arXiv subjects

Sebastian A. R. Ellis

Publications and source records attributed to Sebastian A. R. Ellis.

At least 19 recordsLinked to original sources

arXiv2026

Ultralight dark matter detection with trapped-ion interferometry

We explore how recent advances in the manipulation of single-ion wave packets open new avenues for detecting weak magnetic fields sourced by ultralight dark matter. A trapped ion in a ``Schrödinger cat'' state can be prepared with its spin and motional degrees of freedom entangled and be used as a matter-wave interferometer that is sensitive to the Aharonov-Bohm-like phase shift accumulated by the ion over its trajectory. The result of the spin-motion entanglement is a parametrically-enhanced sensitivity to weak magnetic fields as compared with an un-entangled ion in a trap. Taking into account the relevant boundary conditions, we demonstrate that a single trapped ion can probe unexplored regions of kinetically-mixed dark-photon dark matter parameter space in the $10^{-15}~\text{eV} \lesssim m_{A'} \lesssim 10^{-14}$~eV mass window. We also show how such a table-top quantum device will also serve as a complementary probe of axion-like particle dark matter in the same mass window.

hep-ph↗

arXiv2026

Improving Neutrino Oscillation Measurements through Event Classification

Precise neutrino energy reconstruction is essential for next-generation long-baseline oscillation experiments, yet current methods remain limited by large uncertainties in neutrino-nucleus interaction modeling. Even so, it is well established that different interaction channels produce systematically varying amounts of missing energy and therefore yield different reconstruction performance--information that standard calorimetric approaches do not exploit. We introduce a strategy that incorporates this structure by classifying events according to their underlying interaction type prior to energy reconstruction. Using supervised machine-learning techniques trained on labeled generator events, we leverage intrinsic kinematic differences among quasi-elastic scattering, meson-exchange current, resonance production, and deep-inelastic scattering processes. A cross-generator testing framework demonstrates that this classification approach is robust to microphysics mismodeling and, when applied to a simulated DUNE $ν_μ$ disappearance analysis, yields improved accuracy and sensitivity at the 10-20% level. These results highlight a practical path toward reducing reconstruction-driven systematics in future oscillation measurements.

hep-ph↗

arXiv2026

Crowdsourcing Gravitational Waves from Superradiant Axions

Black hole superradiance is a powerful probe of ultralight axions. If nature contains a boson with a mass of order $10^{-12}\,$eV, $\textit{mere vacuum fluctuations}$ will lead to its efficient production around spinning stellar mass black holes, forming a gravitational atom that both drains the black hole spin and decays to produce near-monochromatic gravitational waves. Existing superradiance constraints derive primarily from spin measurements of a handful of identified black holes. Here we instead present a detailed study of the population level effect: gravitational waves arising from both the 100 million black holes in the Milky Way and the stochastic signal from axion clouds throughout the universe. We study the impact of a broad range of systematic uncertainties on the black hole properties and compute the projected axion sensitivity for LIGO, as well as the future instruments Einstein Telescope, Cosmic Explorer, and a high-frequency Magnetic Weber Bar. We demonstrate that LIGO can robustly probe axion masses from roughly $10^{-13}\,$eV to $4 \times 10^{-12}\,$eV. If the black hole population extends to masses slightly below $5\,M_{\odot}$ - as hinted for by LIGO inspiral observations - LIGO would approach $10^{-11}\,$eV. Under that same assumption we show that a future high-frequency detector could push considerably higher, potentially beyond $10^{-10}\,$eV in the most optimistic scenarios, reaching towards the lowest masses within the projected sensitivity of axion dark matter searches.

hep-ph↗

arXiv2026

A Prototype Hybrid Mode Cavity for Heterodyne Axion Detection

In the heterodyne approach to axion detection, axion dark matter induces transitions between two modes of a microwave cavity, resulting in a parametrically enhanced signal power. We describe the fabrication and characterization of a prototype normal conducting cavity specifically optimized for heterodyne detection. Corrugations on the cavity walls support linearly polarized hybrid modes which maximize the signal power while strongly suppressing noise. We demonstrate tuning mechanisms which allow one mode's frequency to be scanned across a 4 MHz range, while suppressing cross-coupling noise by at least 80 dB. A future superconducting cavity with identical geometry to our prototype would have the potential to probe orders of magnitude beyond astrophysical bounds.

