Matter: Models and Constraints

We calculate the scattering rate of sub-GeV dark matter in solid-state targets for spin-dependent dark matter -- nucleon interactions. For dark matter particles with mass below 100 MeV, the scattering occurs predominantly through incoherent phonon production. For dark matter heavier than 100 MeV, we match onto the nuclear recoil calculation. To compare the sensitivity of future direct detection experiments with existing constraints, we consider three models with interactions which are mediated by spin-0 or spin-1 particles. This allows us to derive bounds on the cross section from searches for the mediating particle, including bounds from stellar cooling, beam dump experiments, meson factories and dark matter self-interactions. The existing bounds are very stringent, though for $m_\chi\gtrsim 100$ MeV there is parameter space which may be accessible with direct detection, depending on the exposure and background rates.

hep-ph↗

arXiv2025

CODEX-b: Opening New Windows to the Long-Lived Particle Frontier at the LHC

This document is written as a contribution to the European Strategy of Particle Physics (ESPP) update. We offer a detailed overview of current developments and future directions for the CODEX-b detector, which aims to detect long-lived particles beyond the Standard Model. We summarize the scientific motivation for this detector, advances in our suite of simulation and detector optimization frameworks, and examine expected challenges, costs, and timelines in realizing the full detector. Additionally, we describe the technical specifications for the smaller-scale demonstrator detector (CODEX-$\beta$) we have installed in the LHCb experimental cavern.

hep-ex↗

arXiv2024

Broadband phonon production from axion absorption

We show that axion dark matter in the range meV $\lesssim m_a\lesssim$ 100 meV can incoherently excite phonons in crystal targets with unpolarised nuclear spins. This can occur through its coupling to nuclear spins and/or through its induced time-dependent electric dipole moment in nuclei. Due to the random orientation of the nuclear spins, translation symmetry is broken in the phonon effective theory, allowing axion absorption to create phonons with unrestricted momentum. The absorption rate is therefore proportional to the phonon density of states, which generically has support across a wide range of energies, allowing for a broadband detection scheme. We calculate the absorption rate for solid $\text{H}_2$, $\text{D}_2$, $\text{Al}_2\text{O}_3$, $\text{GaAs}$, $\text{H}_2\text{O}$, $\text{D}_2\text{O}$, $\text{Be}$ and $\text{Li}_2 \text{O}$, and find that materials containing light, non-zero spin nuclei are the most promising. The predicted rates for the QCD axion are of the order of a few events / 10 kg-year exposure, setting an ambitious target for the required exposure and background suppression.

hep-ph↗

arXiv2024

Displaced Searches for Axion-Like Particles and Heavy Neutral Leptons at Mu3e

We present strategies for the Mu3e experiment to search for light, weakly coupled particles produced in rare muon decays, focusing on displaced $e^+e^-$ decays within the hollow target. In most scenarios the backgrounds can be fully suppressed with a suitable set of cuts. We furthermore quantify the interplay between displaced and prompt searches at Mu3e and existing constraints, showing how Mu3e has a unique opportunity to probe unexplored parameter space.

hep-ph↗

arXiv2024

Imprints of supersymmetry at a future Z factory

We study the discovery potential of Z branching ratios due to contributions induced by the MSSM electroweak sector, assuming that the squarks and gluinos are heavy. Precision measurements at a future Z factory would yield sensitivity to MSSM that is complementary to direct searches at the LHC, provided that the systematic uncertainties can be reduced to a level comparable to the expected statistical uncertainties.

hep-ph↗

arXiv2024

Technical design report for the CODEX-$\beta$ demonstrator

The CODEX-$\beta$ apparatus is a demonstrator for the proposed future CODEX-b experiment, a long-lived-particle detector foreseen for operation at IP8 during HL-LHC data-taking. The demonstrator project, intended to collect data in 2025, is described, with a particular focus on the design, construction, and installation of the new apparatus.

physics.ins-det↗

arXiv2023

Angling for Insights: Illuminating Light New Physics at Mu3e through Angular Correlations

We examine the capability of Mu3e to probe light new physics scenarios that produce a prompt electron-positron resonance and demonstrate how angular observables are instrumental in enhancing the experimental sensitivity. We systematically investigate the effect of Mu3e's expected sensitivity on the parameter space of the dark photon, as well as on axion-like particles and light scalars with couplings to muons and electrons.

