Direct Detection of Dark Matter -- APPEC Committee Report

Abstract: This Report provides an extensive review of the experimental programme of direct detection searches of particle dark matter. It focuses mostly on European efforts, both current and planned, but does it within a broader context of a worldwide activity in the field. It aims at identifying the virtues, opportunities and challenges associated with the different experimental approaches and search techniques. It presents scientific and technological synergies, both existing and emerging, with some other areas of particle physics, notably collider and neutrino programmes, and beyond. It addresses the issue of infrastructure in light of the growing needs and challenges of the different experimental searches. Finally, the Report makes a number of recommendations from the perspective of a long-term future of the field. They are introduced, along with some justification, in the opening Overview and Recommendations section and are next summarised at the end of the Report. Overall, we recommend that the direct search for dark matter particle interactions with a detector target should be given top priority in astroparticle physics, and in all particle physics, and beyond, as a positive measurement will provide the most unambiguous confirmation of the particle nature of dark matter in the Universe.

• The paper highlights the importance of direct detection experiments by reviewing WIMP-based searches and innovative methods to probe dark matter.
• It details multiple detection techniques such as noble gas detectors, cryogenic bolometers, and scintillating crystals for various interaction types.
• The report advocates for significant infrastructure investments and international collaboration to overcome background challenges and drive future discoveries.

Overview and Recommendations: Direct Detection of Dark Matter

The search for dark matter (DM) is pivotal in advancing our understanding of the Universe, highlighting critical gaps in the Standard Model (SM) and presenting one of the most intriguing puzzles in modern physics. The APPEC Committee Report presents an extensive review of current and future direct detection efforts, primarily emphasizing activities within Europe but situating them within the broader international context.

Key Themes and Findings

1. Dark Matter Constitutes a Large Portion of the Universe: A substantial part, approximately 26%, of the Universe is composed of dark matter. The quest to directly detect this elusive component is high on the physics agenda due to its profound impact on understanding fundamental physical laws.
2. WIMP Searches Dominate Direct Detection Efforts: Weakly Interacting Massive Particles (WIMPs) remain a focal point due to their theoretical simplicity and potential to be directly detected via scattering on detector materials. Despite advanced non-observation at required sensitivities, WIMPs continue to motivate substantial experimental programs. The synergy between direct detection and indirect and collider searches is crucial for a comprehensive exploration.
3. Diverse Detection Techniques and Challenges: Multiple approaches for direct detection include using noble gases like xenon and argon, cryogenic bolometers, and scintillating crystals. Each technique varies in sensitivity to different DM mass ranges and interaction types, such as spin-independent and spin-dependent interactions. The formidable challenge posed by background noises, particularly from neutrinos, underscores the importance of technological advancements and methodological rigor.
4. Advancements and Infrastructure Needs: A decade-long push has improved detection sensitivity by nearly three orders of magnitude each decade, thanks to developments in technology and techniques. However, continued progress will require significant infrastructural investments, notably in enhancing current underground laboratories and forming networks for collaborative research.
5. Global Collaboration Bolstered by Strong European Leadership: Europe hosts several leading DM experiments, prominently identified in the report. Key projects like XENONnT, DarkSide-20k, and CRESST have shown advanced capabilities in probing DM parameter space. The European consortium's collaborative efforts are critical to future discoveries.

Strategic Recommendations

• Prioritization of Direct Detection: Direct detection efforts should remain top priorities due to their potential unambiguous identification and characterization of DM particles.
• Diverse Approach Across Mass and Interaction Ranges: A multipronged strategy should continue to explore the widest range of DM properties, maximizing discovery potential while minimizing model dependencies.
• Investment in Infrastructure: The enhancement of underground laboratories and the establishment of an integrated European Underground Science Laboratory would facilitate long-term advances in the field.
• Support for R&D and New Technologies: Continued support for detector R&D is vital to push technical boundaries, particularly in reducing backgrounds and enhancing detection capabilities.
• Cultivation of International Collaborations: European leadership should be leveraged to foster global partnerships that share technologies, methodologies, and findings across borders.

Theoretical and Future Implications

The report underscores the importance of direct detection to provide the most unambiguous evidence for particle DM. Theoretical advancements complement these efforts through refined models and simulations integrating findings from diverse techniques and cosmic observations.

With global scientific merits and challenges outlined, the report solidifies dark matter research as central to both scientific inquiry and technological advancement in particle physics. This strategic emphasis ensures robust exploration of DM properties in the approaching era of underground and potential space-based investigational platforms.