physics.ins-det↗

arXiv2026

The COSMIC WISPers White Paper: The physics case for Weakly Interacting Slim Particles

Axions and other very weakly interacting slim particles (WISPs), with masses below 1 GeV, arise naturally in many extensions of the Standard Model of particle physics. In particular, they could offer a new framework to explain the nature of dark matter and may help address a range of puzzling observations in astrophysics and particle physics. This review provides an overview of ongoing WISP searches and outlines the prospects for the next decade, spanning their theoretical motivation, indirect signatures in astrophysical observations, and dedicated laboratory experiments. It is based on the work carried on by the EU-funded COST Action ``Cosmic WISPers in the Dark Universe: Theory, astrophysics, and experiments'' (CA21106, https://www.cost.eu/actions/CA21106). This network plays a key role in coordinating and supporting WISP searches across Europe, while also contributing to the development of a roadmap aimed at securing European leadership in this research area. It is emphasized that Europe is currently pursuing a rich, diverse, and cost-effective experimental program, with the potential to deliver one or more transformative discoveries.

hep-ph↗

arXiv2025

Soft Gravitons, Hard Truths: Infrared Safety of Particle Processes in a Gravitational-Wave Background

Gravitational waves are thought to propagate unattenuated through matter due to a cancellation between graviton absorption and stimulated emission inferred from leading-order soft-graviton arguments. We revisit this reasoning and show that it fails for the converse problem: the effect of a gravitational-wave background on matter. For unstable particles, real graviton emission \emph{and} absorption appear to enhance decay rates. By extending the soft-graviton framework describing real and virtual processes in a gravitational wave background, and resumming them to all orders, we show that inclusive decay rates remain essentially unchanged. The mutual transparency between matter and gravitational radiation thus follows from infrared safety, and not from a fortuitous cancellation in the lowest-order approximation of exclusive rates.

hep-ph↗

arXiv2025

Magnets are Weber Bar Gravitational Wave Detectors

When a gravitational wave (GW) passes through a DC magnetic field, it couples to the conducting wires carrying the currents which generate the magnetic field, causing them to oscillate at the GW frequency. The oscillating currents then generate an AC component through which the GW can be detected - thus forming a resonant mass detector or a Magnetic Weber Bar. We quantify this claim and demonstrate that magnets can have exceptional sensitivity to GWs over a frequency range demarcated by the mechanical and electromagnetic resonant frequencies of the system; indeed, we outline why a magnetic readout strategy can be considered an optimal Weber bar design. The concept is applicable to a broad class of magnets, but can be particularly well exploited by the powerful magnets being deployed in search of axion dark matter, for example by DMRadio and ADMX-EFR. Explicitly, we demonstrate that the MRI magnet that is being deployed for ADMX-EFR can achieve a broadband GW strain sensitivity of $\sim$$10^{-20}/\sqrt{\text{Hz}}$ from a few kHz to about 10 MHz, with a peak sensitivity down to $\sim$$10^{-22}/\sqrt{\text{Hz}}$ at a kHz exploiting a mechanical resonance.

hep-ph↗

arXiv2025

Classical (and Quantum) Heuristics for Gravitational Wave Detection

We derive a lower bound on the sensitivity of generic mechanical and electromagnetic gravitational wave detectors. We consider both classical and quantum detection schemes, although we focus on the former. Our results allow for a simple reproduction of the sensitivities of a variety of experiments, including optical interferometers, resonant bars, optomechanical sensors, and electromagnetic conversion experiments. In the high-frequency regime, all detection schemes we consider can be characterised by their stored electromagnetic energy and the signal transfer function, which we provide. We discuss why high-frequency gravitational wave searches are especially difficult and primordial gravitational wave backgrounds might not be detectable above the sensitivity window of existing interferometers.

gr-qc↗

arXiv2025

Dark Matter Nuclear Magnetic Resonance is Sensitive to Dark Photons and the Axion-Photon Coupling

