Illuminating Dark Matter (2025)

The Simons Symposium on Illuminating Dark Matter, held 1–7 June 2025, was the third and final meeting of the series. As with its predecessors in 2018 and 2023, the central theme was the search for the identity of particle dark matter. In addition, however, the organizers of this year’s Symposium, Rouven Essig and Jonathan Feng, took note of the increased synergy between probes of dark matter and neutrinos, and also encouraged the participants to think more broadly about the connections of dark matter to other new phenomena.

This year’s symposium also took place against the backdrop of prioritization exercises in the United States and Europe that are focused on very large and expensive projects, which will, at best, yield results decades in the future. The focus this year, then, was to develop novel ways to discover new particles without the enormous costs and timescales of conventional approaches. These new directions leverage, for example, existing large facilities, new ultrasensitive detector technologies, and data from astrophysical systems, while also taking a new critical look at what constitutes a viable dark matter candidate. The symposium brought together leading particle and astroparticle theorists and experimentalists in a specially-designed program with much of the time reserved for unstructured discussion to facilitate this goal.

A number of participants talked about experiments, both ongoing and proposed, that exploit existing large facilities. Felix Kling discussed FASER (ForwArd Search ExpeRiment), an experiment initiated at the first Simons Symposium on IDM in 2018, which has now been constructed and has been collecting data at the Large Hadron Collider at CERN for 3 years. Kling showed FASER’s world-leading sensitivities to a variety of new particles and the first direct detection of collider neutrinos, which is now motivating upgraded detectors and the Forward Physics Facility, a new underground cavern at CERN. Miriam Diamond discussed MATHUSLA (Massive Timing Hodoscope for Ultra Stable neutraL pArticles), a proposal to construct a large detector on the Earth’s surface near the CMS experiment at CERN. With a decay volume of about 40 m x 40 m x 10 m, MATHUSLA targets a wide variety of beyond-the-Standard-Model ultra-long-lived particles, which may be produced at the High-Luminosity LHC in large numbers, but escape the main detectors. Zhen Liu discussed the possibility of detecting milli-charged particles in particle beam experiments, such as DUNE (Deep Underground Neutrino Experiment) and ArgoNeuT (Argon Neutrino Teststand), demonstrating the synergy between new particle and neutrino experiments. Liu also showed the potential to detect sub-meV dark photons through light-shining-through-walls experiments using high-Q, superconducting RF cavities.

Craig Group reviewed progress on LDMX (Light Dark Matter Experiment), a proposed experiment that would exploit the Linac Coherent Light Source at SLAC to search for missing energy signals, with many orders-of-magnitude improvement in sensitivity over current bounds on, for example, invisibly-decaying dark photons. Group also highlighted the capabilities of Fermilab experiments, such as NOvA, DUNE, and the proposed DarkQuest to search for long-lived particles, along with a new proposal for a Fermilab Facility for Dark Sector Discovery. Kate Scholberg summarized the rapid progress in recent years in discovering and studying coherent elastic neutrino-nucleus scattering (CEvNS) at spallation sources. Such detectors also provide excellent sensitivity to light (MeV-GeV) dark matter, and Scholberg discussed, in particular, the prospects for extremely sensitive detectors at Oak Ridge National Laboratory.

As in previous symposia in this series, a few participants focused on leveraging the incredible progress that has been made with new, ultrasensitive detector technologies. Juan Estrada discussed the remarkable Skipper-CCD technology, capable of sensing even a single electron with sub-electron noise precision. He reviewed the applications of this technology for dark matter direct detection in SENSEI, DAMIC-M, Oscura, and DarkNESS, for dark-sector searches at nuclear reactors and with particle beams, and in telescopes. Masha Baryakhtar described how disordered dielectric materials can be used to enhance the detection of axion and dark-photon dark matter in the ~10 meV to eV mass range. She showed that even a detector whose target material is table salt has sensitivity to new regions of parameter space if coupled with superconducting nanowire single photon detectors (SNSPD) that reads out the photon generated by the dark matter converting in the dielectric medium. Asher Berlin showed that if millicharged particles exist and their charge is not too small, they can accumulate on the Earth, reaching densities many orders of magnitude higher than the ambient density of dark matter in wide regions of parameter space. He reviewed how these particles can be probed using a variety of ideas, before discussing a new idea which is an adaptation of an idea from Cavendish that can be traced back all the way to approximately 1773.

