arXiv subjects

Sophie Middleton

Publications and source records attributed to Sophie Middleton.

12 recordsLinked to original sources

arXiv2026

Expected Sensitivity of the Light Dark Matter eXperiment to Long-Lived Dark Photons and Axion-Like Particles

The Light Dark Matter eXperiment (LDMX) is an electron-beam fixed-target experiment primarily designed to achieve world-leading, model-independent sensitivity to sub-GeV dark matter particles. LDMX aims to identify dark sector particle production through the detection of events with substantial missing energy and momentum, a signature of invisible particles escaping detection. Beyond this primary objective, LDMX offers a complementary search strategy for long-lived, visibly decaying particles, such as dark photons and axion-like particles. We present the first detailed evaluation of the ability of LDMX to identify visibly decaying, long-lived particles that couple to electrons using a detailed simulation, based on the Geant4-toolkit, that incorporates realistic detection efficiencies and background levels. We demonstrate that LDMX can achieve a sensitivity that is competitive with other experiments that are currently running. The models explored in this paper are distinct and complementary to those probed in the LDMX flagship missing-momentum analysis. Through searching for both invisible dark matter and visibly decaying long-lived signatures, LDMX will significantly advance the search for light dark matter and provide a broad exploration of the sub-GeV dark sector.

hep-ex↗

arXiv2025

LDMX -- The Light Dark Matter eXperiment

The Light Dark Matter eXperiment (LDMX) is an electron fixed-target experiment optimized to search for sub-GeV dark matter production through the missing momentum signature. LDMX is designed to operate in End Station A at SLAC, using an 8 GeV electron beam accelerated alongside the LCLS-II drive beam. The design of the apparatus is strongly motivated by the performance requirements of a high-rate missing momentum search and leverages detector technologies and designs from other experiments along with existing facilities at SLAC. LDMX will improve on previous results by up to three orders of magnitude, enabling broad sensitivity to dark sector scenarios including the dark matter interaction strengths motivated by freeze-out of MeV-GeV mass dark matter to the observed relic abundance. With hermetic forward coverage, LDMX also has sensitivity to visible signatures of dark sectors and provides a unique probe of electron-nuclear interactions important to interpreting data from accelerator-based neutrino experiments. This report encompasses the technical design of the LDMX Detector, its simulated performance, and the physics capabilities of the experiment.

hep-ex↗

arXiv2025

Sensitivity of an Early Dark Matter Search using the Electromagnetic Calorimeter as a Target for the Light Dark Matter eXperiment

The Light Dark Matter eXperiment (LDMX) is proposed to employ a thin tungsten target and a multi-GeV electron beam to carry out a missing momentum search for the production of dark matter candidate particles. We study the sensitivity for a complementary missing-energy-based search using the LDMX Electromagnetic Calorimeter as an active target with a focus on early running. In this context, we construct an event selection from a limited set of variables that projects sensitivity into previously-unexplored regions of light dark matter phase space -- down to an effective dark photon interaction strength $y$ of approximately $2\times10^{-13}$ ($5\times10^{-12}$) for a 1MeV (10MeV) dark matter candidate mass.

hep-ex↗

arXiv2025

A New Determination of the (Z,A) Dependence of Coherent Muon-to-Electron Conversion

Should muon-to-electron conversion in the field of a nucleus be found in the current generation of experiments, the measurement of the atomic number dependence of the process will become an important experimental goal. We present a new treatment of the (Z,A) dependence of coherent muon-to-electron conversion in 236 isotopes. Our approach differs from previous treatments in several ways. Firstly, we include the effect of permanent quadrupole deformation on the charged lepton flavor violating matrix elements, using the method of Barrett moments. This method also enables the addition of muonic X-ray nuclear size and shape determinations of the charge distribution to the electron scattering results used previously. Secondly, we employ a Hartree-Bogoliubov model to calculate neutron-related matrix elements for even-even nuclei. This takes into account the quadrupole deformation of the neutron distributions and the fact that neutrons are, in general, in different shell model orbits than protons. The calculated conversion rates differ from previous calculations, particularly in the region of large permanent quadrupole deformation. Finally, we propose an alternative normalization of the muon-to-electron conversion rated, which related more closely to what a given experiment acturally measures, and better separate lepton physics from nuclear physics effects.

