The Dark Matter Scientific Assessment Group (DMSAG) A Joint Sub-panel of HEPAP and AAAC Report on the Direct Detection and Study of Dark Matter July 5, 2007 Table of Contents Table of Contents................................................................................................................ 2 I. Executive Summary......................................................................................................... 4 Scientific Introduction .................................................................................................... 7 Findings & Recommendations...................................................................................... 11 1. Findings................................................................................................................. 11 2. Recommendations................................................................................................. 15 II. Discussions................................................................................................................... 18 1. Theoretical Motivation.............................................................................................. 18 2. Current State of Technology and Issues to be Resolved........................................... 21 2.1 Axion searches.................................................................................................... 22 2.2 WIMP Searches .................................................................................................. 24 2.2.1 Solid State Detectors........................................................................................ 24 2.2.2 Noble Liquids................................................................................................... 29 Xenon........................................................................................................................ 30 Argon and Neon........................................................................................................ 33 2.2.3 Warm Bubble Detectors................................................................................... 38 2.2.4 Direction-Sensitive Detectors .......................................................................... 40 2.2.5 New Techniques............................................................................................... 41 III. Maintaining U. S. Leadership ..................................................................................... 45 IV. Facilities and Space for the Direct Detection Program .............................................. 47 V. Standards of Proof for Dark Matter ............................................................................. 51 VI. Theory......................................................................................................................... 53 1. Overview................................................................................................................... 53 2. Evidence for the Existence of Dark Matter............................................................... 54 2.1 Clusters of galaxies............................................................................................. 54 2.2 Rotation Curves .................................................................................................. 54 2.3 Lensing................................................................................................................ 55 2.4 Hot Gas in Clusters ............................................................................................. 56 2.5 The Cosmic Microwave Background and other Cosmological Measurements.. 57 2.6 Alternatives to Dark Matter ................................................................................ 58 3. Dark Matter Candidates ........................................................................................... 59 3.1 Axion................................................................................................................... 59 3.2 WIMPs ................................................................................................................ 60 3.3 SuperWIMPs....................................................................................................... 61 3.4 Exotic Candidates ............................................................................................... 61 4. Dark Matter Relic Density........................................................................................ 62 4.1 The WIMP miracle ............................................................................................. 62 4.2 Relic density of a thermal WIMP ....................................................................... 62 4.3 Relic density of neutralinos in supersymmetric models ..................................... 64 4.4 Relic density of a superWIMP............................................................................ 66 4.5 Relic density of axions........................................................................................ 66 5. Direct detection of WIMPs and axions..................................................................... 67 5.1 WIMP-nucleon scattering cross sections............................................................ 67 2 5.2 Target detection rates for various supersymmetric models ................................ 68 5.3 Results from DAMA and spin-dependent direct detection................................. 70 5.4 WIMP mass determination ................................................................................. 71 5.5 Direct detection of axion dark matter ................................................................. 73 6. Distribution of dark matter in the galaxy.................................................................. 73 7. Indirect detection of WIMP dark matter................................................................... 77 7.1 Neutrino Telescopes............................................................................................ 77 7.2 Gamma-ray and Cosmic-ray Telescopes ............................................................ 78 7.3 Relation between direct and indirect dark matter detection................................ 79 8. Dark Matter at Colliders ........................................................................................... 81 8.1 WIMPs at colliders ............................................................................................. 81 8.2 SUSY at colliders................................................................................................ 81 8.3 Extracting astrophysical results from collider measurements. ........................... 82 8.4 SuperWIMPs at Colliders ................................................................................... 85 APPENDIX A – Review of Neutron Backgrounds .......................................................... 86 APPENDIX B – Glossary of Experiments ....................................................................... 91 APPENDIX C – DMSAG Panel Charge .......................................................................... 92 APPENDIX D – DMSAG Panel Members ...................................................................... 94 APPENDIX E – Schedule of Panel Meetings................................................................... 95 3 I. Executive Summary DMSAG has been charged to make a detailed examination of the field of direct detection of dark matter and to consider it in the broader context of particle physics and astrophysics. Some specific guidance was provided in the Charge to DMSAG and includes questions such as these abstracted from the Charge Letter (see Appendix C) and listed immediately below. The progress in experimental techniques in the field is in a state of rapid flux and new levels of sensitivity have been reached; many advances have been made during the course of the work of this sub-panel. From present evidence, further advances can be confidently expected in both the near and long term. The funding for the US direct detection program is currently severely limited and constraining relative to the new opportunities and importance of the field. Consequently, answers to many of the questions posed are often conditional on present and soon to be expected progress in funding and technology. The DMSAG response to the Charge questions are contained in a set of Findings and Recommendations summarized at the end of this Section and repeated in distribution through-out the relevant portions of the text. To aid the reader, we cite in the questions below those Findings, Recommendations and Sections most directly related to a particular question; each may appear more than once. For analyses leading to the Findings and Recommendations the reader is directed to the topical Sections indicated in the Table of Contents. • What are the most promising experimental approaches for the direct detection of dark matter using particle detectors in underground laboratories? Cryogenic techniques based on solid state (phonons and ionization in Ge and Si) and noble liquids (in two-phase systems of both liquid Xe and liquid Ar) are presently leading the field and showing the greatest promise for coherent scattering of WIMPs. Methods with single phase liquid argon and warm liquids or gases are showing significant promise for the future. (The leading axion search, ADMX, does not require underground siting.) See Findings #3, 4, and 5 and Recommendations #3, 4, 5 and 8 and Sec. II-2. • What are: the relative advantages, disadvantages,