Yotam Soreq pointed out the interplay between precision and intensity probes in searching for particles beyond the Standard Model. He showed how precision spectroscopy data can probe bosonic (spin-0) scalar and pseudoscalar particles. In some cases, the existing bounds are superseded by precision kaon decays, but for particles producing new hadronic interactions, precision spectroscopy is a powerful probe. He contrasted this with searches at LUXE (a high-intensity photon source) and the Electron-Ion Collider.

Ideas for detecting hard-to-probe dark matter candidates, such as very massive particles or particles with very low interactions with ordinary matter, were also presented. Joseph Bramante discussed how neutron stars can be used to detect thermal dark matter candidates. In particular, these can be captured by neutron stars and, depending on the dark matter properties, thermalize and/or annihilate, in both cases heating the neutron star. Finding a very cold neutron star would disfavor such dark matter candidates. He also discussed how heavy particles could create tracks in ancient mica. Maxim Pospelov also showed how neutron stars uniquely probe a range of dark-sector particles, including very heavy dark matter with a small direct-detection cross section, a dark neutron close in mass to the ordinary neutron, and a dark matter particle whose abundance is generated by the freeze-in mechanism via a massive mediator. Such particles will either heat the neutron star or turn it into a black hole, with both possibilities being constrained. Using astrophysical systems to probe the properties of dark matter particles has been a constant theme throughout the three symposia meetings.

Hai-Bo Yu continued this theme by presenting several intriguing pieces of evidence for the idea that dark matter particles can interact strongly with each other, from explaining the diversity of galaxy rotation curves, to providing an explanation for the gap and spur features observed in the GD-1 stellar stream. He particularly emphasized that self-interacting dark matter can give rise to a range of halo density profiles, with some being cored and others undergoing gravothermal collapse and becoming very cuspy. This prediction can be further tested using observational data from JWST and the Rubin Observatory.

A number of participants gave talks on new ideas for dark matter production and the related problem of baryogenesis. Brian Batell explored the possibility that dark matter is stable as a consequence of the Pauli exclusion principle. A variety of considerations focus attention on a scalar dark matter particle lighter than 10 meV, decaying to fermions, and Batell explored the possibility that this fermion could even be the Standard Model neutrinos, with dark matter decays then enhancing the cosmic neutrino background. Stefania Gori reviewed the classic axion dark matter candidate, which is typically too light and weakly-coupled to be amenable to, for example, direct-detection experiments. Gori then highlighted several interesting generalizations to the more general class of axion-like particles, which are amenable to accelerator probes, as well as axion-mediated dark-matter interactions, which can introduce significant dark-matter self-interactions, as well as spin-dependent interactions in matter and phonon excitations in crystals.

Anson Hook discussed relaxation solutions to the dark matter-baryon coincidence problem in the context of QCD axion dark matter. In a class of models he described, the ratio of dark matter to baryon energy densities is a ratio of beta functions, and the observed value of 5.36 for this ratio could be reproduced by a simple choice of an integer-valued free parameter. Brian Shuve presented a model in which dark matter is produced by the freeze-in mechanism, which also generates the baryon asymmetry. The model predicts new particles with masses at the TeV scale or below, and motivates searches for new electroweak states with mass around 100 GeV, which, remarkably, are not yet excluded for certain decay branching ratios to leptons.

