Illuminating Dark Matter (2023)

The search for dark matter has undergone a drastic shift recently. For decades, the main dark matter candidate has been a weakly interacting massive particle, or “WIMP,” which, despite its meek name, has dominated experimental searches. Nowadays, dark matter physicists have broadened the theoretical possibilities for what constitutes dark matter and have dramatically expanded searches for it. In fact, a large range of experiments and astrophysical probes still need to be developed to probe dark matter candidates with a diverse range of masses and particle properties and to decipher which candidates have been realized by nature.

Dark matter properties, such as self-interactions or dissipative interactions, can impact how cosmic structure forms on small scales. Several participants described the incredible progress in simulating the formation of small-scale structure. Alyson Brooks focused on simulations of dwarf galaxies, contrasting their properties in a universe containing cold, collisionless dark matter with one containing self-interacting dark matter. Phil Hopkins gave an overview of the progress being made in simulating galaxy formation in a variety of dark matter models, including primordial black holes, ultralight, dissipative, warm, strongly self-interacting and atomic dark matter, highlighting the challenges that current simulations face. Frank van den Bosch echoed the challenges of making sufficiently reliable predictions of various dark matter models to compare with the data, focusing on the prospects of using substructure to constrain the nature of dark matter. Mariangela Lisanti showed recent simulations of dissipative dark matter, highlighting the surprising impact that even a subdominant component of such dark matter can have on the formation of galaxies.

Several participants described how astrophysical and cosmological observations can constrain the properties of dark matter. Vasily Belokurov showed how observations of stellar streams can detect and measure small-scale structure and how this has already been used, for example, to measure the mass of the Large Magellanic Cloud. Priyamvada Natarajan described an excess small-scale galaxy-galaxy strong lensing signal from observations of galaxy clusters and what this might imply for the properties of dark matter. Neelima Sehgal highlighted the breadth of science that can be learned from future high-resolution, high-precision observations of the cosmic microwave background from CMB-HD, including what can be discovered about the properties of new, light species and about dark matter. Tomer Volansky discussed the physics of 21-cm observations and how different dark matter properties can impact the 21-cm signal. Masha Baryakhtar described a novel proposal for a modification to optical intensity interferometers that allows for microarcsecond astrometry at angular separations as large as several arcseconds. She discussed how this design, combined with other technological advances, allows for the unprecedented precision in angular resolution of stars and described several applications. Neal Weiner showed the effects from various dark-sector scenarios on big bang nucleosynthesis, the cosmic microwave background, the distribution of structure and the Hubble parameter H0.

One area of continued work is axion dark matter, both theoretically and prospects for covering the motivated part of parameter space that is largely untouched observationally. Aaron Chou gave an overview of the U.S. experimental program in axion detection, from cavity experiments like ADMX and HAYSTAC, to the high mass axion searches like BREAD and MADMAX, to the low mass end featured in the NMR searches as well as DM-Radio. Aaron asked the question of what would seriously be required to cover the whole QCD axion mass range within one generation of physicists, given that the technologies are available but simply need to be scaled up. Ben Safdi focused instead on the theoretical side of axion dark matter in two aspects, first the cosmic string contribution to axion production. Then, however, Safdi argued that axions consistent with UV completions (such as in string theory) that solve the axion quality problem do not generically have the types of structures that produce axion strings or miniclusters, upending two of the types of axion production mechanisms. Yue Zhao discussed how axions could be detected through polarimetric measurements by the Event Horizon Telescope — the axion background near a black hole induces birefringence in linearly polarized photons that can be detection for a large enough axion-like coupling.

