Direct Detection of Dark Matter APPEC Committee Report Version 1.02 (9 October 2020)
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Direct Detection of Dark Matter APPEC Committee Report Version 1.02 (9 October 2020) Committee Members: Julien Billard1, Mark Boulay2, Susana Cebrián3, Laura Covi4, Giuliana Fiorillo5, Anne Green6, Joachim Kopp7, Béla Majorovits8, Kimberly Palladino9;12, Federica Petricca8, Leszek Roszkowski10 (chair), Marc Schumann11 1 Univ Lyon, Université Lyon 1, CNRS/IN2P3, IP2I-Lyon, F-69622, Villeurbanne, France 2 Department of Physics, Carleton University, Ottawa, Canada 3 Centro de Astropartículas y Física de Altas Energías, Universidad de Zaragoza, Zaragoza, Spain 4 Institute for Theoretical Physics, Georg-August University, Goettingen, Germany 5 Physics Department, Università degli Studi “Federico II” di Napoli and INFN Napoli, Naples, Italy 6 School of Physics and Astronomy, University of Nottingham, Nottingham, UK 7 CERN, Geneva, Switzerland and Johannes Gutenberg University, Mainz, Germany 8 Max-Planck-Institute for Physics, Munich, Germany 9 Department of Physics, University of Wisconsin - Madison, Madison, WI, USA 10 Astrocent, Nicolaus Copernicus Astronomical Center PAS and National Centre for Nuclear Research, Warsaw, Poland 11 Institute of Physics, University of Freiburg, Freiburg, Germany 12 Department of Physics, Oxford University, Oxford, UK 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, opportunit- ies and challenges associated with the different experimental approaches and search techniques. It presents scientific and technological synergies, both ex- isting and emerging, with some other areas of particle physics, notably collider and neutrino programmes, and beyond. It addresses the issue of infrastruc- ture 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. DRAFT - NOT FOR DISTRIBUTION Contents 1 Overview and Recommendations...............................4 2 Introduction..........................................9 3 Dark Matter – Evidence, Properties and Candidates..................... 10 3.1 Evidence for Dark Matter.................................. 10 3.1.1 Astronomical and Cosmological Evidence for Non-Baryonic Dark Matter....... 10 3.1.2 Observational Probes of the Nature and Abundance of Dark Matter........... 10 3.2 Particle Physics Candidates................................. 11 3.2.1 General Properties.................................... 11 3.2.2 Thermal WIMP Dark Matter from Freeze-Out..................... 12 3.2.3 Non-thermal and Other Alternatives to the Thermal WIMP DM Paradigm....... 15 3.2.4 Axions.......................................... 16 3.3 Alternatives to Particle Dark Matter............................ 18 4 Underground Searches for WIMPs.............................. 19 4.1 Direct Detection Principles................................. 19 4.2 Interactions......................................... 20 4.3 Experimental Approaches.................................. 21 4.4 Backgrounds and Background Mitigation Strategies.................... 24 4.5 Current Status........................................ 25 4.6 Status and Plans of Experimental Activities........................ 27 4.6.1 Bolometers........................................ 27 4.6.2 Liquid Xenon Detectors................................. 31 4.6.3 Liquid Argon Detectors.................................. 35 4.6.4 Scintillating Crystals, Ionisation Detectors, Bubble Chambers............. 39 4.6.5 Directional Detectors................................... 43 4.7 Comparison of Different Experimental Efforts....................... 45 4.8 SWOT Tables for WIMP Experiments........................... 48 5 Searches for Axions and ALPs................................ 48 5.1 Principles of Detection................................... 48 5.1.1 Haloscopes........................................ 54 5.1.2 Helioscopes........................................ 56 5.1.3 Laboratory experiments.................................. 56 5.2 Cavity Haloscope Experiments............................... 57 5.3 Dielectric Haloscope and Dish Antenna Experiments................... 58 5.4 Experiments for Low Mass Dark Matter Axions and ALPs................ 58 5.5 Helioscope Experiments.................................. 59 5.6 Light-Shining-Through-the-Wall Experiments....................... 59 DRAFT5.7 Vacuum Polarisation - NOT and Bi-refringence FOR Experiments DISTRIBUTION................... 60 5.8 Low-Background Experiments............................... 60 5.9 Current Status, Limits and Projections........................... 61 5.10 Axion Research: R&D Efforts and Synergies with Other Physics Areas.......... 