hep-ph↗

arXiv2025

Experimental Measurements of the Muon $g-2$ and Searches for Charged Lepton Flavor Violation in the Muon Sector

Since its discovery, the muon has proven to be an invaluable probe of the Standard Model (SM). Muons are readily available in tertiary beams in facilities around the world. They do not decay hadronically and have a lifetime of a few $μ$ s; consequently, muon experiments offer clean, high-statistics environments to make precision measurements and search for new physics that could appear through deviations from the SM expectation. The 2020s have seen a renaissance in muon physics highlighted by the high-profile results from the Fermilab Muon $g - 2$ experiment which continues to provide successive measurements of the muon's anomalous magnetic moment with world-leading precision. In addition, a suite of experiments is coming online to search for new physics in the form of charged lepton flavor violation in the muon sector. These experiments will probe effective mass scales of new physics up to $10^4$ TeV/c$^2$, far beyond the reach of direct searches at colliders. This article explores the motivations, recent results, and status of these experiments.

hep-ex↗

arXiv2025

The Advanced Muon Facility: a proposed multi-purpose muon facility at Fermilab

Charged lepton flavor violation (CLFV) is expected in a diverse set of new physics scenarios. The current generation of experiments probe CLFV in the muon sector in three complementary channels: $μ^-N \rightarrow e^- N$ (Mu2e, COMET), $μ^+ \rightarrow e^+ γ$ (MEG-II), and $μ^+ \rightarrow e^+e^+e^-$s (Mu3e). These experiments aim to enhance existing limits by several orders-of-magnitude in the coming decade and offer discovery potential to many new physics models. The proposed Advanced Muon Facility (AMF) would be a multi-purpose muon facility based at Fermilab and introduces an innovative approach based on a muon storage ring to enable a full suite of muon CLFV experiments. AMF would host CLFV experiments with sensitivities orders-of-magnitude beyond the present era. In the event of a signal in these currently planned experiments, AMF would enable additional measurements to elucidate the nature of the new physics observed. The design and R$\&$D for AMF is in its infancy. This article outlines the motivations for AMF, detailing on-going R$\&$D efforts, and highlighting potential synergies with the proposed muon collider.

hep-ex↗

arXiv2024

The Mu2e crystal and SiPM calorimeter: construction status

The Mu2e experiment at Fermilab searches for the neutrino-less conversion of a negative muon into an electron, with a distinctive signature of a mono-energetic electron with energy of 104.967 MeV. The calorimeter is made of two disks of pure CsI crystals, each read out by two custom large area UV-extended SiPMs. It plays a fundamental role in providing excellent particle identification capabilities and an online trigger filter while improving the track reconstruction, requiring better than 10% energy and 500 ps timing resolutions for 100 MeV electrons. In this paper, we present the status of construction and the Quality Control (QC) performed on the produced crystals and photosensors, the development of the rad-hard electronics, and the most important results of the irradiation tests. Construction of the mechanics is also reported. Status and plans for the calorimeter assembly and its first commissioning are described.

physics.ins-det↗

arXiv2023

Photon-rejection Power of the Light Dark Matter eXperiment in an 8 GeV Beam

The Light Dark Matter eXperiment (LDMX) is an electron-beam fixed-target experiment designed to achieve comprehensive model independent sensitivity to dark matter particles in the sub-GeV mass region. An upgrade to the LCLS-II accelerator will increase the beam energy available to LDMX from 4 to 8 GeV. Using detailed GEANT4-based simulations, we investigate the effect of the increased beam energy on the capabilities to separate signal and background, and demonstrate that the veto methodology developed for 4 GeV successfully rejects photon-induced backgrounds for at least $2\times10^{14}$ electrons on target at 8 GeV.

hep-ex↗

arXiv2023

Current Status and Future Prospects for the Light Dark Matter eXperiment

The constituents of dark matter are still unknown, and the viable possibilities span a vast range of masses. The physics community has established searching for sub-GeV dark matter as a high priority and identified accelerator-based experiments as an essential facet of this search strategy. A key goal of the accelerator-based dark matter program is testing the broad idea of thermally produced sub-GeV dark matter through experiments designed to directly produce dark matter particles. The most sensitive way to search for the production of light dark matter is to use a primary electron beam to produce it in fixed-target collisions. The Light Dark Matter eXperiment (LDMX) is an electron-beam fixed-target missing-momentum experiment that realizes this approach and provides unique sensitivity to light dark matter in the sub-GeV range. This contribution provides an overview of the theoretical motivation, the main experimental challenges, how LDMX addresses these challenges, and projected sensitivities. We further describe the capabilities of LDMX to explore other interesting new and standard physics, such as visibly-decaying axion and vector mediators or rare meson decays, and to provide timely electronuclear scattering measurements that will inform the modeling of neutrino-nucleus scattering for DUNE.