New detector technologies are also being developed. One example is the development of ultra-sensitive gravitational wave detectors, or optomechanics more generally, for dark matter detection. This is bringing in tools that have been developed for other areas of physics for application to fundamental physics. Diego Blas gave an overview of the motivation for the detection of high-frequency gravitational waves, as well as their detection. He discussed the possibility of utilizing cavities normally employed for axion detection instead for high frequency gravitational waves and explored the strain sensitivity. Hartmut Grote discussed detection of ultralight dark matter with gravitational wave interferometers. Marianna Safronova gave an overview of quantum technologies to search for dark matter, both in the laboratory and space, including clocks and atom interferometers. Highly sensitive detectors can search for the extremely faint whispers of dark matter. Javier Tiffenberg described the progress in Skipper-CCD detectors and a new multi-amplifier-sensing architecture that allows for the reduction of the readout time by orders of magnitude. He also introduced a novel CCD-like sensor concept with the capability to provide millisecond time resolution and to suppress dark counts by several orders of magnitude.

Model building for dark matter relic abundance — and implications for high energy collider experiments — continues to be an area of active interest and research. Joshua Ruderman explored a novel model for producing the dark matter relic abundance through a pandemic, exponentially growing mechanism. Josef Pradler discussed the strongly interacting massive particle (SIMP) paradigm for dark matter production when there are additional states available at the time of dark matter freeze-out. Lisa Randall discussed several models of dark matter, how the correct relic abundance can be generated in them, and how they might impact structure formation. Michele Papucci explored the consequences of hidden sector dark matter for collider explorations of hidden sectors through long-lived particles, through experiments such as CODEX-b and FASER.

Dark matter remains a vibrant and diverse field of research focused on observational consequences, both astrophysically and through laboratory probes. The symposium offered the opportunity for experts from diverse fields, including numerical N-body simulations, astrophysical and cosmological observations, particle theory, instrumentation, and collider phenomenology to discuss the common directions in the search for the nature of dark matter.

Masha Baryakhtar
University of Washington

Precision Astrometry with Extended Path Intensity Interferometry
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Masha Baryakhtar will present a proposal for a modification to optical intensity interferometers that allows for microarcsecond astrometry at angular separations as large as several arcseconds. The modification introduces an additional, adjustable path length into the optics, which creates a primary interference fringe for widely separated sources. Combined with other recent technological advances in spectroscopy and fast single-photon detection, this design could allow for unprecedented precision in angular resolution of stars. Promising applications include astrometric microlensing of stars, binary-orbit characterization, exoplanet detection and galactic acceleration measurements.
 

Vasily Belokurov
University of Cambridge

Detecting and Measuring Small-Scale Dark Matter Substructure with Stellar Streams
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Diego Blas
Universitat Autònoma de Barcelona

Detecting (High Frequency) Gravitational Waves in a Box
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In the presence of a background electromagnetic (EM) field, the passage of gravitational waves (GWs) generate different modes in EM cavities by either their effect on the background field or by interacting with the boundaries of the cavity. In this talk, Diego Blas will describe this process in some detail and show that these set-ups provide some of the best bounds for GWs in the MHz-GHz band. Blas will finish by briefly describing the way GWs interact with spin systems.
 

Alyson Brooks
Rutgers University

Dwarf Galaxy Formation in CDM vs SIDM
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After assembling the largest and most realistic sample of simulated dwarf galaxies to date, Alyson Brooks and collaborators are now re-running a subset in SIDM to investigate the predicted changes in galaxy formation. They use an observationally motived, velocity-dependent cross-section for interaction. Brooks will present preliminary results, including for the diversity of rotation curves and the shapes of galaxies and their halos.
 

Aaron Chou
Fermilab

How to Discover Axion Dark Matter for Real
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Direct detection of waves of axion dark matter presents an extreme challenge. After decades of R&D on nearly quantum-limited receivers, experiments such as ADMX, HAYSTAC and CAPP-12TB have achieved sensitivity to the dimensionless axion-photon coupling of ~0.3. The coverage can be best characterized as tiny slivers of axion mass when compared to the many orders of magnitude range of unprobed axion masses. Even if the current frequency scan rate could be kept up (it cannot), it would take 300 years to cover the remaining parameter space. Furthermore, attempting to reach a slightly weaker but still very theoretically reasonable coupling of 0.1 would increase the required integration time by another factor of 100. In this talk, Aaron Chou will survey some experimental strategies to improve signal/noise ratio and increase the frequency scan speed. This will hopefully provoke a discussion of what it is going to take to get serious about discovering the quantum chromodynamics (QCD) axion within our lifetimes.
 