62 2 5.10.1 High field large aperture magnets............................. 62 5.10.2 Development of (sub-)quantum limited and single photon detectors:.......... 64 5.10.3 Low loss RF technology and cavities........................... 65 5.10.4 Optical cavities and lasers................................ 65 5.10.5 Nuclear magnetic resonance technologies and spin coupling.............. 65 5.10.6 X-ray detectors...................................... 65 5.10.7 Cryogenic engineering.................................. 65 5.11 SWOT Tables for Axion/ALPs Experiments........................ 66 6 Broader Context........................................ 67 6.1 WIMP Searches....................................... 68 6.1.1 Indirect Searches..................................... 68 6.1.2 Searches at the Large Hadron Collider.......................... 71 6.1.3 Astrophysical Probes of Nature of DM......................... 73 6.2 Other Physics at Direct DM Detection Experiments.................... 73 6.2.1 Neutrino Physics..................................... 73 6.2.2 Neutrinoless Double Beta Decay and Other Rare Nuclear Decays............ 74 6.2.3 Other Aspects of Physics Beyond the Standard Model.................. 74 6.3 Other Constraints on Axions and ALPs.......................... 75 6.4 Infrastructure and Deep Underground Laboratories.................... 76 7 List of Recommendations................................... 80 Glossary and Acronyms 81 References............................................... 84 DRAFT - NOT FOR DISTRIBUTION 3 1 Overview and Recommendations The nature of invisible dark matter (DM) that constitutes some 26% of the mass-energy balance of the Universe remains one of the most fundamental puzzles in physics today. The most compelling solution to the DM enigma is provided by postulating some new elementary particle that must be outside of the spectrum of the Standard Model (SM) – which in fact provides one of the strongest arguments in support of “new physics” beyond the SM (BSM). The hypothetical relic particle is probably cold, i.e., non-relativistic, but otherwise in principle not much is known about its basic properties, with the allowed mass range spanning nearly fifty decades, while non-particle DM, for instance in the form of (primordial) black holes, can be even much heavier. Likewise, so far only gravitational effects of DM have been observed. Nevertheless, there are good reasons to expect that a particle DM relic exhibits also much less feeble interactions, up to the (electro)weak ones of the SM, as is the case of weakly interacting massive particles (WIMPs) in a large variety of BSM models, or intermediate ones, in the case of axions and, more generally, pseudoscalar axion-like particles (ALPs). A decades-long, intense, trans-national and increasingly global worldwide experimental WIMP search is conducted following three main strategies: direct detection (DD) of the scattering of a DM particle off a target in deep underground detectors, indirect detection (ID) of exotic products of DM pair annihilation (or possibly decay) in the Galactic halo and beyond, and their production in accelerators or fixed-target experiments. The underlying principle to search for axions and ALPs in haloscopes (DM axions), helioscopes (solar axions) and laboratory experiments is to primarily make use of axion-photon conversion in the presence of a strong magnetic field, but also other complementary couplings to elec- trons or nuclei are used. The prime scientific objectives of DD, in both WIMP and axion/ALP searches, are: (i) to detect a signal of DM particle direct interaction with the detector, and (ii) to determine its mass and interaction cross section, or else (iii) to experimentally exclude the broadest accessible ranges of both quantities. While direct detection of a DM particle will constitute a historical landmark in the exploration of the invisible Universe, it must be emphasised that this will be only the first step in a long quest to unravel the true nature of the particle constituting DM. This will in general require employing a multi-channel, mul- timessenger approach combining information also from ID, collider searches, astrophysics, cosmology and astronomy. Recommendation 1. The search for dark matter with the aim of detecting a direct signal of DM particle interactions with a detector should be given top priority in astroparticle physics, as a positive measurement will provide the most unambiguous confirmation of the particle nature of dark matter in the Universe. Until recently most experimental efforts concentrated on exploring: (i) the mass range from a few GeV=c2 up to