We demonstrate that nuclear magnetic resonance based searches for dark matter (DM) have intrinsic and powerful sensitivity to dark photons and the axion-photon coupling. The reason is conceptually straightforward. An instrument such as CASPEr-Gradient begins with a large sample of nuclear spins polarised in a background magnetic field. In the presence of axion DM coupled to nucleons, the spin ensemble feels an effective magnetic field $\mathbf{B} \propto \nabla a$ that tilts the spins, generating a potentially observable precession. If the magnetic field is real rather than effective, the system responds identically. A real field can be generated by a kinetically mixed dark photon within the shielded region the sample is placed or an axion coupled to photons through its interaction with the background magnetic field. We show that all three signals are detectable and distinguishable. If CASPEr-Gradient were to reach the QCD axion prediction of the axion-nucleon coupling, it would simultaneously be sensitive to kinetic mixings of $ε\simeq 3 \times 10^{-16}$ and axion-photon couplings of $g_{aγγ} \simeq 2 \times 10^{-16}\,{\rm GeV}^{-1}$ for $m \simeq 1\,μ{\rm eV}$.

hep-ph↗

arXiv2025

Momentum and Matter Matter for Axion Dark Matter Matters on Earth

We investigate the implications of matter effects to searches for axion Dark Matter on Earth. The finite momentum of axion Dark Matter is crucial to elucidating the effects of Earth on both the axion Dark Matter field value and its gradient. We find that experiments targeting axion couplings compatible with canonical solutions of the strong CP puzzle are likely not affected by Earth's matter effects. However, experiments sensitive to lighter axions with stronger couplings can be significantly affected, with a significant part of the parameter space suffering from a reduced axion field value, and therefore decreased experimental sensitivity. In contrast, the spatial gradient of the axion field can be enhanced along Earth's radial direction, with important implications for ongoing and planned experiments searching for axion Dark Matter.

hep-ph↗

arXiv2024

Dielectric Haloscopes as Gravitational Wave Detectors

We argue that dielectric haloscopes like MADMAX, originally designed for detecting axion dark matter, are also very promising gravitational wave detectors. Operated in resonant mode at frequencies around $\mathcal{O}(10\,\text{GHz})$, these detectors benefit from enhanced gravitational wave to photon conversion at the surfaces of a stack of thin dielectric disks. Since the gravitational wave is relativistic, there is an additional enhancement of the signal compared to the axion case due to increased conversion probability of gravitational waves to photons in the vacuum between the disks. A gravitational wave search using a dielectric haloscope imposes stringent requirements on the disk thickness and placement, but relaxed requirements on the disk smoothness. An advantage is the possibility of a broadband or hybrid resonant/broadband operation mode, which extends the frequency range down to $\mathcal{O}(100\,\text{MHz})$. We show that strain sensitivities down to $10^{-21} \text{Hz}^{-1/2} \times (10\,\text{GHz}/f)$ will be possible in the coming years for the broadband setup, while a resonant setup optimized for gravitational waves could even reach $3\times 10^{-23} \text{Hz}^{-1/2} \times (10\,\text{GHz}/f)$ with current technology.

hep-ph↗

arXiv2024

Resonant Conversion of Gravitational Waves in Neutron Star Magnetospheres

High frequency gravitational waves are the subject of rapidly growing interest in the theoretical and experimental community. In this work we calculate the resonant conversion of gravitational waves into photons in the magnetospheres of neutron stars via the inverse Gertsenshtein mechanism. The resonance occurs in regions where the vacuum birefringence effects cancel the classical plasma contribution to the photon dispersion relation, leading to a massless photon in the medium which becomes kinematically matched to the graviton. We set limits on the amplitude of a possible stochastic background of gravitational waves using X-ray and IR flux measurements of neutron stars. Using Chandra ($2-8\,\text{keV}$) and NuSTAR ($3-79\,\text{keV}$) observations of RX J1856.6-3754, we set strain limits $h_c^{\rm lim} \simeq 10^{-26} - 10^{-24}$ in the frequency range $ 5\times 10^{17}\, {\rm Hz} \lesssim f \lesssim 2\times 10^{19}\,\text{Hz}$. Our limits are many orders of magnitude stronger than existing constrains from individual neutron stars at the same frequencies. We also use recent JWST observations of the Magnetar 4U 0142+61 in the range $2.7\times 10^{13}\, {\rm Hz} \lesssim f \lesssim 5.9\times 10^{13}\, {\rm Hz} $, setting a limit $h_{\rm c}^{\rm lim} \simeq 5 \times 10^{-19}$. These constraints are in complementary frequency ranges to laboratory searches with CAST, OSQAR and ALPS II. We expect these limits to be improved both in reach and breadth with a more exhaustive use of telescope data across the full spectrum of frequencies and targets.