Hartmut Grote
Cardiff University

Searching for Ultra-Light Dark Matter with Laser Interferometry
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Phil Hopkins
California Institute of Technology

Progress and Cautions from Astrophysical Simulations
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Phil Hopkins will summarize some recent work from the astrophysical simulations side. Great progress has been made in the last few years on explicitly simulating both different dark-sector models (ultralight, primordial black holes, dissipative, warm, strongly self-interacting, atomic/mirror, etc.) and physics of interest for many potential signature sectors (galaxy formation/structure and stellar feedback but also cosmic rays and gamma-ray astrophysics, line emission processes, ultra-high-redshift galaxies, etc.). Unsurprisingly, many of the supposed signatures (various high-energy “excesses,” unusual galaxy dynamics/mass profiles, massive high-redshift galaxies) can be explained without new dark-sector physics (though they do not rule them out). However, it is also the case that many dark matter models produce non-linear phenomenology which strongly differs from the “conventional wisdom” and can be non-monotonic in parameters like the cross-section, mass, etc. And it continues to be difficult for astrophysical simulations to reproduce the densest observed galaxy populations.
 

Mariangela Lisanti
Princeton University and Flatiron Institute

Simulations of Strongly Dissipative Dark Matter and Its Impact on Galaxy Formation
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Priyamvada Natarajan
Yale University

Persistent Excess of Galaxy-Galaxy Strong Lensing in Clusters

An excess of observed small-scale galaxy-galaxy strong lensing events has been reported in cluster lenses. The data presents an order of magnitude discrepancy with predictions of lambda cold dark matter (LCDM) from simulations. Upon comparison across multiple simulation suites, with varying resolutions and feedback physics implementations, we find that this tension with LCDM persists. This might signal a hitherto unknown issue with either the simulation methods or our assumptions regarding the standard cosmological model may be wanting. Priyamvada Natarajan will present new results exploring implications for alternative dark matter models to account for this small-scale discrepancy.
 

Michele Papucci
California Institute of Technology

Searches for Long-Lived Particles
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Josef Pradler
University of Vienna & Austrian Academy of Sciences

SIMPle Thoughts
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Josef Pradler will discuss the dynamics of strongly interacting massive particles (SIMPs) when they freeze-out in the presence of further available states in the spectrum and identify new avenues for the relic density generation.
 

Joshua Ruderman
New York University

Exponentially Growing Dark Matter
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Ben Safdi
University of California Berkeley

The Post-Inflationary QCD Axion Mass Prediction
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Ben Safdi will present updated results of numerical simulations to determine the mass of the quantum chromodynamics (QCD) axion when the Peccei–Quinn (PQ) symmetry is broken after inflation. These results expand upon https://arxiv.org/abs/2108.05368, which was the first analysis to use adaptive mesh refinement (AMR) to evolve the axion string network in the early universe. Safdi will discuss a new suite of simulations, with roughly an order of magnitude increase in dynamical range, and possible sources of systematic uncertainty, such as axions produced during domain wall collapse. Additionally, Safdi will comment on the dependence of the axion mass prediction on the ultraviolet (UV) completion, such as field theory versus extra dimension UV completions, along with work using the suite of AMR simulations to understand how axion-like-particle strings may be constrained by the visible radiation that they inject into the early universe.
 

Marianna Safronova
University of Delaware

New Ideas in Ultralight Dark Matter Searches in the Laboratory and in Space
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The extraordinary advances in quantum control of matter and light have been transformative in the development of quantum technologies enabling new searches for ultralight dark matter. Deployment of quantum technologies in space presents fantastic opportunities for paradigm-changing discoveries and enables exploration. Marianna Safronova will discuss new ideas in ultralight dark matter searches with atom-based quantum technologies and a nuclear clock in the laboratory and in space.
 

