arXiv subjects

Patrick J. Fox

Publications and source records attributed to Patrick J. Fox.

At least 19 recordsLinked to original sources

arXiv2026

For Whom the Xenon Recoils: Magnetic Inelastic Dark Baryons

We develop a theory of a dark baryon dark matter candidate that interacts with nuclei dominantly through inelastic scattering mediated by a transition magnetic dipole operator. Models are presented that can elegantly explain both the scattering rate and the size of the inelastic splitting to be consistent with the one event observed at LZ\@. Elastic scattering is suppressed due to accidental symmetries within the strongly-coupled sector. The nuclear response is dominated by a spin-dependent structure factor, thus we find a large range of masses, $1 \; {\rm TeV} \lesssim m_\chi \lesssim 50 \; {\rm TeV}$, and mass splitting of $100 \; {\rm keV} \lesssim \delta \lesssim 500 \; {\rm keV}$ can fit the data for a variety of possible dark matter velocity distributions. The models predict that some nuclear scattering events will be accompanied by a simultaneous photon signal with energy $\delta$, which not only distinguishes this model from other inelastic dark matter explanations but also enables directional detection in conventional direct detection experiments. We briefly comment on the dark matter abundance, indirect detection signals (including the models' significantly weaker constraints from dark matter annihilation in the Sun), and the associated collider signals that arise from the meson sector of the theory.

hep-ph↗

arXiv2026

Time-dependent signals of new physics at the LHC

The Large Hadron Collider (LHC) is sensitive to signals of beyond the Standard Model physics through a variety of channels including missing energy and resonance searches. In most searches, the new physics and the Standard Model backgrounds are assumed to be invariant in time, up to systematic effects from the experiment. However, new physics with a time variation would provide an additional handle to separate signal from background. Such a time variation may come from ultralight dark matter coupling to an oscillating background field. In this paper, we consider an interaction of dark matter with quarks and an additional heavy particle, and show that the sensitivity of a search that uses timing information at the LHC can be up to a factor of two stronger compared to one that does not use time information.

hep-ph↗

arXiv2026

New physics in multi-lepton tau decays

Dark particles with lepton-flavor-violating couplings to the tau lepton can induce rare neutrinoless $\tau$ decays with large final state multiplicities. We study models where transitions of the type $\tau^\pm\to \ell^\pm\,\phi$, with $\phi$ a light new particle, initiate a chain of decays in the dark sector that terminate with decays into electrons, muons, or pions. These decay cascades appear as rare five or even seven-body $\tau$ decays with multiple reconstructable resonances. We survey several representative models: kinetically mixed dark photon, gauged $L_i-L_j$ models, and other more exotic charge assignments such as chiral $U(1)'$ extensions of the Standard Model. The main new ingredient is the possibility of flavor violation at very high scales. In these models, a number of channels that have not yet been searched for experimentally, such as $\tau\to 5\mu$, $\tau\to 3\mu\,2e$, $\tau\to \mu\,4e$, and hadronic channels like $\tau\to \mu\,4\pi$, typically dominate over the previously-considered signatures such as $\tau \to 3\mu$. While some of the models, such as the gauged $L_i-L_j$ ones, also contain more challenging channels with missing energy due to decays to neutrinos, they can still be searched for via fully visible channels.

hep-ph↗

arXiv2025

$L_\mu-L_\tau$ gauge bosons in beam dumps and supernovae

We study the phenomenology of a sub-GeV $L_\mu-L_\tau$ gauge boson. We find discrepancies with existing literature in sensitivity projections for the upcoming SHiP experiment and in the treatment of supernovae cooling constraints. We present a quantitative analysis of different production modes in beam dumps and compare our results to previous work. In the context of supernovae, we re-evaluate the standard supernova cooling bounds from SN1987A and analyze additional supernova-based probes: diffusive cooling, constraints from the existence of low-energy supernovae, and the absence of a high-energy neutrino signal from SN1987A.