hep-ph↗

arXiv2024

Resonant Conversion of Wave Dark Matter in the Ionosphere

We consider resonant wave-like dark matter conversion into low-frequency radio waves in the Earth's ionosphere. Resonant conversion occurs when the dark matter mass and the plasma frequency coincide, defining a range $m_{ \text{DM} } \sim 10^{-9} - 10^{-8}$ eV where this approach is best suited. Owing to the non-relativistic nature of dark matter and the typical variational scale of the Earth's ionosphere, the standard linearized approach to computing dark matter conversion is not suitable. We therefore solve a second-order boundary-value problem, effectively framing the ionosphere as a driven cavity filled with a positionally-varying plasma. An electrically-small dipole antenna targeting the generated radio waves can be orders of magnitude more sensitive to dark photon and axion-like particle dark matter in the relevant mass range. The present study opens up a promising way of testing hitherto unexplored parameter space which could be further improved with a dedicated instrument.

hep-ph↗

arXiv2023

Quadratic Coupling of the Axion to Photons

We show that the QCD axion couples to the electromagnetic kinetic term at one loop. The result is that if axions make up dark matter, they induce temporal variation of the fine structure constant $α$, which is severely constrained. We recast these constraints on the QCD axion parameter space. We also discuss how to generalise our finding to axion-like particles, and the resulting constraints.

hep-ph↗

arXiv2023

Signals of Millicharged Dark Matter in Light-Shining-Through-Wall Experiments

We discuss a novel detection technique for millicharged dark matter that makes use of existing light-shining-through-wall (LSW) experiments searching for massive dark photons. Since millicharged particles interact with both the visible and dark sectors, a small background of such particles enables the search for visible signals even in the limit of a massless dark photon. Furthermore, for sufficiently large couplings, a small dark matter subcomponent consisting of millicharged particles can have a terrestrial abundance that is significantly enhanced with respect to its galactic abundance. We leverage this fact to show that in certain parts of parameter space, future runs of the LSW Dark SRF experiment can be used to set the strongest limits on millicharged relics.

hep-ph↗

arXiv2023

Detecting High-Frequency Gravitational Waves with Microwave Cavities

We give a detailed treatment of electromagnetic signals generated by gravitational waves (GWs) in resonant cavity experiments. Our investigation corrects and builds upon previous studies by carefully accounting for the gauge dependence of relevant quantities. We work in a preferred frame for the laboratory, the proper detector frame, and show how to resum short-wavelength effects to provide analytic results that are exact for GWs of arbitrary wavelength. This formalism allows us to firmly establish that, contrary to previous claims, cavity experiments designed for the detection of axion dark matter only need to reanalyze existing data to search for high-frequency GWs with strains as small as $h\sim 10^{-22}-10^{-21}$. We also argue that directional detection is possible in principle using readout of multiple cavity modes. Further improvements in sensitivity are expected with cutting-edge advances in superconducting cavity technology.

hep-ph↗

arXiv2023

MAGO$\,$2.0: Electromagnetic Cavities as Mechanical Bars for Gravitational Waves

Superconducting cavities can operate analogously to Weber bar detectors of gravitational waves, converting mechanical to electromagnetic energy. The significantly reduced electromagnetic noise results in increased sensitivity to high-frequency signals well outside the bandwidth of the lowest mechanical resonance. In this work, we revisit such signals of gravitational waves and demonstrate that a setup similar to the existing "MAGO" prototype, operating in a scanning or broadband manner, could have sensitivity to strains of $\sim 10^{-22} - 10^{-18}$ for frequencies of $\sim 10 \ \text{kHz} - 1 \ \text{GHz}$.

hep-ph↗

arXiv2022

Snowmass Theory Frontier: Astrophysics and Cosmology

We summarize progress made in theoretical astrophysics and cosmology over the past decade and areas of interest for the coming decade. This Report is prepared as the TF09 "Astrophysics and Cosmology" topical group summary for the Theory Frontier as part of the Snowmass 2021 process.

hep-ph↗