A hallmark of these symposia has been to encourage the participants to discuss whatever topic they are most excited about, even if it does not fall directly under the dark matter rubric. Glennys Farrar focused on the origin of ultra-high energy cosmic rays, presenting an intriguing series of arguments for why they originate from binary neutron star mergers, in particular that they are accelerated in the magnetized turbulent outflows occurring during the merger event. She described predictions for the cosmic-ray spectrum and composition, as well as future tests of this hypothesis. Subir Sarkar presented a critical challenge to the standard cosmological model. Given the wealth of cosmological data, he argued that we must test whether the universe is isotropic and homogeneous, a fundamental assumption. He described how data show that the inferred accelerated expansion rate from Type Ia supernovae is likely an artifact of us being in a bulk flow, and how the rest frame of distant matter does not seem to be the rest frame inferred from the cosmic microwave background.

To ensure ample time for discussion, the schedule did not allow for the organizers and all participants to give a formal talk. Instead, Essig and Feng, as well as the remaining participants Céline Boehm, Gaia Lanfranchi, and Sebastian Trojanowski, participated extensively in the discussions that took place during and after the talks, and during the coffee breaks, meal times, and in the afternoons.

To summarize, the third Illuminating Dark Matter Simons Symposium once again brought together ambitious and interactive leaders in the field of dark matter. The many new ideas generated during it will undoubtedly lead to new progress in our quest to identify dark matter. We are grateful for the extraordinary financial and administrative support provided by the Simons Foundation to enable this progress.

Masha Baryakhtar
University of Washington

Order and Disorder: Dielectrics for Wave Dark Matter Detection
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Theories that seek to explain the outstanding puzzles of the Standard Model of particle physics often predict ultralight, feebly-interacting particles. These ultralight particles—scalars, axions, and dark photons—are often produced in the early universe. I will focus on my experimental proposals based on dielectric metamaterials, in which axion and dark photon dark matter can efficiently convert to detectable single photons. While an ordered stack provides the deepest dark matter reach, embracing the randomness of a disordered powder allows for a robust, broadband search.
Brian Batell
University of Pittsburgh

Cosmic Stability of Dark Matter from Pauli Blocking
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I will discuss a novel sub-eV scalar dark matter (DM) candidate whose stability is due to the Pauli exclusion of its fermionic decay products. The stability of the DM condensate against decays, scatterings, and parametric resonance will be examined. Scattering can populate an interacting thermal dark sector component to energies far above the DM mass which can be probed through precision cosmology. If the lightest neutrino stabilizes the DM, the cosmic neutrino background (CnuB) can be significantly altered from the standard cosmology and thus be probed in the future by CnuB detection experiments.
Asher Berlin
Fermilab

Cavendish Tests of Millicharged Particles
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A terrestrial population of millicharged particles that interact significantly with normal matter can arise if, e.g., they make up a dark matter subcomponent or if they are light enough to be produced in cosmic ray air showers. Such particles may have evaded detection to date if, through repeated scatters, they rapidly thermalize down to terrestrial temperatures, well below the thresholds of most existing dark matter detectors. I will discuss how reinterpretations of Cavendish tests of Coulomb’s Law, first performed in the late 18th century, provide some of the strongest bounds on this largely unexplored parameter space. I will also propose a simple modification to explore an even larger region of new parameter space, with sensitivity to the irreducible millicharge density generated from cosmic rays.
Joe Bramante
Queen’s University

Prospects for Neutron Stars as Dark Matter Detectors
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I will review work over the past decade on the possibility that thermal measurements of neutron stars could serve as an incisive, next generation search for dark matter interactions.
Craig Group
University of Virginia

The Status of LDMX and Related Opportunities
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The Light Dark Matter eXperiment (LDMX) is a proposed electron-beam fixed-target experiment that would take place at SLAC and run parasitically with unused electrons from the LCLS-II electron gun. The small-scale experiment promises unprecedented sensitivity to the dark sector in the 1-100 MeV mass scale. Physics opportunities and the status of the planning effort for LDMX will be covered. Other opportunities, related to the beam upgrade at Fermilab, will also be mentioned.
Miriam Diamond
University of Toronto