hep-ph↗

arXiv2025

Radio Killed the Axion Star: Constraining Axion Properties with Radio Telescopes

Axion dark matter or any ultralight bosonic dark matter can go through Bose-Einstein condensation due to the large phase density, leading to the formation of axion stars or solitons in dark matter halo centers. The formation rate is enhanced in the presence of the substructures expected in the post-inflationary scenario for the QCD axion or axion-like particles. An axion star will continue to grow until a critical mass is reached, after which it collapses and then explodes, with the emission of relativistic axions, in a process called an ``axinovae.'' There can also be accompanying photon emission due to the stimulated decay of axions in the coherent compact axion star. In axion models with a modest enhancement ($\kappa\sim \mathcal{O}(10)$) of the axion-photon coupling $g_{a\gamma}= \kappa \alpha/(2\pi f_a)$ axinovae will contain a significant flux of radio photons. We determine the range of parameters over which axinovae can be detectable with radio transient searches.

hep-ph↗

arXiv2025

Long-lived Axion-Like Particles from Tau Decays

Axion-like particles (ALPs) are well-motivated examples of light, weakly coupled particles in theories beyond the Standard Model. In this work, we study long-lived ALPs coupled exclusively to leptons in the mass range between $2 m_e$ and $m_\tau - m_e$. For anarchic flavor structure the leptophilic ALP production in tau decays or from ALP-tau bremsstrahlung is enhanced thanks to derivative couplings of the ALP and can surpass production from electron and muon channels, especially for ALPs heavier than $m_\mu$. Using past data from high-energy fixed-target experiments such as CHARM and BEBC we place new constraints on the ALP decay constant $f_a$, reaching scales as high as $\mathcal{O}(10^8)$~GeV in lepton-flavor-violating channels and $f_a \sim \mathcal{O}(10^2)$~GeV in lepton-flavor-conserving ones. We also present projections for the event-rate sensitivity of current and future detectors to ALPs produced at the Fermilab Main Injector, the CERN SPS, and in the forward direction of the LHC. We show that SHiP will be sensitive to $f_a$ values that are over an order of magnitude above the existing constraints.

hep-ph↗

arXiv2025

Direct Detection of Ultralight Dark Matter via Charged Lepton Flavor Violation

We propose a dark matter direct-detection strategy using charged particle decays at accelerator-based experiments. If ultralight $(m_\phi \ll \text{eV})$ dark matter has a misalignment abundance, its local field oscillates in time at a frequency set by its mass. If it also couples to flavor-changing neutral currents, rare exotic decays such as $\mu \to e \phi'$ and $\tau\to e(\mu)\phi'$ inherit this modulation. Focusing on such charged lepton flavor-violating decays, we show that sufficient event samples can enable detection of ultralight dark matter candidates at Mu3e, Belle-II, and FCC-ee.

hep-ph↗

arXiv2024

Long-lived vectors from electromagnetic cascades at SHiP

We simulate dark-vector, $V$, production from electromagnetic cascades at the recently approved SHiP experiment. The cascades (initiated by photons from $\pi^0\rightarrow \gamma \gamma$) can lead to 3-4 orders of magnitude increase of the event rate relative to using primary production alone. We provide new SHiP sensitivity projections for dark photons and electrophilic gauge bosons, which are significantly improved compared to previous literature. The main gain in sensitivity occurs for long-lived dark vectors with masses below $\sim 50-300~{\rm MeV}$. The dominant production mode in this parameter space is low-energy annihilation $e^+ e^- \rightarrow V(\gamma)$. This motivates a detailed study of backgrounds and efficiencies in the SHiP experiment for sub-GeV signals.