hep-ex↗

arXiv2023

Recent Results from BABAR: Dark Matter, Axion-like Particles and Heavy Neutral Leptons, a contribution to the 2023 Electroweak session of the 57th Rencontres de Moriond

Three independent searches for new physics using data collected at BABAR are presented. Firstly, two searches for dark matter and baryogenesis: $B^{0}\rightarrow Λ+ ψ_{D}$ and $B^{+} \rightarrow p + ψ_{D}$ are detailed, where $ψ_{D}$ is a new dark fermion. Neither signal is observed and new upper limits on the branching fractions, at the 90 $\%$ confidence level (C.L), are placed at $\mathcal{O}(10^{-5} - 10^{-6})$ across the mass range $1.0< m_{ψ_{D}}<4.3$ GeV/c$^{2}$. Secondly, new limits on the coupling, $g_{aW}$, of an axion-like particle ($a$) to the $W$ boson, at the 90 $\%$ C.L, are presented at $\mathcal{O}(10^{-5})$ GeV$^{-1}$ for $a$ masses in the mass range 0.175 $<m_{a}<$ 4.78 GeV/c$^{2}$. Thirdly, a model-independent search for heavy neutral leptons (HNL) found new upper limits at the 95 $\%$ C.L on the extended Pontecorvo-Maki-Nakagawa-Sakata (PMNS) matrix element, $|U_{τ4}|^{2}$, which depend on the HNL mass hypothesis and vary from $2.31 \times 10^{-2}$ to $5.04 \times 10^{-6}$, across the mass range $100 < m_{4} < 1300$ MeV/c$^{2}$, with more stringent limits on higher HNL masses.

hep-ex↗

arXiv2022

Experimental Searches For Heavy Neutral Leptons

The highly successful Standard Model is not complete. It does not explain the baryonic asymmetry in the Universe, the existence of dark matter or the non-zero masses of the neutrinos. Extensions of the Standard Model that propose the existence of additional Heavy Neutral Leptons (HNLs) are well-motivated and can explain several of these phenomena. In addition, light sterile neutrinos of $\mathcal{O}(\text{eV}/c^{2})$ can explain experimentally observed oscillation anomalies. The Neutrino Minimal Standard Model proposes HNLs with masses $\mathcal{O}(\text{keV}/c^{2} - \text{GeV}/c^{2})$, while more exotic models predict very large masses, up to the GUT scale. Due to the multitude of models which hypothesize HNLs, the mass range to be explored by experiments is large. Experimental searches for HNLs can be conducted at existing neutrino, beam dump and collider-based experiments, and, depending on the signature, can constrain mixing between additional neutrinos and any of the three active neutrinos.

hep-ex↗

arXiv2022

Experimental Searches for Muon to Electron Conversion in a Nucleus: COMET, DeeMe, and Mu2e. A Contributed paper for Snowmass 21

Searches for charged lepton flavor violation (CLFV) offer deep probes for a range of new physics scenarios, such as super-symmetric models, theories involving scalar leptoquarks or additional Higgs doublets, and models explaining the neutrino mass hierarchy and the matter-antimatter asymmetry of the universe via leptogenesis. The coherent, neutrinoless conversion of a muon to an electron in the field of a nucleus, $μ^{-}+N\rightarrow e^{-} +N$, is one example of a muonic CLFV process which has sensitivity to this new physics. This paper details three experiments: COMET, DeeMe and Mu2e which will search for $μ^{-}+N\rightarrow e^{-} +N$ in the coming decade. These experiments offer sensitivity up to an effective new physics mass scale of $\mathcal{O}(10^{4}$ TeV/c$^{2}$), going far beyond what can be achieved in direct, collider-based, searches. The theoretical motivation, designs and anticipated timelines for these three experiments are presented. These experiments are a crucial part of a global search for CLFV. Continued support for all planned experimental searches for muonic CLFV is strongly encouraged.

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