Neelima Sehgal
Stony Brook University

Dark Matter Discoveries from CMB-HD, a Stage-5 CMB Facility
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Cosmic microwave background (CMB) experiments have contributed powerful constraints on the fundamental physics of the universe. Upcoming CMB experiments such as the Simons Observatory and CMB-S4 are poised to extend this progress even further. However, CMB experiments still have a wealth of information to offer beyond these near-term facilities regarding the properties of dark matter, inflation, light relic particles and dark energy. A much lower-noise and higher-resolution wide-area CMB survey can cross a number of critical fundamental physics thresholds and open a relatively untapped window of late-time CMB anisotropies. Here Neelima Sehgal will discuss CMB-HD, a Stage-5 CMB facility, and focus in particularly on the dark matter science it can enable.
 

Javier Tiffenberg
Fermilab

A New Lamp Post

The non-destructive readout capability of the Skipper Charge-Coupled Device (CCD) has demonstrated to be a powerful technique in reducing the noise limitation of conventional devices, enabling single-electron counting. Instruments based on Skipper-CCD technology, such as SENSEI and DAMIC-M, are leading the field in the search of light-DM candidates that interact with electrons. However, the noise reduction technique of the Skipper-CCDs requires taking several measurements of the same pixel charge, and this has a significant impact on the occurrence of pile-up events with two or more electrons produced by dark current during readout, which can take several hours. Javier Tiffenberg will present a new multi-amplifier-sensing architecture that allows for the reduction of the readout time by orders of magnitude. Tiffenberg will report results from working sensors at Fermilab that utilize this design. Furthermore, Tiffenberg will introduce a novel CCD-like sensor concept with the capability to provide millisecond time resolution and suppress dark counts by two orders of magnitude, opening a new avenue for direct detection experiments. Tiffenberg will discuss the science reach of these new sensors and the plans for their fabrication.
 

Frank van den Bosch
Yale University

Prospects of Using Dark Matter Substructure to Constrain the Nature of Dark Matter
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The abundance and demographics of dark matter substructure depends on the nature of dark matter. Hence, numerous studies are underway to try and constrain the mass function and density profiles of dark matter subhalos, using a variety of techniques, ranging from counting satellite galaxies, to gravitational lensing, to gaps in stellar streams. Making the most of this ongoing work requires that, given a model for the dark matter, we can make reliable predictions for comparison with the data. Frank van den Bosch will give a short overview of what is considered to be significant hurdles in this enterprise.
 

Neal Weiner
New York University

Dark-Sector Physics Below the MeV Scale
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Neal Weiner will discuss the effects on big bang nucleosynthesis (BBN), cosmic microwave background (CMB), structure, Hubble parameter H0 from various dark-sector scenarios, including neutrino heating/cooling, dark sector steps, etc.
 

Yue Zhao
University of Utah

Probing Axions with Event Horizon Telescope Polarimetric Measurements
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With high spatial resolution, polarimetric imaging of a supermassive black hole, like M87* or Sgr A*, by the Event Horizon Telescope can be used to probe the existence of ultralight bosonic particles, such as axions. Such particles can accumulate around a rotating black hole through the superradiance mechanism, forming an axion cloud. When linearly polarized photons are emitted from an accretion disk near the horizon, their position angles oscillate due to the birefringent effect when traveling through the axion background. Recently, the polarization properties of the radiation near the supermassive black hole M87* are measured in four individual days. This is exactly what is needed to test the existence of a dense axion cloud. Yue Zhao will apply the azimuthal distribution of EVPA measured by the EHT and study the axion-photon coupling. The EHT data can rule out a considerable portion of the axion parameter space for the axion mass window from 10^−21 to 10^−20 eV, which was unexplored by previous experiments.