US Cosmic Visions: New Ideas in Dark Matter 2017: Community Report

TL;DR

The paper argues that while flagship experiments (G2) will cover key regions of axion and WIMP parameter space, a broad, cost-effective portfolio of small-scale experiments is essential to explore motivated ultra-light and hidden-sector DM candidates. It outlines two primary frameworks—ultralight bosonic DM (including QCD axions) and hidden-sector DM with new mediators—and details concrete experimental paths across direct detection, ultralight searches, and accelerator-based production. By mapping thermal and quasi-thermal relic targets to observable signals and emphasizing complementarity among techniques, the report argues for rapid, coordinated investments in theory and technology development. The goal is to illuminate diverse DM possibilities, leverage existing facilities, and accelerate progress toward identifying the particle nature of dark matter with broad, cross-disciplinary impact.

Abstract

This white paper summarizes the workshop "U.S. Cosmic Visions: New Ideas in Dark Matter" held at University of Maryland on March 23-25, 2017.

Figures (23)

• Figure 1: Mass ranges for dark matter and mediator particle candidates, experimental anomalies, and search techniques described in this document. All mass ranges are merely representative; for details, see the text. The QCD axion mass upper bound is set by supernova constraints, and may be significantly raised by astrophysical uncertainties. Axion-like dark matter may also have lower masses than depicted. Ultralight Dark Matter and Hidden Sector Dark Matter are broad frameworks. Mass ranges corresponding to various production mechanisms within each framework are shown and are discussed in Sec. \ref{['sec:sciencecase']}. The Beryllium-8, muon $(g-2)$, and small-scale structure anomalies are described in \ref{['WG4sec:WG4']}. The search techniques of Coherent Field Searches, Direct Detection, and Accelerators are described in Secs. \ref{['sec:WG2experiments']}, \ref{['sec:WG1experiments']}, and \ref{['sec:WG3experiments']}, respectively, and Nuclear and Atomic Physics and Microlensing searches are described in Sec. \ref{['WG4sec:WG4']}.
• Figure 2: Schematic illustration of the complementarity of different types of experiments in exploring QCD axion DM and ultralight DM more generally. The horizontal axis illustrates the observationally allowed mass range for ultralight DM, with an arrow highlighting the viable mass range for the QCD axion specifically. Indicative ranges of sensitivity for different techniques are illustrated by dark blue arrows for coherent field, new-force, and X-ray helioscope techniques (see Sec. \ref{['sec:WG2experiments']}, while a red arrow indicates the range of DM masses that can be explored by absorption in direct-detection experiments (see Sec. \ref{['sec:WG1experiments']}).
• Figure 3: Schematic illustration of the complementarity of different types of experiments in exploring sharp targets and general regions of interest for hidden-sector DM. Anomalies in data (see Section \ref{['sssec:HS-opportunities']}) highlight regions of interest in mediator mass and/or coupling to visible or dark matter; the red arrows highlight the suggested regions of mediator mass. Blue horizontal arrows for production mechanisms (see Sections \ref{['sssec:HS-thermal']}-\ref{['sssec:HS-freezein']}) indicate the parameter regions over which they are viable (dashed), regions in which they motivate a sharp parameter-space target (solid arrow), and, in the case of asymmetric DM, a "natural" range where the DM and baryon number densities are comparable (thick band). Blue and red vertical arrows highlight directions in "theory space" that have significant impact on detection strategies, while the green vertical arrows indicate the models to which different experimental approaches are most sensitive. Direct detection is discussed in Section \ref{['sec:WG1experiments']}, accelerator-based experiments in Section \ref{['sec:WG3experiments']}, and cosmology and nuclear and atomic physics probes in Section \ref{['WG4sec:WG4']}.
• Figure 4: Ideas to probe low-mass DM via scattering off, or absorption by, nuclei (NR) or electrons (ER).
• Figure 5: Sample processes considered in this section to detect DM, $\chi$. Top left: DM-nucleus scattering. Top middle: DM-electron scattering. Top right: DM-nucleus scattering with emission of a photon. Bottom left: Absorption by an electron of a bosonic DM particle (a vector $A'$, scalar $\phi$, or pseudoscalar $a$). Bottom middle: Absorption by an electron of a bosonic DM particle, made possible by emission of a phonon $\Phi$. Bottom right: Emission of multiple phonons in DM scattering off helium.