Searches for Long-Lived Particles with MATHUSLA
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Long-Lived Particles (LLPs) beyond the Standard Model emerge in numerous theoretical frameworks that address key open questions in physics — including dark matter, as well as the hierarchy problem, neutrino masses, and the baryon asymmetry of the universe. The LHC may be producing a significant number of neutral LLPs with masses above a GeV, yet these elusive particles could be escaping detection by the main LHC experiments. To bridge this gap, we have recently produced a Conceptual Design Report for the MATHUSLA detector (MAssive Timing Hodoscope for Ultra-Stable neutraLpArticles), a dedicated surface-based experiment positioned above CMS designed to operate during the High-Luminosity LHC era. MATHUSLA consists of multiple layers of plastic scintillators with wavelength-shifting fibers coupled to silicon photomultipliers, surrounding a large, air-filled decay volume. In this talk, we will present the 40-square-meter MATHUSLA design, optimized for low construction cost while maintaining world-leading sensitivity to LLPs. Additionally, we will discuss our detailed Monte Carlo studies of rare Standard Model background processes in MATHUSLA, and show results from “demonstrator modules” that have been constructed at the University of Victoria and the University of Toronto.
Rouven Essig
Stony Brook University

Conclusion
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Juan Estrada
Fermilab

Dark Matter with Low Threshold Sensors Underground, in Beams and in Telescopes
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I will discuss current ideas and plans for exploring the dark sector using skipper- CCDs and other low-threshold silicon imagers in underground experiments, particle beams, and telescopes. There will be a special focus on developments centered at Fermilab, including opportunities for utilizing the future beam facilities at the lab.
Glennys Farrar
New York University

The Origin of Ultrahigh Energy Cosmic Rays
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Glennys Farrar will give a succinct overview of what is known and what remains uncertain about the spectrum, composition and arrival directions of ultrahigh-energy cosmic rays detected on Earth. Two features of the data compellingly single out binary neutron star mergers as the source of UCRs. A simple analysis of how UCRs are accelerated, initialized to the magnetic fields produced in the merger, predicts a spectrum in quantitative agreement with observation. Smoking-gun predictions of the scenario are 1) coincidences between EHE neutrinos and the gravitational wave produced by the BNS merger and 2) the highest energy UCRs are more massive than iron, having originated as r-process elements.
Jonathan Feng
UC Irvine

Intro
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Stefania Gori
University of Califonia, Santa Cruz

Axion Mediated Dark Matter: from Models to Experimental Tests
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The direct detection of sub-GeV dark matter (DM) remains a significant challenge due to the low recoil energies involved. In this talk, we focus on the scattering rate of sub-GeV DM particles interacting via spin-dependent couplings with nucleons, in solid-state targets. For DM masses below 100 MeV, the dominant scattering process involves incoherent multiphonon production, which offers a promising avenue to extend sensitivity to this low-mass regime. We evaluate the potential of upcoming experiments to detect such interactions and compare their sensitivity to existing constraints, including those from stellar cooling limits, beam dump experiments, and meson factory searches that probe the mediating particle. Incorporating these bounds, we estimate that future detectors could observe a few scattering events per kilogram per year of exposure.
Anson Hook
University of Maryland

Predicting the Dark Matter–Baryon Abundance Ratio
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We discuss relaxation solutions to the dark matter–baryon coincidence problem in the context of QCD axion dark matter. In relaxation solutions, a moduli dynamically adjusts the mass of dark matter and baryons until their energy densities are O(1) the same. Because the QCD axion is heavily connected to QCD, scanning the QCD axion mass inherently also scans the proton mass. In the context of relaxation solutions, this implies that the ratio of dark matter to baryon abundances (ΩDM/ΩB) is a ratio of beta functions, showing that these models can only accommodate discrete values of ΩDM/ΩB, thereby “predicting” the ratio of the dark matter to baryon abundances.