hep-ph↗

arXiv2024

Muon-induced baryon number violation

The search for charged-lepton flavor violation in muon capture on nuclei is a powerful probe of heavy new physics. A smoking gun signal for $\mu \rightarrow e$ conversion is a monochromatic electron with energy almost equal to the muon mass. We show that light new physics can mimic this signature and that it can also lead to electrons above the $\mu \rightarrow e$ signal peak. A concrete example of such light new physics is $\mu^- $-nucleon annihilation into a light dark sector, which can produce an energetic $e^-$ as well as $e^+e^-$ byproducts. Due to the size of the muon mass, the exotic muon capture process can be kinematically allowed, while the otherwise stringent constraints, e.g., from proton decay, are kinematically forbidden. We also discuss other relevant constraints, including those from the stability of nuclei and muon capture in the interior of neutron stars.

hep-ph↗

arXiv2024

Axion Stars: Mass Functions and Constraints

The QCD axion and axion-like particles, as leading dark matter candidates, can also have interesting implications for dark matter substructures if the Peccei-Quinn symmetry is broken after inflation. In such a scenario, axion perturbations on small scales will lead to the formation of axion miniclusters at matter-radiation equality, and subsequently the formation of axion stars. Such compact objects open new windows for indirect searches for axions. We compute the axion star mass function based on recent axion minicluster studies and Bose star simulations. Applying this mass function, we find post-inflation axion-like particles with masses $1.8 \times 10^{-21}~{\rm eV}<m_a< 3.3 \times 10^{-17}~{\rm eV}$ are constrained by the lack of dynamical heating of stars in ultrafaint dwarfs. We also find that current microlensing surveys are insensitive to QCD axion stars. While we focus on the gravitational detectability of axion stars, our result can be directly applied to other interesting signatures of axion stars, e.g. their decay to photons, that require as input the abundance, mass, and density distribution of axion stars.

hep-ph↗

arXiv2024

Earth-Catalyzed Detection of Magnetic Inelastic Dark Matter with Photons in Large Underground Detectors

Inelastic dark matter with moderate splittings, $\mathcal{O}({\rm few} \; {\rm to} \; 150)$ keV, can upscatter to an excited state in the Earth, with the excited state subsequently decaying, leaving a distinctive monoenergetic photon signal in large underground detectors. The photon signal can exhibit sidereal-daily modulation, providing excellent separation from backgrounds. Using a detailed numerical simulation, we examine this process as a search strategy for magnetic inelastic dark matter with the dark matter mass near the weak scale, where the upscatter to the excited state and decay proceed through the same magnetic dipole transition operator. At lower inelastic splittings, the scattering is dominated by moderate mass elements in the Earth with high spin, especially $^{27}$Al, while at larger splittings, $^{56}$Fe becomes the dominant target. We show that the proposed large volume gaseous detector CYGNUS will have excellent sensitivity to this signal. Xenon detectors also provide excellent sensitivity through the inelastic nuclear recoil signal, and if a future signal is seen, we show that the synergy among both types of detection can provide strong evidence for magnetic inelastic dark matter. In the course we have calculated nuclear response functions for elements relevant for scattering in the Earth, which are publicly available on GitHub.

hep-ph↗

arXiv2024

Extracting Halo Independent Information from Dark Matter Electron Scattering Data

Direct detection experiments and the interpretation of their results are sensitive to the velocity structure of the dark matter in our galactic halo. In this work, we extend the formalism that deals with such astrophysics-driven uncertainties, originally introduced in the context of dark-matter-nuclear scattering, to include dark-matter-electron scattering interactions. Using mock data, we demonstrate that the ability to determine the correct dark matter mass and velocity distribution is depleted for recoil spectra which only populate a few low-lying bins, such as models involving a light mediator. We also demonstrate how this formalism allows one to test the compatibility of existing experimental data sets (e.g. SENSEI and EDELWEISS), as well as make predictions for possible future experiments (e.g. GaAs-based detectors).

hep-ph↗

arXiv2024

Dark fluxes from electromagnetic cascades

We study dark sector production in electromagnetic (EM) cascades. This problem requires accurate simulations of Standard Model (SM) and dark sector processes, both of which impact angular and energy distributions of emitted particles that ultimately determine flux predictions in a downstream detector. We describe the minimal set of QED processes which must be included to faithfully reproduce a SM cascade, and identify a universal algorithm to generate a dark sector flux given a Monte-Carlo simulation of a SM shower. We provide a new tool, $\texttt{PETITE}$, which simulates EM cascades with associated dark vector production, and compare it against existing literature and "off the shelf" tools. The signal predictions at downstream detectors can strongly depend on the nontrivial interplay (and modelling) of SM and dark sector processes, in particular multiple Coulomb scattering and positron annihilation. We comment on potential impacts of these effects for realistic experimental setups.

