Illuminating Dark Matter (2018)

The identity of dark matter is one of the great scientific mysteries of our time. The field is currently undergoing a transformation. The odds-on favorites from earlier decades, WIMPs—while not excluded—are being increasingly squeezed by the lack of positive signals in direct detection or at the LHC. The other classic candidates, sterile neutrinos and axions, are being re-examined, with qualitatively new possibilities (especially as concerns detection) emerging. In the last decade, the community has dramatically broadened the range of dark matter theories it has studied, motivating new searches, including experiments that are remarkably small, cheap, and fast, but nevertheless provide sensitive probes of these new ideas. At the same time, the era of precision astrophysics and cosmology is placing powerful new constraints on dark matter candidates through the observation of dark matter clustering on small scales, an example of the microscopic dynamics of particles imprinting itself on macroscopic scales. These developments together make clear that the field is unlikely to look very similar in one decade to how it looks today.

The Simons Symposium on Illuminating Dark Matter sought to move the discussion forward at this pivotal point in time. It brought together 23 researchers to take a fresh look at dark matter. The meeting’s participants spanned the wide range of fields that are now connected with dark matter, with interests ranging from astrophysics and cosmology to particle physics, and even to condensed matter and atomic physics. Each participant contributed a 30-minute talk, which was followed by ample time for discussion. Participants were encouraged to consider different time periods (short, medium, long) and how their subfields could progress during each of these periods. In the afternoon, the Symposium included organized, but free-ranging, discussions, in which participants discussed hot and controversial topics in more depth and with more explanation than is usual at conferences. In this way, the Symposium, which was simultaneously informal and intense, bridged many divides, for example, between astrophysicists and particle physicists, and between theorists and experimentalists.

A major topic of discussion at the Symposium was the extent to which the small-scale structure of the distribution of dark matter in the universe could constrain or motivate new particle dark matter properties. Manoj Kaplinghat argued that a variety of disagreements between small-scale structure simulations and observations, particularly the diversity of halo distributions, could be taken as evidence for strongly self-interacting dark matter. Alyson Brooks, Phil Hopkins and Julio Navarro showed the promise—and limitations—of simulations for constraining dark matter interactions and structure. Through debate and discussion, it became clear that baryon and dark matter dynamics are still difficult to disentangle in their impact on the dark matter distribution of halos like the Milky Way and dwarf satellites. The field of simulations is not yet at the point that robust upper limits on dark matter self-interactions (in the presence of baryons) can be quoted, but it was emphasized in the discussion that a sign of precision in simulations would be robust upper bounds on dark matter interaction strengths that monotonically decrease with time. Jo Bovy showed that a particularly promising avenue for determining the clustering of dark matter on the smallest scales are narrow stellar streams in the halo of the Milky Way. Neal Dalal and Neal Weiner discussed how sensitive new gravitational lensing measurements could also dramatically increase our understanding of the small scale structure of dark matter. Because different dark matter candidates leave different imprints in the small scale structure, one might be able to differentiate between dark matter candidates this way.

Particularly interesting are light dark matter particles and dark sectors with masses in the meV to GeV range, which could explain many puzzles. Moreover, several mechanisms exist that can naturally generate dark matter with the correct relic abundance in this mass range. Recent years have seen an explosion of new ideas to detect such light dark matter particles. These ideas have often emerged from theoretical physicists thinking across several disciplines, including particle physics, condensed matter physics, cosmology, and atomic, molecular, and optical physics. But these ideas also require expert experimentalists and instrumentalists to sharpen these ideas and bring them to fruition. These developments are allowing physicists to explore vast new regions of dark-matter parameter space.

Several concrete experimental proposals have been developed to tackle the detection of these theories of dark matter. Rouven Essig discussed techniques, focused on electronic excitation and ionization in atoms and in semiconductors, to detect dark matter with mass in the MeV to GeV range. Javier Tiffenberg showed that an experiment using CCDs based on these ideas was near to being realized. Rafael Lang emphasized that, although it is important to pursue hidden sector dark matter, the WIMP endures and it is important to pursue the detection of this candidate all the way down to the neutrino background. Adam Ritz discussed how sub-MeV dark matter can be accelerated in the Sun to higher velocities and then be probed in direct-detection experiments on Earth. Roni Harnik discussed how dark matter detectors can be used to probe non-standard neutrino interactions, and how neutrino detectors can probe novel dark matter candidates.

In addition to detecting the presence of dark matter through its scattering off normal matter, dark matter can also be discovered by producing it in particle accelerators and colliders. This approach has been known for a long time, but the growth of interest in light dark matter has opened a vast array of new possibilities. Bertrand Echenard presented a comprehensive review of sub-GeV dark matter searches at accelerators, surveying the many new initiatives around the world with an emphasis on the proposed LDMX experiment, searching for invisible dark mediator decays with unprecedented sensitivity. Mauro Raggi focused on the possibility of discovering dark matter and dark sectors with positron beams, for example, through resonant searches for \(e^+ e^- \to X \to e^+ e^-\), where \(X\) is a new particle. In particular, PADME is starting to take data and may be able to definitively test many new physics explanations of the \(6.8\sigma\) beryllium 8 anomaly. Jonathan Feng reviewed motivations for the new emphasis on light dark sectors, emphasized the genericity of non-renormalizable portal interactions, and described FASER, a small and inexpensive proposed experiment that will extend the LHC’s sensitivity to light and weakly-interacting new particles.

While much of the discussion focused on the myriad of new experiments and probes being proposed (by both theorists and experimentalists), several talks emphasized that rich model building avenues remain. Kathryn Zurek considered a simple dark sector of asymmetric dark matter in the absence of a dark analogue of electromagnetism, and showed that huge bound states, as heavy as \(10^{19}\) GeV, are observationally and cosmologically viable and give rise to unique experimental signatures. Tomer Volansky discussed what the properties needed for dark matter to explain the recent 21 cm observation and also how dissipative dark matter could enhance the growth rate of supermassive black holes. Josh Ruderman sought to build a simple hidden sector dark photon model that also could explain the recent 21 cm observation. Jessie Shelton focused on cosmological and terrestrial signatures of hidden sectors with a dark radiation bath.

Finally, new progress was reported on a number of classic dark matter candidates. The recent observations of gravitational waves by LIGO has renewed interest in primordial black hole (PBH) dark matter. Bernard Carr reviewed the fascinating history of PBHs, the three open windows at intermediate sublunar, and asteroid masses, and stressed the interesting implications of PBHs even if they account for only some of the dark matter. Alex Kusenko presented a new paradigm for production of black holes in the early universe and noted that PBH dark matter can contribute to r-process nucleosynthesis, as well as lead to striking signatures, such as kilonova without gravitational wave counterparts and fast radio bursts. Aaron Chou reviewed the classic motivations for axion dark matter, presented “hot off the press” results from ADMX that have reached the DFSZ limit for masses around \(2 \, \mu\text{eV}\), and discussed a number of new ideas from quantum metrology to enable higher mass searches. Finally, Kev Abazajian discussed sterile neutrinos, the original dark fermions, and shed light on the tantalizing 3.5 keV line seen from galaxies and clusters of galaxies and its natural explanation in the context of sterile neutrino dark matter.

A PDF of the full agenda may be downloaded here.

A PDF of the participant list may be downloaded here.