The original composite axion model has only a single integer degree of freedom N, the size of the gauge group, and we show that when N = 8, the observed value of ΩDM/ΩB = 5.36 is reproduced to within its percent level error bars. Novel tests of this model include more precise measurements of ΩDM/ΩB, a better lattice determination of the dependence of the proton mass on the high-energy QCD gauge coupling, as well as more traditional tests such as fifth force experiments.
Felix Kling
DESY

From FASER to the FPF: Looking Forward for Illuminate Dark Matter at the LHC
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Physics searches and measurements at high-energy collider experiments traditionally focus on the high-pT region. However, for light and weakly-coupled particles, this emphasis may be misguided, as light particles tend to be highly collimated around the beam line, allowing sensitive searches with small detectors. The FASER experiment was specifically designed to capitalize on this opportunity, expanding the LHC’s physics potential by searching for feebly interacting particles and studying neutrino interactions at TeV energies. The proposed Forward Physics Facility aims to extend this program in the HL-LHC era. In this talk, I will provide an overview over this emerging forward physics program and its potential to offer unique insights for dark sector searches and neutrino measurements.
Zhen Liu
University of Minnesota

Beam/Power Sourced Millicharged Particles and Dark Photons
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In this talk, I will first present recent progress in using Beam to source millicharged particles, as well as new search constraints and projections from various technologies, ranging from Liquid Argon to Skipper CCD. In the second part, I will present recent developments in a light-shining-through-wall experiment with high- quality superconducting RF cavities, known as Dark SRF.
Maxim Pospelov
University of Minnesota

Neutron Stars as Probes of Dark Sector
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Neutron stars are born in the violent supernova explosions, when the temperatures reach tens of MeV, replicating in some aspects the environment of the very early Universe. We investigate the faith of MeV dark matter particles in the freeze-in regime that get produced and gravitationally trapped in the newly born neutron stars. We show that the late time annihilation of dark matter back to the Standard Model states will lead to an additional source of heat generation that may be incompatible with observation of stars’ surface temperatures. This sets nontrivial constraints on the space of dark matter models.
Subir Sarkar
Oxford University

A Challenge to the Standard Cosmological Model
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In the ΛCDM cosmological model, the Universe is assumed to be isotropic and homogeneous when averaged on large scales. That the Cosmic Microwave Background has a dipole anisotropy is interpreted as due to our peculiar (non- Hubble) motion because of local inhomogeneity. There must then be a corresponding dipole in the sky distribution of sources at high redshift. Using catalogues of radio sources and quasars, we find that this expectation is rejected at

>5σ, i.e., the distribution of distant matter is not isotropic in the ‘CMB frame.’ This calls into question the standard practice of boosting to this frame to analyse cosmological data, in particular to infer acceleration of the Hubble expansion rate using Type Ia supernovae, which is then interpreted as due to a cosmological constant Λ. We find that the inferred acceleration is in fact anisotropic (in the direction of the CMB hotspot) and likely a general relativistic effect because of our being embedded in a coherent bulk flow — rather than due to dark energy.
Kate Scholberg
Duke University

Dark Matter Searches at Spallation Sources
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I’ll describe experimental searches for dark matter at spallation sources. I’ll review current results and discuss prospects for the future.
Brian Shuve
Harvey Mudd College

Baryogenesis from Dark Matter Freeze-In
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Models of freeze in feature a departure from equilibrium over an extended period of the early universe’s history, and they can naturally give rise to the observed matter- antimatter asymmetry. I will present recent developments in freeze-in baryogenesis, including a novel, testable mechanism for the simultaneous production of dark matter and baryogenesis via dark matter oscillations. I will highlight the connections between the early universe dynamics and phenomenology, including tests at colliders, and in cosmological and astrophysical observables.
Yotam Soreq
Israel Institute of Technology

The Interplay Between Precision and Intensity for BSM Searches
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In this talk, we will discuss novel methods to hunt for new physics beyond the standard model at different energy and length scales, from atomic physics to future colliders.
Haibo Yu
UC Riverside

Astrophysical Signatures of Gravothermal Collapse in Dark Matter Halos
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Haibo Yu will present recent high-resolution N-body simulations of self-interacting dark matter and examine their implications across a wide range of astrophysical observations. Yu will highlight novel signatures associated with gravothermal collapse in dark matter halos. Yu will explore the prospects for detecting these signatures through observations of strong gravitational lensing systems, stellar streams, and supermassive black holes. These studies open new avenues for probing the fundamental nature of dark matter.