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

New Physics Searches at Kaon and Hyperon Factories

Rare meson decays are among the most sensitive probes of both heavy and light new physics. Among them, new physics searches using kaons benefit from their small total decay widths and the availability of very large datasets. On the other hand, useful complementary information is provided by hyperon decay measurements. We summarize the relevant phenomenological models and the status of the searches in a comprehensive list of kaon and hyperon decay channels. We identify new search strategies for under-explored signatures, and demonstrate that the improved sensitivities from current and next-generation experiments could lead to a qualitative leap in the exploration of light dark sectors.

hep-ph↗

arXiv2023

Recurrent Axinovae and their Cosmological Constraints

Axion-like dark matter whose symmetry breaking occurs after the end of inflation predicts enhanced primordial density fluctuations at small scales. This leads to dense axion minihalos (or miniclusters) forming early in the history of the Universe. Condensation of axions in the minihalos leads to the formation and subsequent growth of axion stars at the cores of these halos. If, like the QCD axion, the axion-like particle has attractive self-interactions there is a maximal mass for these stars, above which the star rapidly shrinks and converts an $\mathcal{O}(1)$ fraction of its mass into unbound relativistic axions. This process would leave a similar (although in principle distinct) signature in cosmological observables as a decaying dark matter fraction, and thus is strongly constrained. We place new limits on the properties of axion-like particles that are independent of their non-gravitational couplings to the standard model.

hep-ph↗

arXiv2022

TF08 Snowmass Report: BSM Model Building

We summarize the state of Beyond the Standard Model (BSM) model building in particle physics for Snowmass 2021, focusing mainly on several whitepaper contributions to BSM model building (TF08) and closely related areas.

hep-ph↗

arXiv2022

DIMUS: Super-Compact Dimuonium Spectroscopy Collider at Fermilab

While dimuonium $(μ^+μ^-)$ has not yet been observed, it is of utmost fundamental interest. By virtue of the larger mass, dimuonium has greater sensitivity to beyond the standard model effects than its cousins positronium or muonium, both discovered long ago, while not suffering from large QCD uncertainties. Dimuonium atoms can be created in $e^+e^-$ collisions with large longitudinal momentum, allowing them to decay a small distance away from the beam crossing point and avoid prompt backgrounds. We envision a unique cost-effective and fast-timeline opportunity for copious production of $(μ^+μ^-)$ atoms at the production threshold via a modest modification of Fermilab's existing FAST/NML facility to arrange collisions of 408 MeV electrons and positrons at a 75$^{\rm o}$ angle. This compact 23 m circumference collider (DIMUS) will allow for precision tests of QED and open the door for searches for new physics coupled to the muon. Fermilab's FAST/NML is perfectly suited for DIMUS as there are existing SRF accelerators and infrastructure, capable of producing high energy, high current electron and positron beams, sufficient for $O(10^{32})\mathrm{cm}^2\mathrm{s}^{-1}$ luminosity and $\sim$0.5 million dimuons per year. The expansion will require installation of a second SRF cryomodule, positron production and accumulation system, fast injection/extraction kickers and two small circumference intersecting rings. An approximately meter-sized detector with several layers of modern pixelated silicon detector and crystal-based electromagnetic calorimeters will ensure observation of the decays of dimuonium to electron-positron pairs in presence of the Bhabba scattering background. An expansion of the system to would extend the physics program of DIMUS to include precision studies of rare processes with muons, pions, and $η$ mesons produced in $e^{+}e^{-}$ collisions.

hep-ex↗