hep-ph↗

arXiv2023

A Robust Search for Lepton Flavour Violating Axions at Mu3e

We propose a search at Mu3e for lepton flavor violating axion(-like) particles in $\mu\to 3e + a$ decays. By requiring an additional $e^+e^-$ pair from internal conversion, one can circumvent the calibration challenges which plague the $\mu \to e+a$ channel for axions lighter than 20 MeV. Crucially, the corresponding reduction in signal rate is to a large extent compensated for by Mu3e's ability to resolve highly collimated tracks. For phase I of Mu3e, we project a sensitivity to decay constants as high as $6\times 10^9$ GeV which probes uncharted parameter space in scenarios of axion dark matter. The sensitivity to axions which couple primarily to right-handed leptons can be further improved by leveraging the polarisation of the muon beam.

hep-ph↗

arXiv2023

Dark Matter scattering in low threshold detectors

The scattering of sub-GeV dark matter in direct detection experiments happens at characteristic wavelengths comparable or larger than the interparticle spacing. Collective effects in the target material must therefore be accounted for when calculating the scattering rate. For dark matter-nucleon couplings, this implies matching onto the appropriate phonon effective theory and calculating single and multi-phonon scattering amplitudes. For dark matter-electron couplings, we make use of the energy loss formalism to predict the scattering rate. Combining both techniques allows us to derive a formula for the Migdal effect in crystals, which differs from prior calculations performed in atomic systems.

hep-ph↗

arXiv2023

The Postdoc Accord in Theoretical High Energy Physics

We present the results of a survey meant to assess the opinion of the high-energy physics theory (HET) community on the January 7th postdoc acceptance deadline - specifically, whether there is a preference to shift the deadline to later in January or February. This survey, which served for information-gathering purpose only, is part of a community conversation on the optimal timing of an acceptance deadline and whether the community would be better served by a later date. In addition, we present an analysis of data from the postdoc Rumor Mill, which gives a picture of the current hiring landscape in the field. We emphasize the importance of preserving a universal deadline, and the current results of our survey show broad support for a shift to a later date. A link to the survey, frequently asked questions, a running list of supporters, and next steps can be found on our companion web page.

hep-ph↗

arXiv2023

Scouting for dark showers at CMS and LHCb

We assess the capabilities of the CMS and LHCb searches for low-$p_T$ displaced dimuon pairs to discover hidden valley models, using a newly-developed benchmark model that realizes a range of dimuon vertex topologies. We show that the data scouting techniques used in these searches provide unique sensitivity and we make some additional suggestions to further extend the scope of future experimental searches.

hep-ph↗

arXiv2022

A guide to hunting long-lived particles at the LHC

This article is a pedagogical review to searches for long-lived particles at the LHC, primarily aimed at experimentalists and theorists seeking to initiate and/or deepen their research in this field. We cover general theory priors and example models, the main experimental techniques for long-lived particles and some of the subtleties involved with both estimating signal efficiencies and background rates.

hep-ph↗

arXiv2022

Geometry Optimization for Long-lived Particle Detectors

The proposed designs of many auxiliary long-lived particle (LLP) detectors at the LHC call for the instrumentation of a large surface area inside the detector volume, in order to reliably reconstruct tracks and LLP decay vertices. Taking the CODEX-b detector as an example, we provide a proof-of-concept optimization analysis that demonstrates the required instrumented surface area can be substantially reduced for many LLP models, while only marginally affecting the LLP signal efficiency. This optimization permits a significant reduction in cost and installation time, and may also inform the installation order for modular detector elements. We derive a branch-and-bound based optimization algorithm that permits highly computationally efficient determination of optimal detector configurations, subject to any specified LLP vertex and track reconstruction requirements. We outline the features of a newly-developed generalized simulation framework, for the computation of LLP signal efficiencies across a range of LLP models and detector geometries.

hep-ph↗

arXiv2022

Machine-Learning Compression for Particle Physics Discoveries

In collider-based particle and nuclear physics experiments, data are produced at such extreme rates that only a subset can be recorded for later analysis. Typically, algorithms select individual collision events for preservation and store the complete experimental response. A relatively new alternative strategy is to additionally save a partial record for a larger subset of events, allowing for later specific analysis of a larger fraction of events. We propose a strategy that bridges these paradigms by compressing entire events for generic offline analysis but at a lower fidelity. An optimal-transport-based $\beta$ Variational Autoencoder (VAE) is used to automate the compression and the hyperparameter $\beta$ controls the compression fidelity. We introduce a new approach for multi-objective learning functions by simultaneously learning a VAE appropriate for all values of $\beta$ through parameterization. We present an example use case, a di-muon resonance search at the Large Hadron Collider (LHC), where we show that simulated data compressed by our $\beta$-VAE has enough fidelity to distinguish distinct signal morphologies.

hep-ph↗